Category Theory in Context
Emily Riehl

Chapter 6 is adapted with permission from Chapter 1 of Categorical Homotopy Theory,by Emily Riehl, Cambridge University Press. © Emily Riehl 2014

To Peter Johnstone, whose beautiful Part III lectures provided my first acquaintance withcategory theory and form the skeleton of this book.
andTo Martin Hyland, who guided my initial explorations of this subject’s frontiers andinspired my aspirations to think categorically.

The aim of theory really is, to a great extent, thatof systematically organizing past experience insuch a way that the next generation, our students
and their students and so on, will be able toabsorb the essential aspects in as painless a way
as possible, and this is the only way in which you
can go on cumulatively building up any kind of
scientific activity without eventually coming to adead end.
M.F. Atiyah, “How research is carried out”
[Ati74]

Contents
Preface
Sample corollaries
A tour of basic categorical notions
Note to the reader
Notational conventions
Acknowledgments

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x
xi
xv
xvi
xvi

Chapter 1. Categories, Functors, Natural Transformations
1.1. Abstract and concrete categories
1.2. Duality
1.3. Functoriality
1.4. Naturality
1.5. Equivalence of categories
1.6. The art of the diagram chase1.7. The 2-category of categories

1
3
9
13
23
29
36
44

Chapter 2. Universal Properties, Representability, and the Yoneda Lemma
2.1. Representable functors
2.2. The Yoneda lemma
2.3. Universal properties and universal elements
2.4. The category of elements

49
50
55
62
66

Chapter 3. Limits and Colimits
3.1. Limits and colimits as universal cones
3.2. Limits in the category of sets
3.3. Preservation, reflection, and creation of limits and colimits
3.4. The representable nature of limits and colimits
3.5. Complete and cocomplete categories
3.6. Functoriality of limits and colimits
3.7. Size matters3.8. Interactions between limits and colimits

73
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84
90
93
99
106
109
110

Chapter 4. Adjunctions
4.1. Adjoint functors
4.2. The unit and counit as universal arrows
4.3. Contravariant and multivariable adjoint functors
4.4. The calculus of adjunctions
4.5. Adjunctions, limits, and colimits
4.6. Existence of adjoint functors

115
116
122
126
132
136
143

Chapter 5.

153

Monads and their Algebras
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CONTENTS

5.1.
5.2.
5.3.
5.4.
5.5.
5.6.

Monads from adjunctions
Adjunctions from monads
Monadic functors
Canonical presentations via free algebras
Recognizing categories of algebras
Limits and colimits in categories of algebras

154
158
166
168
173
180

Chapter 6. All Concepts are Kan Extensions
6.1. Kan extensions
6.2. A formula for Kan extensions
6.3. Pointwise Kan extensions
6.4. Derived functors as Kan extensions
6.5. All concepts

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190
193
199
204
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Epilogue: Theorems in Category Theory
E.1. Theorems in basic category theory
E.2. Coherence for symmetric monoidal categories
E.3. The universal property of the unit interval
E.4. A characterization of Grothendieck toposes
E.5. Embeddings of abelian categories

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217
219
221
222
223

Bibliography

225

Catalog of Categories

229

Glossary of Notation

231

Index

233

Preface
Atiyah described mathematics as the “science of analogy.” In this vein, the purview
of category theory is mathematical analogy. Category theory provides a cross-disciplinary
language for mathematics designed to delineate general phenomena, which enables thetransfer of ideas from one area of study to another. The category-theoretic perspective canfunction as a simplifying1 abstraction, isolating propositions that hold for formal reasons
from those whose proofs require techniques particular to a given mathematical discipline.2
A subtle shift in perspective enables mathematical content to be described in languagethat is relatively indifferent to the variety of objects being considered. Rather than characterize the objects directly, the categorical approach emphasizes the transformations between
objects of the same general type. A fundamental lemma in category theory implies that any
mathematical object can be characterized by its universal property—loosely by a representation of the morphisms to or from other objects of a similar form. For example, tensor
products, “free” constructions, and localizations are characterized by universal properties inappropriate categories, or mathematical contexts. A universal property typically expresses
one of the mathematical roles played by the object in question. For instance, one universal
property associated to the unit interval identifies self-homeomorphisms of this space with
re-parameterizations of paths. Another highlights the operation of gluing two intervals end
to end to obtain a new interval, the construction used to define composition of paths.
Certain classes of universal properties define blueprints which specify how a new
object may be built out of a collection of existing ones. A great variety of mathematicalconstructions fit into this paradigm: products, kernels, completions, free products, “gluing”
constructions, and quotients are all special cases of the general category-theoretic notion oflimits or colimits, a characterization that makes it easy to define transformations to or fromthe objects so-defined. The input data for these constructions are commutative diagrams,
which are themselves a vehicle for mathematical definitions, e.g., of rings or algebras,
representations of a group, or chain complexes.
Important technical differences between particular varieties of mathematical objects
can be described by the distinctive properties of their categories: that rings have all limits
and colimits while fields have few, that a continuous bĳection defines an isomorphism ofcompact Hausdorff spaces but not of generic topological spaces. Constructions that convert
mathematical objects of one type into objects of another type often define transformations
between categories, called functors. Many of the basic objects of study in modern algebraic
topology and algebraic geometry involve functors and would be impossible to define without
category-theoretic language.
1In his mathematical notebooks, Hilbert formulated a “24th problem” (inspired by his work on syzygies) todevelop a criterion of simplicity for evaluating competing proofs of the same result [TW02].
2For example, the standard properties of induced representations (Frobenius reciprocity, transitivity of induction, even the explicit formula) are true of any construction defined as a left Kan extension; character tables,
however, are non-formal.
ix

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PREFACE

Category theory also contributes new proof techniques, such as diagram chasing orarguments by duality; Steenrod called these methods “abstract nonsense.”3 The aim of
this text is to introduce the language, philosophy, and basic theorems of category theory.
A complementary objective is to put this theory into practice: studying functoriality inalgebraic topology, naturality in group theory, and universal properties in algebra.
Practitioners often assert that the hard part of category theory is to state the correct
definitions. Once these are established and the categorical style of argument is sufficiently
internalized, proving the theorems tends to be relatively easy.4 Indeed, the proofs of several
propositions appearing in this text are left as exercises, with confidence that the reader will
eventually find it more efficient to supply their own arguments than to read the author’s.5 The
relative simplicity of the proofs of major theorems occasionally leads detractors to assertthat there are no theorems in category theory. This is not at all the case! Counterexamples
abound in the text that follows. A short list of further significant theorems, beyond thescope of a first course but not too far to be out of the reach of comprehension, appears as
an epilogue.
Sample corollaries
It is difficult to preview the main theorems in category theory before developing
fluency in the language needed to state them. (A reader possessing such fluency mightwish to glance ahead to §E.1.) Instead, here are a few corollaries, results in other areasof mathematics that follow trivially as special cases of general categorical results that are
proven in this text.
As an application of the theory of equivalence between categories:
Corollary 1.5.13. In a path-connected space, any choice of basepoint yields an isomorphic fundamental group.
A fundamental lemma in category theory has the following two results as corollaries:
Corollary 2.2.9. Every row operation on matrices with n rows is defined by left multiplication by some n × n matrix, namely the matrix obtained by performing the row operationon the identity matrix.
Corollary 2.2.10. Any group is isomorphic to a subgroup of a permutation group.
A special case of a general result involving the interchange of limits and colimits is:
Corollary 3.8.4. For any pair of sets X and Y and any function f : X × Y → R
sup inf f (x, y) ≤ inf sup f (x, y)
x∈X y∈Yy∈Y x∈X

3Lang’s Algebra [Lan02, p. 759] supports the general consensus that this was not intended as an epithet:In the forties and fifties (mostly in the works of Cartan, Eilenberg, MacLane, andSteenrod, see [CE56]), it was realized that there was a systematic way of developing
certain relations of linear algebra, depending only on fairly general constructions
which were mostly arrow-theoretic, and were affectionately called abstract nonsenseby Steenrod.
4A famous exercise in Lang’s Algebra asks the reader to “Take any book on homological algebra, and prove
all the theorems without looking at the proofs given in that book” [Lan84, p. 175]. Homological algebra is the
subject whose development induced Eilenberg and Mac Lane to introduce the general notions of category, functor,
and natural transformation.
5In the first iteration of the course that inspired the writing of these lecture notes, the proofs of several major
theorems were also initially left to the exercises, with a type-written version appearing only after the problem set
was due.

A TOUR OF BASIC CATEGORICAL NOTIONS

xi

whenever these infima and suprema exist.
The following five results illustrate a few of the many corollaries of a common theorem, which describes one consequence of a type of “duality” enjoyed by certain pairs ofmathematical constructions:
Corollary 4.5.4. For any function f : A → B, the inverse image function f −1 : PB → PA
between the power sets of A and B preserves both unions and intersections, while the direct
image function f∗ : PA → PB only preserves unions.
Corollary 4.5.5. For any vector spaces U, V, W,
U ⊗ (V ⊕ W)  (U ⊗ V) ⊕ (U ⊗ W) .
Corollary 4.5.6. For any cardinals α, β, γ, cardinal arithmetic satisfies the laws:
α × (β + γ) = (α × β) + (α × γ)

(β × γ)α = βα × γα

αβ+γ = αβ × αγ .

Corollary 4.5.7. The free group on the set X t Y is the free product of the free groups on
the sets X and Y.
Corollary 4.5.8. For any R–S bimodule M, the tensor product M ⊗S − is right exact.
Finally, a general theorem that recognizes categories whose objects bear some sort of“algebraic” structure has a number of consequences, including:Corollary 5.6.2. Any bĳective continuous function between compact Hausdorff spaces
is a homeomorphism.
This is not to say that category theory necessarily provides a more efficient proof ofthese results. In many cases, the proof that general consensus designates the “most elegant”
reflects the categorical argument. The point is that the category-theoretic perspective allows
for an efficient packaging of general arguments that can be used over and over again andeliminates contextual details that can safely be ignored. For instance, our proof that thetensor product commutes with the direct sum of vector spaces will not make use of any
bases, but appeals instead to the universal properties of the tensor product and direct sum
constructions.
A tour of basic categorical notions. . . the science of mathematics exemplifies theinterdependence of its parts.
Saunders Mac Lane, “Topology and logic as asource of algebra” [ML76]

A category is a context for the study of a particular class of mathematical objects.
Importantly, a category is not simply a type signature, it has both “nouns” and “verbs,”
containing specified collections of objects and transformations, called morphisms,6 between
them. Groups, modules, topological spaces, measure spaces, ordinals, and so forth form
categories, but these classifications are not the main point. Rather, the action of packaging
each variety of objects into a category shifts one’s perspective from the particularitiesof each mathematical sub-discipline to potential commonalities between them. A basic
observation along these lines is that there is a single categorical definition of isomorphism
6The term “morphism” is derived from homomorphism, the name given in algebra to a structure-preserving
function. Synonyms include “arrow” (because of the notation “→”) and “map” (adopting the standard mathematical colloquialism).

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PREFACEthat specializes to define isomorphisms of groups, homeomorphisms of spaces, orderisomorphisms of posets, and even isomorphisms between categories (see Definition 1.1.9).
Mathematics is full of constructions that translate mathematical objects of one kind
into objects of another kind. A construction that converts the objects in one category into
objects in another category is functorial if it can be extended to a mapping on morphisms
in such a way that composites and identity morphisms are preserved. Such constructions
define morphisms between categories, called functors. Functoriality is often a key property: for instance, the chain rule from multivariable calculus expresses the functorialityof the derivative (see Example 1.3.2(x)). In contrast with earlier numerical invariants in
topology, functorial invariants (the fundamental group, homology) tend both to be moreeasily computable and also provide more precise information. While the Euler characteristic can distinguish between the closed unit disk and its boundary circle, an easy proof
by contradiction involving the functoriality of their fundamental groups proves that any
continuous endomorphism of the disk must have a fixed point (see Theorem 1.3.3).
On occasion, functoriality is achieved by categorifying an existing mathematical construction. “Categorification” refers to the process of turning sets into categories by adding
morphisms, whose introduction typically demands a re-interpretation of the elements of thesets as related mathematical objects. A celebrated knot invariant called the Jones polynomial must vanish for any knot diagram that presents the unknot, but its categorification, afunctor7 called Khovanov homology, detects the unknot in the sense that any knot diagram
whose Khovanov homology vanishes must represent the unknot. Khovanov homology
converts an oriented link diagram into a chain complex whose graded Euler characteristic
is the Jones polynomial.
A functor may describe an equivalence of categories, in which case the objects in one
category can be translated into and reconstructed from the objects of another. For instance,
there is an equivalence between the category of finite-dimensional vector spaces and linear
maps and a category whose objects are natural numbers and whose morphisms are matrices(see Corollary 1.5.11). This process of conversion from college linear algebra to highschool linear algebra defines an equivalence of categories; eigenvalues and eigenvectors
can be developed for matrices or for linear transformations, it makes no difference.
Treating categories as mathematical objects in and of themselves, a basic observation isthat the process of formally “turning around all the arrows” in a category produces anothercategory. In particular, any theorem proven for all categories also applies to these opposite
categories; the re-interpretation of the result in the opposite of an opposite category yields
the statement of the dual theorem. Categorical constructions also admit duals: for instance,
in Zermelo–Fraenkel set theory, a function f : X → Y is defined via its graph, a subsetof X × Y isomorphic to X. The dual presentation represents a function via its cograph, a
Y-indexed partition of X t Y. Categorically-proven properties of the graph representation
will dualize to describe properties of the cograph representation.
Categories and functors were introduced by Eilenberg and Mac Lane with the goal ofgiving precise meaning to the colloquial usage of “natural” to describe families of isomorphisms. For example, for any triple of k-vector spaces U, V, W, there is an isomorphism(0.0.1)

Vectk (U ⊗k V, W)  Vectk (U, Hom(V, W))between the set of linear maps U ⊗k V → W and the set of linear maps from U to the vector
space Hom(V, W) of linear maps from V to W. This isomorphism is natural in all three
7Morally, one could argue that functoriality is the main innovation in this construction, but making thisfunctoriality precise is somewhat subtle [CMW09].

A TOUR OF BASIC CATEGORICAL NOTIONS

xiii

variables, meaning it defines an isomorphism not simply between these sets of maps but
between appropriate set-valued functors of U, V, and W. Chapter 1 introduces the basic
language of category theory, defining categories, functors, natural transformations, andintroducing the principle of duality, equivalences of categories, and the method of proof bydiagram chasing.
In fact, the isomorphism (0.0.1) defines the vector space U ⊗k V by declaring thatlinear maps U ⊗k V → W correspond to linear maps U → Hom(V, W), i.e., to bilinear
maps U × V → W. This definition is sufficiently robust that important properties of
the tensor product—for instance its symmetry and associativity—can be proven without
reference to any particular construction (see Proposition 2.3.9 and Exercise 2.3.ii). The
advantages of this approach compound as the mathematical objects so-described become
more complicated.
In Chapter 2, we study such definitions abstractly. A characterization of the morphisms
either to or from a fixed object describes its universal property; the cases of “to” or “from”
are dual. By the Yoneda lemma—which, despite its innocuous statement, is arguably themost important result in category theory—every object is characterized by either of its
universal properties. For example, the Sierpinski space is characterized as a topologicalspace by the property that continuous functions X → S correspond naturally to open
subsets of X. The complete graph on n vertices is characterized by the property thatgraph homomorphisms G → Kn correspond to n-colorings of the vertices of the graph
G with the property that adjacent vertices are assigned distinct colors. The polynomial
ring Z[x1 , . . . , xn ] is characterized as a commutative unital ring by the property that ringhomomorphisms Z[x1 , . . . , xn ] → R correspond to n-tuples of elements (r1 , . . . , rn ) ∈ R.
Modern algebraic geometry begins from the observation that a commutative ring can be
identified with the functor that it represents.
The idea of probing a fixed object using morphisms abutting to it from other objects
in the category gives rise to a notion of “generalized elements” (see Remark 3.4.15). The
elements of a set A are in bĳection with functions ∗ → A with domain a singleton set; ageneralized element of A is a morphism X → A with generic domain. In the category of
directed graphs, a parallel pair of graph homomorphisms φ, ψ : A ⇒ B can be distinguishedby considering generalized elements of A whose domain is the free-living vertex or thefree-living directed edge.8 A related idea leads to the representation of a topological space
via its singular complex.
The Yoneda lemma implies that a general mathematical object can be represented asa functor valued in the category of sets. A related classical antecedent is a result that
comforted those who were troubled by the abstract definition of a group: namely that anygroup is isomorphic to a subgroup of a permutation group (see Corollary 2.2.10). A deep
consequence of these functorial representations is that proofs that general categoricallydescribed constructions are isomorphic reduce to the construction of a bĳection between
their set-theoretical analogs (for instance, see the proof of Theorem 3.4.12).
Chapter 3 studies a special case of definitions by universal properties, which come intwo dual forms, referred to as limits and colimits. For example, aggregating the data of thecyclic p-groups Z/pn and homomorphisms between them, one can build more complicatedabelian groups. Limit constructions build new objects in a category by “imposing equations”
on existing ones. For instance, the diagram of quotient homomorphisms· · ·  Z/pn  · · ·  Z/p3  Z/p2  Z/p
8The incidence relation in the graph A can be recovered by also considering the homomorphisms between
these graphs.

xiv

PREFACE

has a limit, namely the group Z p of p-adic integers: its elements can be understood as
tuples of elements (an ∈ Z/pn )n∈ω that are compatible modulo congruence. There is a
categorical explanation for the fact that Z p is a commutative ring and not merely an abeliangroup: each of these quotient maps is a ring homomorphism, and so this diagram and also
its limit lifts to the category of rings.9
By contrast, colimit constructions build new objects by “gluing together” existing ones.
The colimit of the sequence of inclusions
Z/p ,→ Z/p2 ,→ Z/p3 ,→ · · · ,→ Z/pn ,→ · · ·is the Prüfer p-group Z[ 1p ]/Z, an abelian group which can be presented via generators andrelations asDE(0.0.2)Z[ 1p ]/Z B g1 , g2 , . . . pg1 = 0, pg2 = g1 , pg3 = g2 , . . . .
The inclusion maps are not ring homomorphisms (failing to preserve the multiplicative
identity) and indeed it turns out that the Prüfer p-group does not admit any non-trivial
multiplicative structure.
Limits and colimits are accompanied by universal properties that generalize familiar
universal properties in analysis. A poset (A, ≤) may be regarded as a category whose objects
are the elements a ∈ A and in which a morphism a → a0 is present if and only if a ≤ a0 .
The supremum of a collection of elements {ai }i∈I , an example of a colimit in the category(A, ≤), has a universal property: namely to prove thatsup ai ≤ ai∈I

is equivalent to proving that ai ≤ a for all i ∈ I. The universal property of a generic colimit
is a generalization of this, where the collection of morphisms (ai → a)i∈I is regarded asdata, called a cone under the diagram, rather than simply a family of conditions. Limits
have a dual universal property that specializes to the universal property of the infimum ofa collection of elements in a poset.
Chapter 4 studies a generalization of the notion of equivalence of categories, in whicha pair of categories are connected by a pair of opposite-pointing translation functors called
an adjunction. An adjunction expresses a kind of “duality” between a pair of functors, firstrecognized in the case of the construction of the tensor product and hom functors for abeliangroups (see Example 4.3.11). Any adjunction restricts to define an equivalence between
certain subcategories, but categories connected by adjunctions need not be equivalent.
For instance, there is an adjunction connecting the poset of subsets of Cn and the posetof subsets of the ring C[x1 , . . . , xn ] that restricts to define an equivalence between Zariski
closed subsets and radical ideals (see Example 4.3.2). Another adjunction encodes a duality
between the constructions of the suspension and of the loop space of a based topologicalspace (see Example 4.3.14).
When a “forgetful” functor admits an adjoint, that adjoint defines a “free” (or, lesscommonly, the dual “cofree”) construction. Such functors define universal solutions to
optimization problems, e.g., of adjoining a multiplicative unit to a non-unital ring. The
existence of free groups or free rings have implications for the constructions of limits in thesecategories (namely, Theorem 4.5.2); the dual properties for colimits do not hold because
there are no “cofree” groups or rings in general. A category-theoretic re-interpretationof the construction of the Stone–Čech compactification of a topological space defines a
9The lifting of the limit is considerably more subtle than the lifting of the diagram. Results of this nature
motivate Chapter 5.

NOTE TO THE READER

xv

left adjoint to any limit-preserving functor between any pair of categories with similar
set-theoretic properties (see Theorem 4.6.10 and Example 4.6.12).
Many familiar varieties of “algebraic” objects—such as groups, rings, modules, pointed
sets, or sets acted on by a group—admit a “free–forgetful” adjunction with the category ofsets. A special property of these adjoint functors explains many of the common featuresof the categories of algebras that are presented in this manner. Chapter 5 introduces
the categorical approach to universal algebra, which distinguishes the categories of rings,
compact Hausdorff spaces, and lattices from the set-theoretically similar categories offields, generic topological spaces, and posets. The former categories, but not the latter, arecategories of algebras over the category of sets.
The notion of algebra is given a precise meaning in relation to a monad, an endofunctor
that provides a syntactic encoding of algebraic structure that may be borne by objects in the
category on which it acts. Monads are also used to construct categories whose morphisms
are partially-defined or non-deterministic functions, such as Markov kernels (see Example
5.2.10), and are separately of interest in computer science. A key result in categoricaluniversal algebra is a vast generalization of the notion of a presentation of a group via
generators and relations, such as in (0.0.2), which demonstrates that an algebra of any
variety can be presented canonically as a coequalizer10 of a pair of maps from a freealgebra on the “relations” to a free algebra on the “generators.”
The concluding Chapter 6 introduces a general formalism that can be used to redefine
all of the basic categorical notions introduced in the first part of the text. Special cases ofKan extensions define representable functors, limits, colimits, adjoint functors, and monads,
and their study leads to a generalization of, as well as a dualization of, the Yoneda lemma.
In the most important cases, a Kan extension can be computed by a particular formula,which specializes to give the construction of a representation for a group induced from a
representation for a subgroup (see Example 6.2.8), to provide a new way to think about
the collection of ultrafilters on a set (see Example 6.5.12), and to define an equivalence of
categories connecting sheaves on a space with étale spaces over that space (see Exercise
6.5.iii).
A brief detour introduces derived functors, which are certain special Kan extensions thatare of great importance in homological algebra and algebraic topology. A recent categorical
discovery reveals that a common mechanism for constructing “point-set level” derived
functors yields total derived functors with superior universal properties (see Propositions
6.4.12 and 6.4.13). A final motivation for the study of Kan extensions reaches beyond thescope of this book. The calculus of Kan extensions facilitates the extension of basic category
theory to enriched, internal, fibered, or higher-dimensional contexts, which provide natural
homes for more sophisticated varieties of mathematical objects whose transformations havesome sort of higher-dimensional structure.
Note to the reader
The text that follows is littered with examples drawn from a broad range of mathematicalareas. The examples are included for color or historical context but are never essential forunderstanding the abstract category theory. In principle, one could study category theory
immediately after learning some basic set theory and logic, as no other prerequisites arestrictly required, but without some level of mathematical maturity it would be difficult to
see what the point of it all is. We hope that the majority of examples are comprehensible inoutline, even if the details are unfamiliar, but if this is not the case, it is not worth stressing
10A coequalizer is a generalization of a cokernel to contexts that may lack a “zero” homomorphism.

xvi

PREFACEover. Inevitably, given the diversity of mathematical tastes and experiences, the examples
presented here will seldom be optimized for any particular individual, and indeed, eachreader is encouraged to search for their own contexts in which to explore categorical ideas.
Notational conventions
An arrow symbol “→,” either in a display or in text, is only ever used to denote amorphism in an appropriate category. In particular, the objects surrounding it necessarily
lie in a common category. Double arrows “⇒” are reserved for natural transformations,the notation used to suggest the intuition that these are some variety of “2-dimensional”
morphisms. The symbol “7→,” read as “maps to,” appears occasionally when defining afunction between sets by specifying its action on particular elements. The symbol “ ” isused in a less technical sense to mean something along the lines of “yields” or “leads to”
or “can be used to construct.” If the presence of certain morphisms implies the existenceof another morphism, the latter is often depicted with a dashed arrow “d” to suggest thecorrect order of inference.11
We use “⇒” as an abbreviation for a parallel pair of morphisms, i.e., for a pair ofmorphisms with common source and target, and “” as an abbreviation for an opposing
pair of morphisms with sources and targets swapped.
Italics are used occasionally for emphasis and to highlight technical terms. Boldface
signals that a technical term is being defined by its surrounding text.
The symbol “=” is reserved for genuine equality (with “B” used for definitional
equality), with “” used instead for isomorphism in the appropriate ambient category, by
far the more common occurrence.
Acknowledgments
Many of the theorems appearing here are standard fare for a first course on category
theory, but the examples are not. Rather than rely solely on my own generative capacity, I
consulted a great many people while preparing this text and am grateful for their generosity
in sharing ideas.
To begin, I would like to thank the following people who responded to a call for
examples on the n-Category Café and the categories mailing list: John Baez, Martin
Brandenburg, Ronnie Brown, Tyler Bryson, Tim Campion, Yemon Choi, Adrian Clough,Samuel Dean, Josh Drum, David Ellerman, Tom Ellis, Richard Garner, Sameer Gupta,
Gejza Jenča, Mark Johnson, Anders Kock, Tom LaGatta, Paul Levy, Fred Linton, Aaron
Mazel-Gee, Jesse McKeown, Kimmo Rosenthal, Mike Shulman, Peter Smith, Arnaud
Spiwack, Ross Street, John Terilla, Todd Trimble, Mozibur Ullah, Enrico Vitale, DavidWhite, Graham White, and Qiaochu Yuan.
In particular, Anders Kock suggested a more general formulation of “the chain rule
expresses the functoriality of the derivative” than appears in Example 1.3.2(x). The expression of the fundamental theorem of Galois theory as an isomorphism of categories
that appears as Example 1.3.15 is a favorite exercise of Peter May’s. Charles Blair suggested Exercise 1.3.iii and a number of expository improvements to the first chapter. I
learned about the unnatural isomorphism of Proposition 1.4.4 from Mitya Boyarchenko.
Peter Haine suggested Example 1.4.6, expressing the Riesz representation theorem as anatural isomorphism of Banach space-valued functors; Example 3.6.2, constructing thepath-components functor, and Example 3.8.6. He also contributed Exercises 1.2.v, 1.5.v,
1.6.iv, 3.1.xii, and 4.5.v and served as my LATEX consultant.
11Readers who dislike this convention can simply connect the dots.

ACKNOWLEDGMENTS

xvii

John Baez reminded me that the groupoid of finite sets is a categorification of thenatural numbers, providing a suitable framework in which to prove certain basic equations
in elementary arithmetic; see Example 1.4.9 and Corollary 4.5.6. Juan Climent Vidal
suggested using the axiom of regularity to define the non-trivial part of the equivalence of
categories presented in Example 1.5.6 and contributed the equivalence of plane geometries
that appears as Exercise 1.5.viii. Samuel Dean suggested Corollary 1.5.13, Fred Linton
suggested Corollary 2.2.9, and Ronnie Brown suggested Example 3.5.8. Ralf Meyer
suggested Example 2.1.5(vi), Example 5.2.6(ii), Corollary 4.5.8, and a number of exercises
including 2.1.ii and 2.4.v, which he used when teaching a similar course. He also pointed
me toward a simpler proof of Proposition 6.4.12.
Mozibur Ullah suggested Exercise 3.5.vii. I learned of the non-natural objectwise
isomorphism appearing in Example 3.6.5 from Martin Brandenburg who acquired it fromTom Leinster. Martin also suggested the description of the real exponential function asa Kan extension appearing in Example 6.2.7. Andrew Putman pointed out that Lang’s
Algebra constructs the free group on a set using the construction of the General Adjoint
Functor Theorem, recorded as Example 4.6.6. Paul Levy suggested using affine spaces
to motivate the category of algebras over a monad, as discussed in Section 5.2; a similar
example was suggested by Enrico Vitale. Dominic Verity suggested something like Exercise
5.5.vii. Vladimir Sotirov pointed out that the appropriate size hypotheses were missing
from the original statement of Theorem 6.3.7 and directed me toward a more elegant proof
of Lemma 4.6.5. Lyne Moser caught several eleventh hour typos.
Marina Lehner, while writing her undergraduate senior thesis under my direction,
showed me that an entirely satisfactory account of Kan extensions can be given without
the calculus of ends and coends. I have enjoyed, and this book has been enriched by,several years of impromptu categorical conversations with Omar Antolín Camarena. I
am extremely appreciative of the careful readings undertaken by Tobias Barthel, Martin
Brandenburg, Benjamin Diamond, Gabriel Drummond-Cole, Darĳ Grinberg, Peter Haine,
Ralf Meyer, Peter Smith, and Juan Climent Vidal who each sent detailed lists of corrections and suggestions. Ittay Weiss caught several typos in the published version that havebeen corrected in the online text, while Miloslav Štěpán, Kurt Butler, Jure Taslak, Alastair Grant-Stuart, Wenwen Li, Michael Janou Glaeser, and Brian Ceco caught additional
post-publication typos. I would also like to thank John Grafton and Janet Kopito, the acquisitions and in-house editors at Dover Publications, and David Gargaro for his meticulous
copyediting.
I am grateful for the perspicacious comments and questions from those who attended
the first iteration of this course at Harvard—Paul Bamberg, Nathan Gupta, Peter Haine,
Andrew Liu, Wyatt Mackey, Nat Mayer, Selorm Ohene, Jacob Seidman, Aaron Slipper,
Alma Steingart, Nithin Tumma, Jeffrey Yan, Liang Zhang, and Michael Fountaine, who
served as the course assistant—and the second iteration at Johns Hopkins—BenjaminDiamond, Nathaniel Filardo, Alex Grounds, Hanveen Koh, Alex Rozenshteyn, Xiyuan
Wang, Shengpei Yan, and Zhaoning Yang. I would also like to thank the Departments ofMathematics at both institutions for giving me opportunities to teach this course and mycolleagues there, who have created two extremely pleasant working environments. Whilethese notes were being revised, I received financial support from the National Science
Foundation Division of Mathematical Sciences DMS-1551129.
My enthusiasm for mathematical writing and patience for editing were inherited fromPeter May, my PhD supervisor. I appreciate the indulgence of my collaborators, my friends,
and my family while this manuscript was in its final stages of preparation. This book isxviii

PREFACEdedicated to Peter Johnstone and Martin Hyland, whose tutelage I was fortunate to comeunder in a pivotal year spent at Cambridge supported by the Winston Churchill Foundationof the United States.

CHAPTER 1

Categories, Functors, Natural Transformations
Frequently in modern mathematics there occur
phenomena of “naturality”.
Samuel Eilenberg and Saunders Mac Lane,“Natural isomorphisms in group theory”[EM42b]A group extension of an abelian group H by an abelian group G consists of a group E
together with an inclusion of G ,→ E as a normal subgroup and a surjective homomorphism
E  H that displays H as the quotient group E/G. This data is typically displayed in a
diagram of group homomorphisms:
0 → G → E → H → 0.1
A pair of group extensions E and E 0 of G and H are considered to be equivalent whenever
there is an isomorphism E  E 0 that commutes with the inclusions of G and quotient maps
to H, in a sense that is made precise in §1.6. The set of equivalence classes of abeliangroup extensions E of H by G defines an abelian group Ext(H, G).
In 1941, Saunders Mac Lane gave a lecture at the University of Michigan in which
he computed for a prime p that Ext(Z[ 1p ]/Z, Z)  Z p , the group of p-adic integers, whereZ[ 1p ]/Z is the Prüfer p-group. When he explained this result to Samuel Eilenberg, who had
missed the lecture, Eilenberg recognized the calculation as the homology of the 3-spherecomplement of the p-adic solenoid, a space formed as the infinite intersection of a sequenceof solid tori, each wound around p times inside the preceding torus. In teasing apart thisconnection, the pair of them discovered what is now known as the universal coefficient
theorem in algebraic topology, which relates the homology H∗ and cohomology groups H ∗associated to a space X via a group extension [ML05]:(1.0.1)

0 → Ext(Hn−1 (X), G) → H n (X, G) → Hom(Hn (X), G) → 0 .

To obtain a more general form of the universal coefficient theorem, Eilenberg and Mac
Lane needed to show that certain isomorphisms of abelian groups expressed by this groupextension extend to spaces constructed via direct or inverse limits. And indeed this is thecase, precisely because the homomorphisms in the diagram (1.0.1) are natural with respect
to continuous maps between topological spaces.
The adjective “natural” had been used colloquially by mathematicians to mean “definedwithout arbitrary choices.” For instance, to define an isomorphism between a finitedimensional vector space V and its dual, the vector space of linear maps from V to the
1The zeros appearing on the ends provide no additional data. Instead, the first zero implicitly asserts that the
map G → E is an inclusion and the second that the map E → H is a surjection. More precisely, the displayed
sequence of group homomorphisms is exact, meaning that the kernel of each homomorphism equals the image ofthe preceding homomorphism.
12

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

ground field k, requires a choice of basis. However, there is an isomorphism between V
and its double dual that requires no choice of basis; the latter, but not the former, is natural.
To give a rigorous proof that their particular family of group isomorphisms extended toinverse and direct limits, Eilenberg and Mac Lane sought to give a mathematically precise
definition of the informal concept of “naturality.” To that end, they introduced the notion ofa natural transformation, a parallel collection of homomorphisms between abelian groups
in this instance. To characterize the source and target of a natural transformation, they
introduced the notion of a functor.2 And to define the source and target of a functor in
the greatest generality, they introduced the concept of a category. This work, described in“The general theory of natural equivalences” [EM45], published in 1945, marked the birthof category theory.
While categories and functors were first conceived as auxiliary notions, needed togive a precise meaning to the concept of naturality, they have grown into interesting andimportant concepts in their own right. Categories suggest a particular perspective to be
used in the study of mathematical objects that pays greater attention to the maps between
them. Functors, which translate mathematical objects of one type into objects of another,have a more immediate utility. For instance, the Brouwer fixed point theorem translates aseemingly intractable problem in topology to a trivial one (0 , 1) in algebra. It is to these
topics that we now turn.
Categories are introduced in §1.1 in two guises: firstly as universes categorizing
mathematical objects and secondly as mathematical objects in their own right. The first
perspective is used, for instance, to define a general notion of isomorphism that can be
specialized to mathematical objects of every conceivable variety. The second perspective
leads to the observation that the axioms defining a category are self-dual.3 Thus, as exploredin §1.2, for any proof of a theorem about all categories from these axioms, there is a dual
proof of the dual theorem obtained by a syntactic process that is interpreted as “turning
around all the arrows.”
Functors and natural transformations are introduced in §1.3 and §1.4 with examples
intended to shed light on the linguistic and practical utility of these concepts. The categorytheoretic notions of isomorphism, monomorphism, and epimorphism are invariant undercertain classes of functors, including in particular the equivalences of categories, introducedin §1.5. At a high level, an equivalence of categories provides a precise expression of theintuition that mathematical objects of one type are “the same as” objects of another variety:an equivalence between the category of matrices and the category of finite-dimensional
vector spaces equates high school and college linear algebra.
In addition to providing a new language to describe emerging mathematical phenomena,
category theory also introduced a new proof technique: that of the diagram chase. The
introduction to the influential book [ES52] presents commutative diagrams as one of the“new techniques of proof” appropriate for their axiomatic treatment of homology theory.
The technique of diagram chasing is introduced in §1.6 and applied in §1.7 to construct
new natural transformations as horizontal or vertical composites of given ones.

2A brief account of functors and natural isomorphisms in group theory appeared in a 1942 paper [EM42b].
3As is the case for the duality in projective plane geometry, this duality can be formulated precisely as a featureof the first-order theories that axiomatize these structures.

1.1. ABSTRACT AND CONCRETE CATEGORIES

3

1.1. Abstract and concrete categories
It frames a possible template for any
mathematical theory: the theory should havenouns and verbs, i.e., objects, and morphisms,
and there should be an explicit notion ofcomposition related to the morphisms; the theory
should, in brief, be packaged by a category.
Barry Mazur, “When is one thing equal to someother thing?” [Maz08]

Definition 1.1.1. A category consists of• a collection of objects X, Y, Z, . . .
• a collection of morphisms f, g, h, . . .
so that:• Each morphism has specified domain and codomain objects; the notation f : X → Y
signifies that f is a morphism with domain X and codomain Y.
• Each object has a designated identity morphism 1X : X → X.
• For any pair of morphisms f, g with the codomain of f equal to the domain of g, there
exists a specified composite morphism4 g f whose domain is equal to the domain off and whose codomain is equal to the codomain of g, i.e.,:f : X → Y,

g: Y → Z

gf : X → Z .

This data is subject to the following two axioms:• For any f : X → Y, the composites 1Y f and f 1X are both equal to f .
• For any composable triple of morphisms f, g, h, the composites h(g f ) and (hg) f are
equal and henceforth denoted by hg f .
f : X → Y,

g : Y → Z,

h: Z → W

hg f : X → W .

That is, the composition law is associative and unital with the identity morphisms serving
as two-sided identities.
Remark 1.1.2. The objects of a category are in bĳective correspondence with the identity
morphisms, which are uniquely determined by the property that they serve as two-sided
identities for composition. Thus, one can define a category to be a collection of morphisms
with a partially-defined composition operation that has certain special morphisms, which
are used to recognize composable pairs and which serve as two-sided identities; see [Ehr65,§I.1] or [FS90, §1.1]. But in practice it is not so hard to specify both the objects and themorphisms and this is what we shall do.
It is traditional to name a category after its objects; typically, the preferred choice ofaccompanying structure-preserving morphisms is clear. However, this practice is somewhat
contrary to the basic philosophy of category theory: that mathematical objects should
always be considered in tandem with the morphisms between them. By Remark 1.1.2, thealgebra of morphisms determines the category, so of the two, the objects and morphisms,
the morphisms take primacy.
Example 1.1.3. Many familiar varieties of mathematical objects assemble into a category.
4The composite may be written less concisely as g · f when this adds typographical clarity.

4

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS(i) Set has sets as its objects and functions, with specified domain and codomain,5 as
its morphisms.
(ii) Top has topological spaces as its objects and continuous functions as its morphisms.
(iii) Set∗ and Top∗ have sets or spaces with a specified basepoint6 as objects and basepointpreserving (continuous) functions as morphisms.
(iv) Group has groups as objects and group homomorphisms as morphisms. This example
lent the general term “morphisms” to the data of an abstract category. The categories
Ring of associative and unital rings and ring homomorphisms and Field of fields andfield homomorphisms are defined similarly.
(v) For a fixed unital but not necessarily commutative ring R, ModR is the category ofleft R-modules and R-module homomorphisms. This category is denoted by Vectk
when the ring happens to be a field k and abbreviated as Ab in the case of ModZ , asa Z-module is precisely an abelian group.
(vi) Graph has graphs as objects and graph morphisms (functions carrying vertices tovertices and edges to edges, preserving incidence relations) as morphisms. In thevariant DirGraph, objects are directed graphs, whose edges are now depicted asarrows, and morphisms are directed graph morphisms, which must preserve sources
and targets.
(vii) Man has smooth (i.e., infinitely differentiable) manifolds as objects and smooth maps
as morphisms.
(viii) Meas has measurable spaces as objects and measurable functions as morphisms.
(ix) Poset has partially-ordered sets as objects and order-preserving functions as morphisms.
(x) ChR has chain complexes of R-modules as objects and chain homomorphisms asmorphisms.7(xi) For any signature σ, specifying constant, function, and relation symbols, and forany collection of well formed sentences T in the first-order language associated toσ, there is a category ModelT whose objects are σ-structures that model T, i.e., sets
equipped with appropriate constants, relations, and functions satisfying the axioms
T. Morphisms are functions that preserve the specified constants, relations, andfunctions, in the usual sense.8 Special cases include (iv), (v), (vi), (ix), and (x).
The preceding are all examples of concrete categories, those whose objects haveunderlying sets and whose morphisms are functions between these underlying sets, typicallythe “structure-preserving” morphisms. A more precise definition of a concrete category isgiven in 1.6.17. However, “abstract” categories are also prevalent:
Example 1.1.4.
(i) For a unital ring R, MatR is the category whose objects are positive integers and inwhich the set of morphisms from n to m is the set of m × n matrices with values in5[EM45, p. 239] emphasizes that the data of a function should include specified sets of inputs and potential
outputs, a perspective that was somewhat radical at the time.
6A basepoint is simply a chosen distinguished point in the set or space.
7A chain complex C• is a collection (Cn )n∈Z of R-modules equipped with R-module homomorphisms d : Cn →Cn−1 , called boundary homomorphisms, with the property that d2 = 0, i.e., the composite of any two boundary
maps is the zero homomorphism. A map of chain complexes f : C• → C•0 is comprised of a collection ofhomomorphisms fn : Cn → Cn0 so that d fn = fn−1 d for all n ∈ Z.
8Model theory pays greater attention to other types of morphisms, for instance the elementary embeddings,
which are (automatically injective) functions that preserve and reflect satisfaction of first-order formulae.

1.1. ABSTRACT AND CONCRETE CATEGORIES

5

R. Composition is by matrix multiplication
A

n−
→ m,

B

B·A

m−
→k

n −−→ kwith identity matrices serving as the identity morphisms.
(ii) A group G (or, more generally, a monoid9) defines a category BG with a single object.
The group elements are its morphisms, each group element representing a distinct
endomorphism of the single object, with composition given by multiplication. The
identity element e ∈ G acts as the identity morphism for the unique object in thiscategory.
(12)

BS 3 =e(132)


1$ • dq
D

(13)
(23)

(123)

(iii) A poset (P, ≤) (or, more generally, a preorder10) may be regarded as a category. The
elements of P are the objects of the category and there exists a unique morphism
x → y if and only if x ≤ y. Transitivity of the relation “≤” implies that the requiredcomposite morphisms exist. Reflexivity implies that identity morphisms exist.
(iv) In particular, any ordinal α = {β | β < α} defines a category whose objects are thesmaller ordinals. For example, 0 is the category with no objects and no morphisms. 1
is the category with a single object and only its identity morphism. 2 is the category
with two objects and a single non-identity morphism, conventionally depicted as
0 → 1. ω is the category freely generated by the graph
0 → 1 → 2 → 3 → ···in the sense that every non-identity morphism can be uniquely factored as a composite
of morphisms in the displayed graph; a precise definition of the notion of freegeneration is given in Example 4.1.13.
(v) A set may be regarded as a category in which the elements of the set define theobjects and the only morphisms are the required identities. A category is discrete if
every morphism is an identity.
(vi) Htpy, like Top, has spaces as its objects but morphisms are homotopy classes ofcontinuous maps. Htpy∗ has based spaces as its objects and basepoint-preserving
homotopy classes of based continuous maps as its morphisms.
(vii) Measure has measure spaces as objects. One reasonable choice for the morphisms is
to take equivalence classes of measurable functions, where a parallel pair of functions
are equivalent if their domain of difference is contained within a set of measure zero.
Thus, the philosophy of category theory is extended. The categories listed in Example
1.1.3 suggest that mathematical objects ought to be considered together with the appropriate
notion of morphism between them. The categories listed in Example 1.1.4 illustrate that·

9A monoid is a set M equipped with an associative binary multiplication operation M × M →− M and an identity
element e ∈ M serving as a two-sided identity. In other words, a monoid is precisely a one-object category.
10A preorder is a set with a binary relation ≤ that is reflexive and transitive. In other words, a preorder isprecisely a category in which there are no parallel pairs of distinct morphisms between any fixed pair of objects.
A poset is a preorder that is additionally antisymmetric: x ≤ y and y ≤ x implies that x = y.

6

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

these morphisms are not always functions.11 The morphisms in a category are also calledarrows or maps, particularly in the contexts of Examples 1.1.4 and 1.1.3, respectively.
Remark 1.1.5. Russell’s paradox implies that there is no set whose elements are “all sets.”
This is the reason why we have used the vague word “collection” in Definition 1.1.1. Indeed,in each of the examples listed in 1.1.3, the collection of objects is not a set. Eilenberg and
Mac Lane address this potential area of concern as follows:. . . the whole concept of a category is essentially an auxiliary one;our basic concepts are essentially those of a functor and of a naturaltransformation . . . . The idea of a category is required only by theprecept that every function should have a definite class as domain
and a definite class as range, for the categories are provided as thedomains and ranges of functors. Thus one could drop the category
concept altogether and adopt an even more intuitive standpoint, inwhich a functor such as “Hom” is not defined over the category of“all” groups, but for each particular pair of groups which may be
given. [EM45]
The set-theoretical issues that confront us while defining the notion of a category will
compound as we develop category theory further. For that reason, common practice among
category theorists is to work in an extension of the usual Zermelo–Fraenkel axioms of set
theory, with new axioms allowing one to distinguish between “small” and “large” sets,or between sets and classes. The search for the most useful set-theoretical foundations
for category theory is a fascinating topic that unfortunately would require too long of adigression to explore.12 Instead, we sweep these foundational issues under the rug, notbecause these issues are not serious or interesting, but because they distract from the task
at hand.13
For the reasons just discussed, it is important to introduce adjectives that explicitly
address the size of a category.
Definition 1.1.6. A category is small if it has only a set’s worth of arrows.
By Remark 1.1.2, a small category has only a set’s worth of objects. If C is a small
category, then there are functions
mor C o

dom
id
cod

/
/ ob C

that send a morphism to its domain and its codomain and an object to its identity.
11Reid’s Undergraduate algebraic geometry emphasizes that the morphisms are not always functions, writing“Students who disapprove are recommended to give up at once and take a reading course in category theory
instead” [Rei88, p. 4].
12The preprint [Shu08] gives an excellent overview, though it is perhaps better read after Chapters 1–4.
13If pressed, let us assume that there exists a countable sequence of inaccessible cardinals, meaning uncountablecardinals that are regular and strong limit. A cardinal κ is regular if every union of fewer than κ sets each of
cardinality less than κ has cardinality less than κ, and strong limit if λ < κ implies that 2λ < κ. Inaccessibility
means that sets of size less than κ are closed under power sets and κ-small unions. If κ is inaccessible, then theκ-stage of the von Neumann hierarchy, the set Vκ of sets of rank less than κ, is a model of Zermelo–Fraenkel set
theory with choice (ZFC); the set Vκ is a Grothendieck universe. The assumption that there exists a countablesequence of inaccessible cardinals means that we can “do set theory” inside the universe Vκ , and then enlarge theuniverse if necessary as often as needed.
If ZFC is consistent, these axioms cannot prove the existence of an inaccessible cardinal or the consistency of
the assumption that one exists (by Gödel’s second incompleteness theorem). Nonetheless, from the perspectiveof the hierarchy of large cardinal axioms, the existence of inaccessibles is a relatively mild hypothesis.

1.1. ABSTRACT AND CONCRETE CATEGORIES

7

None of the categories in Example 1.1.3 are small—each has too many objects—but“locally” they resemble small categories in a sense made precise by the following notion:
Definition 1.1.7. A category is locally small if between any pair of objects there is only
a set’s worth of morphisms.
It is traditional to write(1.1.8)C(X, Y)or

Hom(X, Y)

for the set of morphisms from X to Y in a locally small category C.14 The set of arrows
between a pair of fixed objects in a locally small category is typically called a hom-set,
whether or not it is a set of “homomorphisms” of any particular kind. Because the notation(1.1.8) is so convenient, it is also adopted for the collection of morphisms between a fixedpair of objects in a category that is not necessarily locally small.
A category provides a context in which to answer the question “When is one thing thesame as another thing?” Almost universally in mathematics, one regards two objects of thesame category to be “the same” when they are isomorphic, in a precise categorical sensethat we now introduce.
Definition 1.1.9. An isomorphism in a category is a morphism f : X → Y for which
there exists a morphism g : Y → X so that g f = 1X and f g = 1Y . The objects X and Y are
isomorphic whenever there exists an isomorphism between X and Y, in which case one
writes X  Y.
An endomorphism, i.e., a morphism whose domain equals its codomain, that is an
isomorphism is called an automorphism.
Example 1.1.10.
(i) The isomorphisms in Set are precisely the bĳections.
(ii) The isomorphisms in Group, Ring, Field, or ModR are the bĳective homomorphisms.
(iii) The isomorphisms in the category Top are the homeomorphisms, i.e., the continuousfunctions with continuous inverse, which is a stronger property than merely being abĳective continuous function.
(iv) The isomorphisms in the category Htpy are the homotopy equivalences.
(v) In a poset (P, ≤), the axiom of antisymmetry asserts that x ≤ y and y ≤ x imply thatx = y. That is, the only isomorphisms in the category (P, ≤) are identities.
Examples 1.1.10(ii) and (iii) suggest the following general question: In a concrete
category, when are the isomorphisms precisely those maps in the category that induce
bĳections between the underlying sets? We will see an answer in Lemma 5.6.1.
Definition 1.1.11. A groupoid is a category in which every morphism is an isomorphism.
Example 1.1.12.
(i) A group is a groupoid with one object.15(ii) For any space X, its fundamental groupoid Π1 (X) is a category whose objects are
the points of X and whose morphisms are endpoint-preserving homotopy classes ofpaths.
14Mac Lane credits Emmy Noether for emphasizing the importance of homomorphisms in abstract algebra,
particularly the homomorphism onto a quotient group, which plays an integral role in the statement of her firstisomorphism theorem. His recollection is that the arrow notation first appeared around 1940, perhaps due toHurewicz [ML88]. The notation Hom(X, Y) was first used in [EM42a] for the set of homomorphisms between apair of abelian groups.
15This is not simply an example; it is a definition.

8

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

A subcategory D of a category C is defined by restricting to a subcollection of objects
and subcollection of morphisms subject to the requirements that the subcategory D contains
the domain and codomain of any morphism in D, the identity morphism of any object inD, and the composite of any composable pair of morphisms in D. For example, there is asubcategory CRing ⊂ Ring of commutative unital rings. Both of these form subcategoriesof the category Rng of not-necessarily unital rings and homomorphisms that need notpreserve the multiplicative unit.16
Lemma 1.1.13. Any category C contains a maximal groupoid, the subcategory containing
all of the objects and only those morphisms that are isomorphisms.
Proof. Exercise 1.1.ii.

For instance, Finiso , the category of finite sets and bĳections, is the maximal subgroupoid of the category Fin of finite sets and all functions. Example 1.4.9 will explain
how this groupoid can be regarded as a categorification of the natural numbers, providing
a vantage point from which to prove the laws of elementary arithmetic.
Exercises.
Exercise 1.1.i.
(i) Consider a morphism f : x → y. Show that if there exists a pair of morphisms
g, h : y ⇒ x so that g f = 1 x and f h = 1y , then g = h and f is an isomorphism.
(ii) Show that a morphism can have at most one inverse isomorphism.
Exercise 1.1.ii. Let C be a category. Show that the collection of isomorphisms in C defines
a subcategory, the maximal groupoid inside C.
Exercise 1.1.iii. For any category C and any object c ∈ C, show that:(i) There is a category c/C whose objects are morphisms f : c → x with domain c andin which a morphism from f : c → x to g : c → y is a map h : x → y between thecodomains so that the triangle
c

f

x

h

g

/ y

commutes, i.e., so that g = h f .
(ii) There is a category C/c whose objects are morphisms f : x → c with codomain cand in which a morphism from f : x → c to g : y → c is a map h : x → y between thedomains so that the triangle
h

x
f



c

/y
g

commutes, i.e., so that f = gh.
The categories c/C and C/c are called slice categories of C under and over c, respectively.
16To justify our default notion of ring, see Poonen’s “Why all rings should have a 1” [Poo14]. The relationshipbetween unital and non-unital rings is explored in greater depth in §4.6.

1.2. DUALITY

9

1.2. Duality
The dual of any axiom for a category is also anaxiom . . . A simple metamathematical argument
thus proves the duality principle. If any statement
about a category is deducible from the axioms fora category, the dual statement is likely deducible.
Saunders Mac Lane, “Duality for groups” [ML50]

Upon first acquaintance, the primary role played by the notion of a category might
appear to be taxonomic: vector spaces and linear maps define one category, manifolds andsmooth functions define another. But a category, as defined in 1.1.1, is also a mathematicalobject in its own right, and as with any mathematical definition, this one is worthy offurther consideration. Applying a mathematician’s gaze to the definition of a category, thefollowing observation quickly materializes. If we visualize the morphisms in a category
as arrows pointing from their domain object to their codomain object, we might imagine
simultaneously reversing the directions of every arrow. This leads to the following notion.
Definition 1.2.1. Let C be any category. The opposite category Cop has• the same objects as in C, and• a morphism f op in Cop for each a morphism f in C so that the domain of f op is defined
to be the codomain of f and the codomain of f op is defined to be the domain of f : i.e.,f op : X → Y

∈ Cop

!

f:Y →X∈ C.

That is, Cop has the same objects and morphisms as C, except that “each morphism ispointing in the opposite direction.” The remaining structure of the category Cop is given
as follows:op• For each object X, the arrow 1X serves as its identity in Cop .
• To define composition, observe that a pair of morphisms f op , gop in Cop is composable
precisely when the pair g, f is composable in C, i.e., precisely when the codomain ofg equals the domain of f . We then define gop · f op to be ( f · g)op : i.e.,
gop f op : X → Z

g : Z → Y, f : Y → X∈C

f g: Z → X

∈ Cop

!

∈ Cop

!

f op : X → Y, gop : Y → Z∈CThe data described in Definition 1.2.1 defines a category Cop —i.e., the composition
law is associative and unital—if and only if C defines a category. In summary, the process of“turning around the arrows” or “exchanging domains and codomains” exhibits a syntacticalself-duality satisfied by the axioms for a category. Note that the category Cop contains
precisely the same information as the category C. Questions about the one can be answeredby examining the other.
Example 1.2.2.
op(i) MatR is the category whose objects are non-zero natural numbers and in which amorphism from m to n is an m × n matrix with values in R. The upshot is that a reader
who would have preferred the opposite handedness conventions when defining MatR
would have lost nothing by adopting them.
(ii) When a preorder (P, ≤) is regarded as a category, its opposite category is the categorythat has a morphism x → y if and only if y ≤ x. For example, ωop is the category

10

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

freely generated by the graph··· → 3 → 2 → 1 → 0.
(iii) If G is a group, regarded as a one-object groupoid, the category (BG)op  B(Gop )is again a one-object groupoid, and hence a group. The group Gop is called the
opposite group and is used to define right actions as a special case of left actions;
see Example 1.3.9.
This syntactical duality has a very important consequence for the development of
category theory. Any theorem containing a universal quantification of the form “for all
categories C” also necessarily applies to the opposites of these categories. Interpreting theresult in the dual context leads to a dual theorem, proven by the dual of the original proof,
in which the direction of each arrow appearing in the argument is reversed. The result isa two-for-one deal: any proof in category theory simultaneously proves two theorems, theoriginal statement and its dual.17 For example, the reader may have found Exercise 1.1.iii
redundant, precisely because the statements (i) and (ii) are dual; see Exercise 1.2.i.
To illustrate the principle of duality in category theory, let us consider the following
result, which provides an important characterization of the isomorphisms in a category.
Lemma 1.2.3. The following are equivalent:(i) f : x → y is an isomorphism in C.
(ii) For all objects c ∈ C, post-composition with f defines a bĳectionf∗ : C(c, x) → C(c, y) .
(iii) For all objects c ∈ C, pre-composition with f defines a bĳectionf ∗ : C(y, c) → C(x, c) .
Remark 1.2.4. In language introduced in Chapter 2, Lemma 1.2.3 asserts that isomorphisms in a locally small category are defined representably in terms of isomorphisms in
the category of sets. That is, a morphism f : x → y in an arbitrary locally small category
C is an isomorphism if and only if the post-composition function f∗ : C(c, x) → C(c, y)
between hom-sets defines an isomorphism in Set for each object c ∈ C.
In set theoretical foundations that permit the definition of functions between large sets,
the proof given here applies also to non-locally small categories. In our exposition, the set
theoretical hypotheses of smallness and local smallness will only appear when there are
essential subtleties concerning the sizes of the categories in question. This is not one ofthose occasions.
Proof of Lemma 1.2.3. We will prove the equivalence (i) ⇔ (ii) and conclude theequivalence (i) ⇔ (iii) by duality.
Assuming (i), namely that f : x → y is an isomorphism with inverse g : y → x, then,as an immediate application of the associativity and identity laws for composition in a
category, post-composition with g defines an inverse functiong∗ : C(c, y) → C(c, x)
to f∗ in the sense that the compositesg∗ f∗ : C(c, x) → C(c, x)

andf∗ g∗ : C(c, y) → C(c, y)

17More generally, the proof of a statement of the form “for all categories C1 , C2 , . . . , Cn ” leads to 2n dual
theorems. In practice, however, not all of the dual statements will differ meaningfully from the original; seee.g., §4.3.

1.2. DUALITY

11

are both the identity function: for any h : c → x and k : c → y, g∗ f∗ (h) = g f h = h andf∗ g∗ (k) = f gk = k.
Conversely, assuming (ii), there must be an element g ∈ C(y, x) whose image underf∗ : C(y, x) → C(y, y) is 1y . By construction, 1y = f g. But now, by associativity ofcomposition, the elements g f, 1 x ∈ C(x, x) have the common image f under the functionf∗ : C(x, x) → C(x, y), whence g f = 1 x . Thus, f and g are inverse isomorphisms.
We have just proven the equivalence (i) ⇔ (ii) for all categories and in particular forthe category Cop : i.e., a morphism f op : y → x in Cop is an isomorphism if and only if(1.2.5)opf∗ : Cop (c, y) → Cop (c, x) is an isomorphism for all c ∈ Cop .

Interpreting the data of Cop in its opposite category C, the statement (1.2.5) expresses thesame mathematical content as(1.2.6)f ∗ : C(y, c) → C(x, c) is an isomorphism for all c ∈ C .

That is: Cop (c, x) = C(x, c), post-composition with f op in Cop translates to pre-composition
with f in the opposite category C. The notion of isomorphism, as defined in 1.1.9, isself-dual: f op : y → x is an isomorphism in Cop if and only if f : x → y is an isomorphism
in C. So the equivalence (i) ⇔ (ii) in Cop expresses the equivalence (i) ⇔ (iii) in C.18 
Concise expositions of the duality principle in category theory may be found in [Awo10,§3.1] and [HS97, §II.3]. As we become more comfortable with arguing by duality, dual
proofs and eventually also dual statements will seldom be described in this much detail.
Categorical definitions also have duals; for instance:
Definition 1.2.7. A morphism f : x → y in a category is(i) a monomorphism if for any parallel morphisms h, k : w ⇒ x, f h = f k implies thath = k; or(ii) an epimorphism if for any parallel morphisms h, k : y ⇒ z, h f = k f implies thath = k.
Note that a monomorphism or epimorphism in C is, respectively, an epimorphism ormonomorphism in Cop . In adjectival form, a monomorphism is monic and an epimorphism
is epic. In common shorthand, a monomorphism is a mono and an epimorphism is an
epi. For graphical emphasis, monos are often decorated with a tail “” while epis may be
decorated at their head “.”The following dual statements re-express Definition 1.2.7:(i) f : x → y is a monomorphism in C if and only if for all objects c ∈ C, postcomposition with f defines an injection f∗ : C(c, x) → C(c, y).
(ii) f : x → y is an epimorphism in C if and only if for all objects c ∈ C, pre-composition
with f defines an injection f ∗ : C(y, c) → C(x, c).
Example 1.2.8. Suppose f : X → Y is a monomorphism in the category of sets. Then, inparticular, given any two maps x, x0 : 1 ⇒ X, whose domain is the singleton set, if f x = f x0
then x = x0 . Thus, monomorphisms are injective functions. Conversely, any injectivefunction can easily be seen to be a monomorphism.
Similarly, a function f : X → Y is an epimorphism in the category of sets if and only
if it is surjective. Given functions h, k : Y ⇒ Z, the equation h f = k f says exactly that h isequal to k on the image of f . This only implies that h = k in the case where the image is allof Y.
18A similar translation, as just demonstrated between the statements (1.2.5) and (1.2.6), transforms the proofof (i) ⇔ (ii) into a proof of (i) ⇔ (iii).

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1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

Thus, monomorphisms and epimorphisms should be regarded as categorical analogsof the notions of injective and surjective functions. In practice, if C is a category in which
objects have “underlying sets,” then any morphism that induces an injective or surjectivefunction between these defines a monomorphism or epimorphism; see Exercise 1.6.iii fora precise discussion. However, even in such categories, the notions of monomorphism andepimorphism can be more general, as demonstrated in Exercise 1.6.v.
sr

Example 1.2.9. Suppose that x →− y→− x are morphisms so that rs = 1 x . The map s is asection or right inverse to r, while the map r defines a retraction or left inverse to s. The
maps s and r express the object x as a retract of the object y.
In this case, s is always a monomorphism and, dually, r is always an epimorphism. To
acknowledge the presence of these one-sided inverses, s is said to be a split monomorphism
and r is said to be a split epimorphism.19
Example 1.2.10. By the previous example, an isomorphism is necessarily both monic andepic, but the converse need not hold in general. For example, the inclusion Z ,→ Q is bothmonic and epic in the category Ring, but this map is not an isomorphism: there are no ringhomomorphisms from Q to Z.
Since the notions of monomorphism and epimorphism are dual, their abstract categorical properties are also dual, such as exhibited by the following lemma.
Lemma 1.2.11.
(i) If f : x  y and g : y  z are monomorphisms, then so is g f : x  z.
(ii) If f : x → y and g : y → z are morphisms so that g f is monic, then f is monic.
Dually:(i’) If f : x  y and g : y  z are epimorphisms, then so is g f : x  z.
(ii’) If f : x → y and g : y → z are morphisms so that g f is epic, then g is epic.
Proof. Exercise 1.2.iii.

Exercises.
Exercise 1.2.i. Show that C/c  (c/(Cop ))op . Defining C/c to be (c/(Cop ))op , deduce
Exercise 1.1.iii(ii) from Exercise 1.1.iii(i).
Exercise 1.2.ii.
(i) Show that a morphism f : x → y is a split epimorphism in a category C if and only
if for all c ∈ C, the post-composition function f∗ : C(c, x) → C(c, y) is surjective.
(ii) Argue by duality that f is a split monomorphism if and only if for all c ∈ C, thepre-composition function f ∗ : C(y, c) → C(x, c) is surjective.
Exercise 1.2.iii. Prove Lemma 1.2.11 by proving either (i) or (i’) and either (ii) or (ii’), thenarguing by duality. Conclude that the monomorphisms in any category define a subcategoryof that category and dually that the epimorphisms also define a subcategory.
Exercise 1.2.iv. What are the monomorphisms in the category of fields?
Exercise 1.2.v. Show that the inclusion Z ,→ Q is both a monomorphism and an epimorphism in the category Ring of rings. Conclude that a map that is both monic and epic neednot be an isomorphism.
Exercise 1.2.vi. Prove that a morphism that is both a monomorphism and a split epimorphism is necessarily an isomorphism. Argue by duality that a split monomorphism that is
an epimorphism is also an isomorphism.
19The axiom of choice asserts that every epimorphism in the category of sets is a split epimorphism.

1.3. FUNCTORIALITY

13

Exercise 1.2.vii. Regarding a poset (P, ≤) as a category, define the supremum of a subcollection of objects A ∈ P in such a way that the dual statement defines the infimum. Provethat the supremum of a subset of objects is unique, whenever it exists, in such a way thatthe dual proof demonstrates the uniqueness of the infimum.
1.3. Functoriality. . . every sufficiently good analogy is yearning tobecome a functor.
John Baez, “Quantum Quandaries: A
Category-Theoretic Perspective” [Bae06]

A key tenet in category theory, motivating the very definition of a category, is thatany mathematical object should be considered together with its accompanying notion of
structure-preserving morphism. In “General theory of natural equivalences” [EM45],
Eilenberg and Mac Lane argue further:. . . whenever new abstract objects are constructed in a specified way
out of given ones, it is advisable to regard the construction of thecorresponding induced mappings on these new objects as an integral
part of their definition.
Categories are themselves mathematical objects, if of a somewhat unfamiliar sort, whichleads to a question: What is a morphism between categories?
Definition 1.3.1. A functor F : C → D, between categories C and D, consists of thefollowing data:• An object Fc ∈ D, for each object c ∈ C.
• A morphism F f : Fc → Fc0 ∈ D, for each morphism f : c → c0 ∈ C, so that thedomain and codomain of F f are, respectively, equal to F applied to the domain orcodomain of f .
The assignments are required to satisfy the following two functoriality axioms:• For any composable pair f, g in C, Fg · F f = F(g · f ).
• For each object c in C, F(1c ) = 1Fc .
Put concisely, a functor consists of a mapping on objects and a mapping on morphisms that preserves all of the structure of a category, namely domains and codomains,
composition, and identities.20
Example 1.3.2.
(i) There is an endofunctor21 P : Set → Set that sends a set A to its power set PA ={A0 ⊂ A} and a function f : A → B to the direct-image function f∗ : PA → PB that
sends A0 ⊂ A to f (A0 ) ⊂ B.
(ii) Each of the categories listed in Example 1.1.3 has a forgetful functor, a general termthat is used for any functor that forgets structure, whose codomain is the category ofsets. For example, U : Group → Set sends a group to its underlying set and a grouphomomorphism to its underlying function. The functor U : Top → Set sends a space
to its set of points. There are two natural forgetful functors V, E : Graph ⇒ Set that
send a graph to its vertex or edge sets, respectively; if desired, these can be combined
20While a functor should be regarded as a mapping from the data of one category to the data of another,parentheses are used as seldom as possible unless demanded for notational clarity.
21An endofunctor is a functor whose domain is equal to its codomain.

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1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

to define a single functor V t E : Graph → Set that carries a graph to the disjoint
union of its vertex and edge sets. These mappings are functorial because in each
instance a morphism in the domain category has an underlying function.
(iii) There are intermediate forgetful functors ModR → Ab and Ring → Ab that forgetsome but not all of the algebraic structure. The inclusion functors Ab ,→ Groupand Field ,→ Ring may also be regarded as “forgetful.” Note that the latter two, butneither of the former, are injective on objects: a group is either abelian or not, but anabelian group might admit the structure of a ring in multiple ways.
(iv) Similarly, there are forgetful functors Group → Set∗ and Ring → Set∗ that takethe basepoint to be the identity and zero elements, respectively. These assignments
are functorial because group and ring homomorphisms necessarily preserve theseelements.
(v) There are functors Top → Htpy and Top∗ → Htpy∗ that act as the identity on objects
and send a (based) continuous function to its homotopy class.
(vi) The fundamental group defines a functor π1 : Top∗ → Group; a continuous functionf : (X, x) → (Y, y) of based spaces induces a group homomorphism f∗ : π1 (X, x) →π1 (Y, y) and this assignment is functorial, satisfying the two functoriality axioms
described above. A precise expression of the statement that “the fundamental groupis a homotopy invariant” is that this functor factors through the functor Top∗ → Htpy∗
to define a functor π1 : Htpy∗ → Group.
(vii) A related functor Π1 : Top → Groupoid assigns an unbased topological space its
fundamental groupoid, the category defined in Example 1.1.12(ii). A continuousfunction f : X → Y induces a functor f∗ : Π1 (X) → Π1 (Y) that carries a pointx ∈ X to the point f (x) ∈ Y. This mapping extends to morphisms in Π1 (X) because
continuous functions preserve paths and path homotopy classes.
(viii) For each n ∈ Z, there are functors Zn , Bn , Hn : ChR → ModR . The functor Zn
computes the n-cycles defined by ZnC• = ker(d : Cn → Cn−1 ). The functor Bn
computes the n-boundary defined by BnC• = im(d : Cn+1 → Cn ). The functor
Hn computes the nth homology HnC• B ZnC• /BnC• . We leave it to the reader toverify that each of these three constructions is functorial. Considering all degrees
simultaneously, the cycle, boundary, and homology functors assemble into functors
Z∗ , B∗ , H∗ : ChR → GrModR from the category of chain complexes to the categoryof graded R-modules. The singular homology of a topological space is defined byprecomposing H∗ with a suitable functor Top → ChR .
(ix) There is a functor F : Set → Group that sends a set X to the free group on X. Thisis the group whose elements are finite “words” whose letters are elements x ∈ X or
their formal inverses x−1 , modulo an equivalence relation that equates the words xx−1
and x−1 x with the empty word. Multiplication is by concatenation, with the empty
word serving as the identity. This is one instance of a large family of “free” functors
studied in Chapter 4.
(x) The chain rule expresses the functoriality of the derivative. Let Euclid∗ denote thecategory whose objects are pointed finite-dimensional Euclidean spaces (Rn , a)—or, better, open subsets thereof—and whose morphisms are pointed differentiable
functions. The total derivative of f : Rn → Rm , evaluated at the designated basepointa ∈ Rn , gives rise to a matrix called the Jacobian matrix defining the directional
derivatives of f at the point a. If f is given by component functions f1 , . . . , fm : Rn →R, the (i, j)-entry of this matrix is ∂x∂ j fi (a). This defines the action on morphismsof a functor D : Euclid∗ → MatR ; on objects, D assigns a pointed Euclidean space1.3. FUNCTORIALITY

15

its dimension. Given g : Rm → Rk carrying the designated basepoint f (a) ∈ Rm tog f (a) ∈ Rk , functoriality of D asserts that the product of the Jacobian of f at a with
the Jacobian of g at f (a) equals the Jacobian of g f at a. This is the chain rule frommultivariable calculus.22(xi) Any commutative monoid M can be used to define a functor M − : Fin∗ → Set.
Writing n+ ∈ Fin∗ for the set with n non-basepoint elements, define M n+ to be M n ,the n-fold cartesian product of the set M with itself. By convention, M 0+ is a singleton
set. For any based map f : m+ → n+ , define the ith component of the corresponding
function M f : M m → M n by projecting from M m to the coordinates indexed byelements in the fiber f −1 (i) and then multiplying these using the commutative monoid
structure; if the fiber is empty, the function M f inserts the unit element in the ith
coordinate. Note each of the sets M n itself has a basepoint, the n-tuple of unit
elements, and each of the maps in the image of the functor are based. It follows thatthe functor M − lifts along the forgetful functor U : Set∗ → Set.
There is a special property satisfied by this construction that allows one to extract
the commutative monoid M from the functor Fin∗ → Set. This observation was usedby Segal to introduce a suitable notion of “commutative monoid” into algebraic
topology [Seg74].
More examples of functors will appear shortly, but first we illustrate the utility ofknowing that the assignment of a mathematical object of one type to mathematical objects ofanother type is functorial. Applying the functoriality of the fundamental group construction
π1 : Top∗ → Group, one can prove:
Theorem 1.3.3 (Brouwer Fixed Point Theorem). Any continuous endomorphism of a 2dimensional disk D2 has a fixed point.
Proof. Assuming f : D2 → D2 is such that f (x) , x for all x ∈ D2 , there is acontinuous function r : D2 → S 1 that carries a point x ∈ D2 to the intersection of the rayfrom f (x) to x with the boundary circle S 1 . Note that the function r fixes the points on theboundary circle S 1 ⊂ D2 . Thus, r defines a retraction of the inclusion i : S 1 ,→ D2 , whichiris to say, the composite S 1 →− D2 →− S 1 is the identity.

f (x)
•

4
x
• • r(x)Pick any basepoint on the boundary circle S 1 and apply the functor π1 to obtain acomposable pair of group homomorphisms:
π1 (i)

π1 (r)

π1 (S 1 ) −−−→ π1 (D2 ) −−−→ π1 (S 1 ) .
By the functoriality axioms, we must have
π1 (r) · π1 (i) = π1 (ri) = π1 (1S 1 ) = 1π1 (S 1 ) .
However, a computation involving covering spaces reveals that π1 (S 1 ) = Z, while π1 (D2 ) =0, the trivial group. The composite endomorphism π1 (r) · π1 (i) of Z must be zero, since it
factors through the trivial group. Thus, it cannot equal the identity homomorphism, which
22Taking a more sophisticated perspective, the derivative defines the action on morphisms of a functor from thecategory Man∗ to the category of real vector spaces that sends a pointed manifold to its tangent space.

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1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

carries the generator 1 ∈ Z to itself (0 , 1). This contradiction proves that the retraction r
cannot exist, and so f must have a fixed point.23
Functoriality also plays a key role in the emerging area of topological data analysis.
Example 1.3.4 (in search of a clustering functor). A clustering algorithm is a functionthat converts a finite metric space into a partition of its points into sets of “clusters.”
An impossibility theorem of Kleinberg proves that there are no clustering algorithms thatsatisfy three reasonable axioms [Kle03].24 A key insight of Carlsson and Mémoli is thatthese axioms can be encoded as morphisms in a category of finite metric spaces in such a
way that what is desired is not a clustering function but a clustering functor into a suitable
category [CM13]. Ghrist’s Elementary Applied Topology [Ghr14, p. 216] describes thismove as follows:
What is the good of this? Category theory is criticized as an esotericlanguage: formal and fruitless for conversation. This is not so.25
The virtue of reformulating (the negative) Theorem [of Kleinberg]
functorially is a clearer path to a positive statement. If the goal isto have a theory of clustering; if clustering is, properly, a nontrivial
functor; if no nontrivial functors between the proposed categories
exist; then, naturally, the solution is to alter the domain or codomain
categories and classify the ensuing functors. One such modification
is to consider a category of persistent clusters.
One pair of categories considered in [CM13] are the categories FinMetric, of finite
metric spaces and distance non-increasing functions, and Cluster, of clusters and refinements. An object in Cluster is a partitioned set. Given a function f : X → Y, the preimagesof a partition of the set Y define a partition of X. A morphism in Cluster is a function
f : X → Y of underlying sets so that the given partition on X refines the partition on X
defined by the preimages of the given partition on Y.
Carlsson and Mémoli observe that the only scale-invariant functors FinMetric →Cluster either assign each metric space the discrete partition (into singletons) or the indiscrete partition (into a single cluster); both cases fail to satisfy Kleinberg’s surjectivity
condition. This suggests that clusters should be replaced by a notion of “persistent” clusters.
A persistent cluster on X is a functor from the poset ([0, ∞), ≤) to the poset of clusters on
X, where φ ≤ ψ if and only if the partition φ refines the partition ψ. The idea is that when
the parameter r ∈ [0, ∞) is small, the partition on X might be very fine, but the clusters areallowed to coalesce as one “zooms out,” i.e., as r increases.
There is a category PCluster whose objects are persistent clusters and whose morphisms are functions of underlying sets f : X → Y that define morphisms in Cluster for eachr ∈ [0, ∞). Carlsson and Mémoli prove that there is a unique functor FinMetric → PCluster,which takes the metric space with two points of distance r to the persistent cluster with one
cluster for t ≥ r and two clusters for 0 ≤ t < r and satisfies two other reasonable conditions;see [CM13] for the details.
23The same argument, with the nth homotopy group functor πn : Top∗ → Group in place of π1 , proves that any
continuous endomorphism of an n-dimensional disk has a fixed point.
24Namely, there are no clustering algorithms that are invariant under rigid scaling, consistent under alterations
to the distance function that “sharpen” the point clusters, and have the property that some distance function realizes
each possible partition.
25Emphasis his.

1.3. FUNCTORIALITY

17

The functors defined in 1.3.1 are called covariant so as to distinguish them fromanother variety of functor that we now introduce.
Definition 1.3.5. A contravariant functor F from C to D is a functor F : Cop → D.26
Explicitly, this consists of the following data:• An object Fc ∈ D, for each object c ∈ C.
• A morphism F f : Fc0 → Fc ∈ D, for each morphism f : c → c0 ∈ C, so that thedomain and codomain of F f are, respectively, equal to F applied to the codomain or
domain of f .
The assignments are required to satisfy the following two functoriality axioms:• For any composable pair f, g in C, F f · Fg = F(g · f ).
• For each object c in C, F(1c ) = 1Fc .
Notation 1.3.6. To avoid unnatural arrow-theoretic representations, a morphism in thedomain of a functor F : Cop → D will always be depicted as an arrow f : c → c0 in C,
pointing from its domain in C to its codomain in C. Similarly, its image will always be
depicted as an arrow F f : Fc0 → Fc in D, pointing from its domain to its codomain. Notethat these conventions require that the domain and codomain objects switch their relative
places, from left to right, but in examples, for instance in the case where C and D are
concrete categories, these positions are the familiar ones. Graphically, the mapping on
morphisms given by a contravariant functor is depicted as follows:
Cop

F

/D

c

7→

Fc
O

f


c0

7→

Ff

7→

Fc0

In accordance with this convention, if f : c → c0 and g : c0 → c00 are morphisms in C,
their composite will always be written as g f : c → c00 . The image of this morphism under
the contravariant functor F : Cop → D is F(g f ) : Fc00 → Fc, the composite F f · Fg of
Fg : Fc00 → Fc0 and F f : Fc0 → Fc.
In summary, even in the presence of opposite categories, we always make an effort todraw arrows pointing in the “correct way” and depict composition in the usual order.27
Example 1.3.7.
(i) The contravariant power set functor P : Setop → Set sends a set A to its power set PA
and a function f : A → B to the inverse-image function f −1 : PB → PA that sends
B0 ⊂ B to f −1 (B0 ) ⊂ A.
op(ii) There is a functor (−)∗ : Vectk → Vectk that carries a vector space to its dual vector
space V ∗ = Hom(V, k). A vector in V ∗ is a linear functional on V, i.e., a linear map
26In this text, a contravariant functor F from C to D will always be written as F : Cop → D. Some mathematicians
omit the “op” and let the context or surrounding verbiage convey the variance. We think this is bad practice,as the co- or contravariance is an essential part of the data of a functor, which is not necessarily determinedby its assignation on objects. More to the point, we find that this notational convention helps mitigate theconsequences of temporary distraction. Seeing F : Cop → D written on a chalkboard immediately conveys that F
is a contravariant functor from C to D, even to the most spaced-out observer. A similar principle will motivate
other notational conventions introduced in Definition 3.1.15 and Notation 4.1.5.
27Of course, technically there is no meaning to the phrase “opposite category”: every category is the opposite
of some other category (its opposite category). But in practice, there is no question which of Set and Setop is the“opposite category,” and sufficiently many of the other cases can be deduced from this one.

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V → k. This functor is contravariant, with a linear map φ : V → W sent to the linearω
map φ∗ : W ∗ → V ∗ that pre-composes a linear functional W −→ k with φ to obtain aφωlinear functional V →
− W−
→ k.
(iii) The functor O : Topop → Poset that carries a space X to its poset O(X) of open
subsets is contravariant on the category of spaces: a continuous map f : X → Y
gives rise to a function f −1 : O(Y) → O(X) that carries an open subset U ⊂ Y to its
preimage f −1 (U), which is open in X; this is the definition of continuity. A similarfunctor C : Topop → Poset carries a space to its poset of closed subsets.
(iv) There is a contravariant functor Spec : CRingop → Top that sends a commutative ring
R to its set Spec(R) of prime ideals given the Zariski topology. The closed subsets
in the Zariski topology are those subsets VI ⊂ Spec(R) of prime ideals containing
a fixed ideal I ⊂ R. This construction is contravariantly functorial: for any ringhomomorphism φ : R → S and prime ideal p ⊂ S , the inverse image φ−1 (p) ⊂ R
defines a prime ideal of R, and the inverse image function φ−1 : Spec(S ) → Spec(R)is continuous with respect to the Zariski topology.
(v) For a generic small category C, a functor Cop → Set is called a (set-valued) presheaf
on C. A typical example is the functor O(X)op → Set whose domain is the poset
O(X) of open subsets of a topological space X and whose value at U ⊂ X is the set ofcontinuous real-valued functions on U. The action on morphisms is by restriction.
This presheaf is a sheaf, if it satisfies an axiom that is introduced in Definition 3.3.4.
(vi) Presheaves on the category ∆, of finite non-empty ordinals and order-preserving
maps, are called simplicial sets. ∆ is also called the simplex category. The
ordinal n + 1 = {0, 1, . . . , n} may be thought of as a direct version of the topologicaln-simplex and, with this interpretation in mind, is typically denoted by “[n]” byalgebraic topologists.
The following result, which appears immediately after functors are first defined in[EM42b], is arguably the first lemma in category theory.
Lemma 1.3.8. Functors preserve isomorphisms.
Proof. Consider a functor F : C → D and an isomorphism f : x → y in C with inverseg : y → x. Applying the two functoriality axioms:
F(g)F( f ) = F(g f ) = F(1 x ) = 1F x .
Thus, Fg : Fy → F x is a left inverse to F f : F x → Fy. Exchanging the roles of f and g(or arguing by duality) shows that Fg is also a right inverse.

Example 1.3.9. Let G be a group, regarded as a one-object category BG. A functor
X : BG → C specifies an object X ∈ C (the unique object in its image) together with anendomorphism g∗ : X → X for each g ∈ G. This assignment must satisfy two conditions:(i) h∗ g∗ = (hg)∗ for all g, h ∈ G.
(ii) e∗ = 1X , where e ∈ G is the identity element.
In summary, the functor BG → C defines an action of the group G on the object X ∈ C.
When C = Set, the object X endowed with such an action is called a G-set. When
C = Vectk , the object X is called a G-representation. When C = Top, the object X iscalled a G-space. Note the utility of this categorical language for defining several analogous
concepts simultaneously.
The action specified by a functor BG → C is sometimes called a left action. A rightaction is a functor BGop → C. As before, each g ∈ G determines an endomorphism1.3. FUNCTORIALITY

19g∗ : X → X in C and the identity element must act trivially. But now, for a pair of elements
g, h ∈ G these actions must satisfy the composition rule (hg)∗ = g∗ h∗ .
Because the elements g ∈ G are isomorphisms when regarded as morphisms in the
1-object category BG that represents the group, their images under any such functor mustalso be isomorphisms in the target category. In particular, in the case of a G-representation
V : BG → Vectk , the linear map g∗ : V → V must be an automorphism of the vector space
V. The point is that the functoriality axioms (i) and (ii) imply automatically that each g∗ isan automorphism and that (g−1 )∗ = (g∗ )−1 ; the proof is a special case of Lemma 1.3.8.
In summary:Corollary 1.3.10. When a group G acts functorially on an object X in a category C, its
elements g must act by automorphisms g∗ : X → X and, moreover, (g∗ )−1 = (g−1 )∗ .
A functor may or may not preserve monomorphisms or epimorphisms, but an argument similar to the proof of Lemma 1.3.8 shows that a functor necessarily preserves split
monomorphisms and split epimorphisms. The retraction or section defines an “equational
witness” for the mono or the epi.
Definition 1.3.11. If C is locally small, then for any object c ∈ C we may define a pair ofcovariant and contravariant functors represented by c:CxC(c,−)/ Set

Cop

C(−,c)/ Set

7→

C(c, x)x

7→

C(x, c)

7→

f∗

7→

C(y, c)

7→

f


y

7→

f∗


C(c, y)fyO

The notation suggests the action on objects: the functor C(c, −) carries x ∈ C to the set
C(c, x) of arrows from c to x in C. Dually, the functor C(−, c) carries x ∈ C to the set
C(x, c).
The functor C(c, −) carries a morphism f : x → y to the post-composition functionf∗ : C(c, x) → C(c, y) introduced in Lemma 1.2.3(ii). Dually, the functor C(−, c) carries fto the pre-composition function f ∗ : C(y, c) → C(x, c) introduced in 1.2.3(iii). Note that
post-composition defines a covariant action on hom-sets, while pre-composition definesa contravariant action. There are no choices involved here; post-composition is always acovariant operation, while pre-composition is always a contravariant one. This is just thenatural order of things.
We leave it to the reader to verify that the assignments just described satisfy the twofunctoriality axioms. Note that Lemma 1.3.8 specializes in the case of represented functors
to give a proof of the implications (i) ⇒ (ii) and (i) ⇒ (iii) of Lemma 1.2.3. These functors
will play a starring role in Chapter 2, where a number of examples in disguise are discussed.
The data of the covariant and contravariant functors introduced in Definition 1.3.11
may be encoded in a single bifunctor, which is the name for a functor of two variables. Its
domain is given by the product of a pair of categories.
Definition 1.3.12. For any categories C and D, there is a category C × D, their product,whose• objects are ordered pairs (c, d), where c is an object of C and d is an object of D,• morphisms are ordered pairs ( f, g) : (c, d) → (c0 , d0 ), where f : c → c0 ∈ C andg : d → d0 ∈ D, and20

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS• in which composition and identities are defined componentwise.
Definition 1.3.13. If C is locally small, then there is a two-sided represented functor
C(−, −) : Cop × C → Set

defined in the evident manner. A pair of objects (x, y) is mapped to the hom-set C(x, y). A
pair of morphisms f : w → x and h : y → z is sent to the function
C(x, y)g( f ∗ ,h∗ )/ C(w, z)

7→

hg f

that takes an arrow g : x → y and then pre-composes with f and post-composes with h toobtain hg f : w → z.
At the beginning of this section, it was suggested that functors define morphisms
between categories. Indeed, categories and functors assemble into a category. Here thesize issues are even more significant than we have encountered thus far. To put a lid
on things, define Cat to be the category whose objects are small categories and whosemorphisms are functors between them. This category is locally small but not small: it
contains Set, Poset, Monoid, Group, and Groupoid as proper subcategories (see Exercises
1.3.i and 1.3.ii). However, none of these categories are objects of Cat.
The non-small categories of Example 1.1.3 are objects of CAT, some category of “large”
categories and functors between them. Russell’s paradox suggests that CAT should not be
so large as to contain itself, so we require the objects in CAT to be locally small categories;
the category CAT defined in this way is not locally small, and so is thus excluded. There isan inclusion functor Cat ,→ CAT but no obvious functor pointing in the other direction.
The category of categories gives rise to a notion of an isomorphism of categories,
defined by interpreting Definition 1.1.9 in Cat or in CAT. Namely, an isomorphism of
categories is given by a pair of inverse functors F : C → D and G : D → C so that thecomposites GF and FG, respectively, equal the identity functors on C and on D. An
isomorphism induces a bĳection between the objects of C and objects of D and likewise
for the morphisms.
Example 1.3.14. For instance:(i) The functor (−)op : CAT → CAT defines a non-trivial automorphism of the categoryof categories. Note that a functor F : C → D also defines a functor F : Cop → Dop .
(ii) For any group G, the categories BG and BGop are isomorphic via the functor (−)−1 that
sends each morphism g ∈ G to its inverse. Any right action can be converted into aleft action by precomposing with this isomorphism, which has the effect of “inserting
inverses in the formula” defining the endomorphism associated to a particular groupelement.
(iii) Similarly, any groupoid is isomorphic to its opposite category via the functor thatacts as the identity on objects and sends a morphism to its unique inverse morphism.
(iv) Any ring R has an opposite ring Rop with the same underlying abelian group but withthe product of elements r and s in Rop defined to be the product s · r of the elements s
and r in R. A left R-module is the same thing as a right Rop -module, which is to say
there is a covariant isomorphism of categories ModR  Rop Mod between the categoryof left R-modules and the category of right Rop -modules.
(v) For any space X, there is a contravariant isomorphism of poset categories O(X) C(X)op that associates an open subset of X to its closed complement.

1.3. FUNCTORIALITY

21(vi) The category MatR is isomorphic to its opposite via an identity-on-objects functor
that carries a matrix to its transpose.
Contrary to the impression created by Examples 1.3.14 (ii), (iii), and (vi), a category
is not typically isomorphic to its opposite category.
Example 1.3.15. Let E/F be a finite Galois extension: this means that F is a finite-index
subfield of E and that the size of the group Aut(E/F) of automorphisms of E fixing every
element of F is at least (in fact, equal to) the index [E : F]. In this case, G B Aut(E/F) iscalled the Galois group of the Galois extension E/F.
Consider the orbit category OG associated to the group G. Its objects are subgroups
H ⊂ G, which we identify with the left G-set G/H of left cosets of H. Morphisms
G/H → G/K are G-equivariant maps, i.e., functions that commute with the left G-action.
By an elementary exercise left to the reader, every morphism G/H → G/K has the formgH 7→ gγK, where γ ∈ G is an element so that γ−1 Hγ ⊂ K.
Let FieldEF denote the subcategory of F/Field whose objects are intermediate fields
F ⊂ K ⊂ E. A morphism K → L is a field homomorphism that fixes the elements of F
pointwise. Note that the group of automorphisms of the object E ∈ FieldEF is the Galois
group G = Aut(E/F).
op
We define a functor Φ : OG → FieldEF that sends H ⊂ G to the subfield of E of elementsthat are fixed by H under the action of the Galois group. If G/H → G/K is induced byγ, then the field homomorphism x 7→ γx sends an element x ∈ E that is fixed by K to anelement γx ∈ E that is fixed by H. This defines the action of the functor Φ on morphisms.
The fundamental theorem of Galois theory asserts that Φ defines a bĳection on objects
opbut in fact more is true: Φ defines an isomorphism of categories OG  FieldEF .
These examples aside, the notion of isomorphism of categories is somewhat unnatural.
To illustrate, consider the category Set∂ of sets and partially-defined functions. A partial
function f : X → Y is a function from a (possibly-empty) subset X 0 ⊂ X to Y; the subset
X 0 is the domain of definition of the partial function f . The composite of two partial
functions f : X → Y and g : Y → Z is the partial function whose domain of definition is theintersection of the domain of definition of f with the preimage of the domain of definitionof g.
There is a functor (−)+ : Set∂ → Set∗ , whose codomain is the category of pointed sets,that sends a set X to the pointed set X+ , which is defined to be the disjoint union of X witha freely-added basepoint. By the axiom of regularity, we might define X+ B X ∪ {X}.28
A partial function f : X → Y gives rise to a pointed function f+ : X+ → Y+ that sends
every point outside of the domain of definition of f to the formally added basepoint of Y+ .
The inverse functor U : Set∗ → Set∂ discards the basepoint and sends a based functionf : (X, x) → (Y, y) to the partial function X\{x} → Y\{y} with the maximal possible domainof definition.
By construction, we see that the composite U(−)+ is the identity endofunctor of thecategory Set∂ . By contrast, the other composite (U−)+ : Set∗ → Set∗ sends a pointed set(X, x) to (X\{x} ∪ {X\{x}}, X\{x}). These sets are isomorphic but they are not identical.
Nor is another set-theoretical construction of the “freely added basepoint” likely to define agenuine inverse to the functor U : Set∗ → Set∂ . It is too restrictive to ask for the categories
Set∂ and Set∗ to be isomorphic.
28In the axioms of Zermelo-Fraenkel set theory, elements of sets (like everything else in its mathematicaluniverse) are themselves sets. The axiom of regularity prohibits a set from being an element of itself. As X < X,
we are free to add the element X as a disjoint basepoint.

22

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

Indeed, there is a better way to decide whether two categories may safely be regarded
as “the same.” To define it, we must relax the identities GF = 1C and FG = 1D between
functors F : C → D and G : D → C that define an isomorphism of categories. This ispossible because the collections Hom(C, C) and Hom(D, D) are not mere (possibly large)
sets: they have higher-dimensional structure. For any pair of categories C and D, thecollection Hom(C, D) of functors is itself a category. To explain this, we introduce what in
French is called a morphisme de foncteurs, the notion that launched the entire subject of
category theory: a natural transformation.
Exercises.
Exercise 1.3.i. What is a functor between groups, regarded as one-object categories?
Exercise 1.3.ii. What is a functor between preorders, regarded as categories?
Exercise 1.3.iii. Find an example to show that the objects and morphisms in the image ofa functor F : C → D do not necessarily define a subcategory of D.
Exercise 1.3.iv. Verify that the constructions introduced in Definition 1.3.11 are functorial.
Exercise 1.3.v. What is the difference between a functor Cop → D and a functor C → Dop ?
What is the difference between a functor C → D and a functor Cop → Dop ?
Exercise 1.3.vi. Given functors F : D → C and G : E → C, show that there is a category,called the comma category F ↓G, which has• as objects, triples (d ∈ D, e ∈ E, f : Fd → Ge ∈ C), and• as morphisms (d, e, f ) → (d0 , e0 , f 0 ), a pair of morphisms (h : d → d0 , k : e → e0 ) sothat the square
Fd

f

Gk

Fh


Fd0

/ Ge

f0


/ Ge0

commutes in C, i.e., so that f 0 · Fh = Gk · f .
Define a pair of projection functors dom : F ↓G → D and cod : F ↓G → E.
Exercise 1.3.vii. Define functors to construct the slice categories c/C and C/c of Exercise
1.1.iii as special cases of comma categories constructed in Exercise 1.3.vi. What are theprojection functors?
Exercise 1.3.viii. Lemma 1.3.8 shows that functors preserve isomorphisms. Find an
example to demonstrate that functors need not reflect isomorphisms: that is, find a functor
F : C → D and a morphism f in C so that F f is an isomorphism in D but f is not an
isomorphism in C.
Exercise 1.3.ix. For any group G, we may define other groups:• the center Z(G) = {h ∈ G | hg = gh ∀g ∈ G}, a subgroup of G,• the commutator subgroup C(G), the subgroup of G generated by elements ghg−1 h−1
for any g, h ∈ G, and• the automorphism group Aut(G), the group of isomorphisms φ : G → G in Group.
Trivially, all three constructions define a functor from the discrete category of groups (withonly identity morphisms) to Group. Are these constructions functorial in• the isomorphisms of groups? That is, do they extend to functors Groupiso → Group?

1.4. NATURALITY23• the epimorphisms of groups29? That is, do they extend to functors Groupepi → Group?
• all homomorphisms of groups? That is, do they extend to functors Group → Group?
Exercise 1.3.x. Show that the construction of the set of conjugacy classes of elements ofa group is functorial, defining a functor Conj : Group → Set. Conclude that any pair ofgroups whose sets of conjugacy classes of elements have differing cardinalities cannot be
isomorphic.
1.4. Naturality
It is not too misleading, at least historically, to saythat categories are what one must define in orderto define functors, and that functors are what one
must define in order to define naturaltransformations.
Peter Freyd, Abelian categories [Fre03]

Any finite-dimensional k-vector space V is isomorphic to its linear dual, the vector
space V ∗ = Hom(V, k) of linear maps V → k, because these vector spaces have the same
dimension. This can be proven through the construction of an explicit dual basis: choosea basis e1 , . . . , en for V and then define e∗1 , . . . , e∗n ∈ V ∗ by
1 i = j∗ei (e j ) = 0 i , j.
The collection e∗1 , . . . , e∗n defines a basis for V ∗ and the map ei 7→ e∗i extends by linearity todefine an isomorphism V  V ∗ .
Now consider a related construction of the double dual V ∗∗ = Hom(Hom(V, k), k)of V. If V is finite dimensional, then the isomorphism V  V ∗ is carried by the dual
opvector space functor (−)∗ : Vectk → Vectk to an isomorphism V ∗  V ∗∗ . The composite∗∗
isomorphism V  V ∗∗ sends the basis e1 , . . . , en to the dual dual basis e∗∗
1 , . . . , en .
As it turns out, this isomorphism has a simpler description. For any v ∈ V, the“evaluation function”
evv

f 7→ f (v) : V ∗ −−→ kdefines a linear functional on V ∗ . It turns out the assignment v 7→ evv defines a linear
isomorphism V  V ∗∗ , this time requiring no “unnatural” choice of basis.30
What distinguishes the isomorphism between a finite-dimensional vector space and its
double dual from the isomorphism between a finite-dimensional vector space and its single
dual is that the former assembles into the components of a natural transformation in thesense that we now introduce.
Definition 1.4.1. Given categories C and D and functors F, G : C ⇒ D, a natural transformation α : F ⇒ G consists of:• an arrow αc : Fc → Gc in D for each object c ∈ C, the collection of which define thecomponents of the natural transformation,
29A non-trivial theorem demonstrates that a homomorphism φ : G → H is an epimorphism in Group if andonly if its underlying function is surjective; see [Lin70].
∗∗∗∗ are the same—it is only our∗
30In fact, e∗∗i (e j ) = e j (ei ) = evei (e j ), and so the two isomorphisms V  V
description that has improved.

24

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

so that, for any morphism f : c → c0 in C, the following square of morphisms in D
Fc
(1.4.2)

αc

Ff


Fc0

/ Gc
Gf

αc0


/ Gc0

commutes, i.e., has a common composite Fc → Gc0 in D.
A natural isomorphism is a natural transformation α : F ⇒ G in which every component αc is an isomorphism. In this case, the natural isomorphism may be depicted as
α : F  G.
In practice, it is usually most elegant to define a natural transformation by saying that“the arrows X are natural,” which means that the collection of arrows defines the components
of a natural transformation, leaving implicit the correct choices of domain and codomainfunctors, and source and target categories. Here X should be a collection of morphisms
in a clearly identifiable (target) category, whose domains and codomains are defined using
a common “variable” (an object of the source category). If this variable is c, one might
say “the arrows X are natural in c” to emphasize the domain object whose component isbeing described. However, the totality of the data of the source and target categories, theparallel pair of functors, and the components should always be considered part of the naturaltransformation. The naturality condition (1.4.2) cannot be stated precisely with any less: it
refers to every object and every morphism in the domain category and is described using
the images in the codomain category under the action of both functors. The “boundarydata” needed to define a natural transformation α is often displayed in a globular diagram:FC ⇓ α ADGThe globular depiction of a natural transformation makes the notions of composable naturaltransformations that are introduced in §1.7 particularly intuitive.
Example 1.4.3.
(i) For vector spaces of any dimension, the map ev : V → V ∗∗ that sends v ∈ V tothe linear function evv : V ∗ → k defines the components of a natural transformationfrom the identity endofunctor on Vectk to the double dual functor. To check that thenaturality squareev / ∗∗
V
V
φ


W

φ∗∗

ev


/ W ∗∗

commutes for any linear map φ : V → W, it suffices to consider the image of a genericvector v ∈ V. By definition, evφv : W ∗ → k carries a functional f : W → k to f (φv).
Recalling the definition of the action of the dual functor of Example 1.3.7(ii) on
morphisms, we see that φ∗∗ (evv ) : W ∗ → k carries a functional f : W → k to f φ(v),which amounts to the same thing.
(ii) By contrast, the identity functor and the single dual functor on finite-dimensional
vector spaces are not naturally isomorphic. One technical obstruction is somewhat

1.4. NATURALITY

25

beside the point: the identity functor is covariant while the dual functor is contravariant.31 More significant is the essential failure of naturality. The isomorphisms
V  V ∗ that can be defined whenever V is finite dimensional require a choice ofbasis, which is preserved by essentially no linear maps, indeed by no non-identity
linear endomorphism.32(iii) There is a natural transformation η : 1Set ⇒ P from the identity to the covariant
power set functor whose components ηA : A → PA are the functions that carry a ∈ A
to the singleton subset {a} ∈ PA.
(iv) For G a group, Example 1.3.9 shows that a functor X : BG → C corresponds to an
object X ∈ C equipped with a left action of G, which suggests a question: What is anatural transformation between a pair X, Y : BG ⇒ C of such functors? Because thecategory BG has only one object, the data of α : X ⇒ Y consists of a single morphism
α : X → Y in C that is G-equivariant, meaning that for each g ∈ G, the diagram
X
g∗


X

α

α

/Y

/Y

g∗

commutes.
(v) The open and closed subset functors described in Example 1.3.7(iii) are naturally
isomorphic when they are regarded as functors O, C : Topop ⇒ Set valued in the
category of sets. The components O(X)  C(X) of the natural isomorphism aredefined by taking an open subset of X to its complement, which is closed. Naturality
asserts that the process of forming complements commutes with the operation oftaking preimages.
(vi) The construction of the opposite group described in Example 1.2.2(iii) defines a(covariant!) endofunctor (−)op : Group → Group of the category of groups; ahomomorphism φ : G → H induces a homomorphism φop : Gop → H op definedby φop (g) = φ(g). This functor is naturally isomorphic to the identity. DefineηG : G → Gop to be the homomorphism that sends g ∈ G to its inverse g−1 ∈ Gop ; thismapping does not define an automorphism of G, because it fails to commute with thegroup multiplication, but it does define a homomorphism G → Gop . Now given any
homomorphism φ : G → H, the diagram
G

ηG

φ


H

/ Gop


ηH

φop

/ H opcommutes because φop (g−1 ) = φ(g−1 ) = φ(g)−1 .
(vii) Define an endofunctor of Vectk by V 7→ V ⊗ V. There is a natural transformationfrom the identity functor to this endofunctor whose components are the zero maps,
but this is the only such natural transformation: there is no basis-independent way todefine a linear map V → V ⊗ V. The same result is true for the category of Hilbert
31A more flexible notion of extranatural transformation can accommodate functors with conflicting variance[ML98a, IX.4]; see Exercise 1.4.vi.
32A proof that there exists no extranatural isomorphism between the identity and dual functors on the categories
of finite-dimensional vector spaces is given in [EM45, p. 234].

26

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

spaces and linear operators between them, in which context it is related to the “nocloning theorem” in quantum physics.33
Another familiar isomorphism that is not natural arises in the classification of finitely
generated abelian groups, objects of a category Abfg . Let T A denote the torsion subgroupof an abelian group A, the subgroup of elements with finite order. In classifying finitely
generated groups, one proves that every finitely generated abelian group A is isomorphic
to the direct sum T A ⊕ (A/T A), the summand A/T A being the torsion-free part of A.
However, these isomorphisms are not natural, as we now demonstrate.
Proposition 1.4.4. The isomorphisms A  T A ⊕ (A/T A) are not natural34 in A ∈ Abfg .
Proof. Suppose the isomorphisms A  T A ⊕ (A/T A) were natural in A. Then thecomposite(1.4.5)A  A/T A  T A ⊕ (A/T A)  A

of the canonical quotient map, the inclusion into the direct sum, and the hypothesized
natural isomorphism would define a natural endomorphism of the identity functor on Abfg .
We shall see that this is impossible.
To derive the contradiction, we first show that every natural endomorphism of theidentity functor on Abfg is multiplication by some n ∈ Z. Clearly the component of
α : 1Abfg ⇒ 1Abfg at Z has this description for some n. Now observe that homomorphismsaZ→− A correspond bĳectively to elements a ∈ A, choosing a to be the image of 1 ∈ Z.
Thus, commutativity of
Z
a


A

αZ =n·−

/Z
a

αA


/A

forces us to define αA (a) = n · a.
In the case where α is the natural transformation defined by (1.4.5), by examining thecomponent at A = Z, we can see that n , 0. Finally, consider A = Z/2nZ. This groupis torsion, so any map, such as αZ/2nZ , which factors through the quotient by its torsion
subgroup is zero. But n , 0 ∈ Z/2nZ, a contradiction.

Example 1.4.6. The Riesz representation theorem can be expressed as a natural isomorphism of functors from the category cHaus of compact Hausdorff spaces and continuous maps to the category Ban of real Banach spaces and continuous linear maps. LetΣ : cHaus → Ban be the functor that carries a compact Hausdorff space X to the Banach
space Σ(X) of signed Baire measures on X and sends a continuous map f : X → Y to themap µ 7→ µ ◦ f −1 : Σ(X) → Σ(Y). Let C ∗ : cHaus → Ban be the functor that carries X tothe linear dual C(X)∗ of the Banach space C(X) of continuous real-valued functions on X.
Now for each µ ∈ Σ(X), there is a linear functional φµ : C(X) → R defined byZg dµ,g ∈ C(X) .
φµ (g) B
X

33The states in a quantum mechanical system are modeled by vectors in a Hilbert space and the observables areoperators on that space. See [Bae06] for more.
34Any finitely generated abelian group A has a short exact sequence 0 → T A → A → A/T A → 0. Proposition
1.4.4 asserts that there is no natural splitting.

1.4. NATURALITY27

For each µ ∈ Σ(X), f : X → Y, and h ∈ C(Y) the equationZZ
h f dµ =h d(µ ◦ f −1 )XYsays that the assignment µ 7→ φµ defines the components of a natural transformationη : Σ ⇒ C ∗ . The Riesz representation theorem asserts that this natural transformation isa natural isomorphism; see [Har83].
Example 1.4.7. Consider morphisms f : w → x and h : y → z in a locally small category
C. Post-composition by h and pre-composition by f define functions between hom-sets
C(x, y)(1.4.8)

h·−

−· f

−· f


C(w, y)/ C(x, z)h·−/ C(w, z)

In Definition 1.3.13 and elsewhere, h · − was denoted by h∗ and − · f was denoted by f ∗ , but
we find this less-concise notation to be more evocative here. Associativity of composition
implies that this diagram commutes: for any g : x → y, the common image is hg f : w → z.
Interpreting the vertical arrows as the images of f under the actions of the functors
C(−, y) and C(−, z), the square (1.4.8) demonstrates that there is a natural transformationh∗ : C(−, y) ⇒ C(−, z)whose components are defined by post-composition with h : y → z. Flipping perspectives
and interpreting the horizontal arrows as the images of h under the actions of the functors
C(x, −) and C(w, −), the square (1.4.8) demonstrates that there is a natural transformationf ∗ : C(x, −) ⇒ C(w, −)whose components are defined by pre-composition with f : w → x.
A final example describes the natural isomorphisms that supply proofs of the fundamental laws of elementary arithmetic.
Example 1.4.9 (a categorification of the natural numbers). For sets A and B, let A × B
denote their cartesian product, let A + B denote their disjoint union, and let AB denote theset of functions from B to A. These constructions are related by natural isomorphisms(1.4.10)

A × (B + C)  (A × B) + (A × C)

(A × B)C  AC × BC

AB+C  AB × AC(AB )C  AB×C

In the first instance, the isomorphism defines the components of a natural transformation
between a pair of functors Set×Set×Set → Set. For the others, the variance in the variables
appearing as “exponents” is contravariant. This is because the assignment (B, A) 7→ AB
defines a functor Setop × Set → Set, namely the two-sided represented functor introducedin Definition 1.3.13.
The displayed natural isomorphisms restrict to the category Finiso of finite sets andbĳections, a category which serves as the domain for the cardinality functor |−| : Finiso → N,
whose codomain is the discrete category of natural numbers.35 Writing a = |A|, b = |B|,
35Mathematical invariants often take the form of a functor from a groupoid to a discrete category.

28

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

and c = |C|, the cardinality functor carries these natural isomorphisms to the equationsa × (b + c) = (a × b) + (a × c)(a × b)c = ac × bc

ab+c = ab × ac(ab )c = a(b×c)

through a process called decategorification. Reversing directions, Finiso is a categorification of the natural numbers, which reveals that the familiar laws of arithmetic follow from
more fundamental natural isomorphisms between various constructions on sets. A slick
proof of each of the displayed natural isomorphisms (1.4.10) appears in Corollary 4.5.6.
Exercises.
Exercise 1.4.i. Suppose α : F ⇒ G is a natural isomorphism. Show that the inverses of
the component morphisms define the components of a natural isomorphism α−1 : G ⇒ F.
Exercise 1.4.ii. What is a natural transformation between a parallel pair of functors between groups, regarded as one-object categories?
Exercise 1.4.iii. What is a natural transformation between a parallel pair of functors
between preorders, regarded as categories?
Exercise 1.4.iv. In the notation of Example 1.4.7, prove that distinct parallel morphisms
f, g : c ⇒ d define distinct natural transformationsf∗ , g∗ : C(−, c) ⇒ C(−, d)andf ∗ , g∗ : C(d, −) ⇒ C(c, −)by post- and pre-composition.
Exercise 1.4.v. Recall the construction of the comma category for any pair of functors
F : D → C and G : E → C described in Exercise 1.3.vi. From this data, construct a
canonical natural transformation α : F dom ⇒ G cod between the functors that form theboundary of the square
cod

F ↓G

D

α

⇒

dom

F

/E
G


/C

Exercise 1.4.vi. Given a pair of functors F : A × B × Bop → D and G : A × C × Cop → D,a family of morphisms
αa,b,c : F(a, b, b) → G(a, c, c)in D defines the components of an extranatural transformation α : F ⇒ G if for any
f : a → a0 , g : b → b0 , and h : c → c0 the following diagrams commute in D:
F(a, b, b)
F( f,1b ,1b )

αa,b,c/ G(a, c, c) F(a, b, b0 ) F(1a ,1b ,g)/ F(a, b, b) F(a, b, b)
G( f,1c ,1c )F(1a ,g,1b0 )αa,b,c

αa,b,cF(a0 , b, b) αa0 ,b,c / G(a0 , c, c) F(a, b0 , b0 ) αa,b0 ,c / G(a, c, c) G(a, c, c)

αa,b,c0/ G(a, c0 , c0 )G(1a ,1c0 ,h)/ G(a, c0 , c)

G(1a ,h,1c )The left-hand square asserts that the components α−,b,c : F(−, b, b) ⇒ G(−, c, c) define anatural transformation in a for each b ∈ B and c ∈ C. The remaining squares assert thatthe components αa,−,c : F(a, −, −) ⇒ G(a, c, c) and αa,b,− : F(a, b, b) ⇒ G(a, −, −) define
transformations that are respectively extranatural in b and in c. Explain why the functors F
and G must have a common target category for this definition to make sense.

1.5. EQUIVALENCE OF CATEGORIES

29

1.5. Equivalence of categories. . . la mathématique est l’art de donner le mêmenom à des choses différentes.
. . . mathematics is the art of giving the same
name to different things.
Henri Poincaré, “L’avenir des mathématiques”[Poi08]There is an analogy between the notion of a natural transformation between a parallel
pair of functors and the notion of a homotopy between a parallel pair of continuous functions
with one important difference: natural transformations are not generally invertible.36 As in
Example 1.1.4(iv), let 1 denote the discrete category with a single object and let 2 denotethe category with two objects 0, 1 ∈ 2 and a single non-identity arrow 0 → 1. There are
two evident functors i0 , i1 : 1 ⇒ 2 whose subscripts designate the objects in their image.
Lemma 1.5.1. Fixing a parallel pair of functors F, G : C ⇒ D, natural transformations
α : F ⇒ G correspond bĳectively to functors H : C × 2 → D such that H restricts along i0
and i1 to the functors F and G, i.e., so thatC

(1.5.2)i0/ C×2 o

F

"  |
D

H

i1

C

G

commutes.
Here i0 denotes the functor defined on objects by c 7→ (c, 0); it may be regarded as the
product of the identity functor on C with the functor i0 .
Proof. Exercise 1.5.i.

For instance, if C = 2, each functor F, G : 2 ⇒ D picks out an arrow of D, which we
also denote by F and G. The directed graph underlying the category 2 × 2 is/••
(1.5.3)•/  •

together with four identity endoarrows not depicted here; the diagonal serves as the common
composite of the edges of the square. The functor H necessarily maps the top and bottom
arrows of (1.5.3) to F and G, respectively. The vertical arrows define the components
α0 and α1 of the natural transformation, and the diagonal arrow witnesses that the square
formed by these four morphisms in D commutes.
If, in (1.5.2), the category 2 were replaced by the category I with two objects and asingle arrow in each hom-set, necessarily an isomorphism, then “homotopies” with thisinterval correspond bĳectively to natural isomorphisms. The category 2 defines the walking
arrow or free arrow, while I defines the walking isomorphism or free isomorphism, ina sense that is explained in Examples 2.1.5(x) and (xi).
36A natural transformation is invertible if and only if each of its constituent arrows is an isomorphism, in whichcase the pointwise inverses assemble into a natural transformation by Exercise 1.4.i.

30

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONSNatural isomorphisms are used to define the notion of equivalence of categories.
An equivalence of categories is precisely a “homotopy equivalence” where the notion ofhomotopy is defined using the category I.
Definition 1.5.4. An equivalence of categories consists of functors F : C  D : G
together with natural isomorphisms η : 1C  GF,  : FG  1D .37 Categories C and D areequivalent, written C ' D, if there exists an equivalence between them.
Unsurprisingly:
Lemma 1.5.5. The notion of equivalence of categories defines an equivalence relation. In
particular, if C ' D and D ' E, then C ' E.
Proof. Exercise 1.5.vi.

Example 1.5.6. The functors (−)+ : Set∂ → Set∗ and U : Set∗ → Set∂ introduced in§1.3 define an equivalence of categories between the category of pointed sets and thecategory of sets and partial functions. The composite U(−)+ is the identity on Set∂ , so
one of the required natural isomorphisms is the identity. There is a natural isomorphismη : 1Set∗  (U−)+ whose components
η(X,x) : (X, x) → (X\{x} ∪ {X\{x}}, X\{x})are defined to be the based functions that act as the identity on X\{x}.
Consider the categories Matk and Vectfd
k of k-matrices and finite-dimensional nonzero k-vector spaces together with an intermediate category Vectbasis
whose objects arekfinite-dimensional vector spaces with chosen basis and whose morphisms are arbitrary (notnecessarily basis-preserving) linear maps. These categories are related by the displayed
sequence of functors:
Matk ok(−)
H/ Vectbasis okU/ Vectfd

Ck(−)
Here U : Vectbasis→ Vectfd: Matk → Vectbasiskk is the forgetful functor. The functor kknsends n to the vector space k , equipped with the standard basis. An m×n-matrix, interpretedwith respect to the standard bases on kn and km , defines a linear map kn → km and thisassignment is functorial. The functor H carries a vector space to its dimension and a linear
map φ : V → W to the matrix expressing the action of φ on the chosen basis of V using thechosen basis of W. The functor C is defined by choosing a basis for each vector space.
Our aim is to show that these functors display equivalences of categories

Matk ' Vectbasis' Vectfdkk .

The composite equivalence Matk ' Vectfd
k expresses an equivalence between concrete andabstract presentations of linear algebra. A direct proof of these equivalences, by defining
suitable natural isomorphisms, is not difficult, but we prefer to give an indirect proof via
a useful general theorem characterizing those functors forming part of an equivalence of
categories. Its statement requires a few definitions:Definition 1.5.7. A functor F : C → D is
• full if for each x, y ∈ C, the map C(x, y) → D(F x, Fy) is surjective;• faithful if for each x, y ∈ C, the map C(x, y) → D(F x, Fy) is injective;
37The notion of equivalence of categories was introduced by Grothendieck in the form of what we would now
call an adjoint equivalence; this definition appears in Proposition 4.4.5. This explains the directions we haveadopted for the natural isomorphisms η and , which are otherwise immaterial (see Exercise 1.4.i).

1.5. EQUIVALENCE OF CATEGORIES

31• and essentially surjective on objects if for every object d ∈ D there is some c ∈ Csuch that d is isomorphic to Fc.
Remark 1.5.8. Fullness and faithfulness are local conditions; a global condition, by contrast, applies “everywhere.” A faithful functor need not be injective on morphisms; neither
must a full functor be surjective on morphisms. A faithful functor that is injective on
objects is called an embedding and identifies the domain category as a subcategory of thecodomain; in this case, faithfulness implies that the functor is (globally) injective on arrows.
A full and faithful functor, called fully faithful for short, that is injective-on-objects defines
a full embedding of the domain category into the codomain category. The domain thendefines a full subcategory of the codomain.
Theorem 1.5.9 (characterizing equivalences of categories). A functor defining an equivalence of categories is full, faithful, and essentially surjective on objects. Assuming theaxiom of choice, any functor with these properties defines an equivalence of categories.
The proof of Theorem 1.5.9 makes repeated use of the following elementary lemma.
Lemma 1.5.10. Any morphism f : a → b and fixed isomorphisms a  a0 and b  b0
determine a unique morphism f 0 : a0 → b0 so that any of—or, equivalently, all of—thefollowing four diagrams commute:
ao
f


b





a0

/ b0

a
f0

f


b

/ a0




/ b0



ao
f0

f


bo





a0

b0

a
f0

f


bo





/ a0

b0

f0

0

Proof. The left-hand diagram defines f . The commutativity of the remaining diagrams is left as Exercise 1.5.iii.
Proof38 of Theorem 1.5.9. First suppose that F : C → D, G : D → C, η : 1C  GF,
and  : FG  1D define an equivalence of categories. For any d ∈ D, the component of thenatural isomorphism d : FGd  d demonstrates that F is essentially surjective. Consider
a parallel pair f, g : c ⇒ c0 in C. If F f = Fg, then both f and g define an arrow c → c0
making the diagram
ηc
/ GFc
c


GF f =GFg

f or g


c0


ηc0


/ GFc0that expresses the naturality of η commute. Lemma 1.5.10 implies that there is a uniquearrow c → c0 with this property, whence f = g. Thus, F is faithful, and by symmetry, so
is G. Given k : Fc → Fc0 , by Lemma 1.5.10, Gk and the isomorphisms ηc and ηc0 define aunique h : c → c0 for which both Gk and GFh make the diagramcηc

Gk or GFh

h


c0

/ GFc


ηc0


/ GFc038The reader is strongly encouraged to stop reading here and attempt to prove this result on their own. Indeed,in the first iteration of the course that produced these lecture notes, this proof was left to the homework, withouteven the time-saving suggestion of Lemma 1.5.10.

32

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

commute. By Lemma 1.5.10 again, GFh = Gk, whence Fh = k by faithfulness of G. Thus,
F is full, faithful, and essentially surjective.
For the converse, suppose now that F : C → D is full, faithful, and essentially surjective
on objects. Using essential surjectivity and the axiom of choice, choose, for each d ∈ D,
an object Gd ∈ C and an isomorphism d : FGd  d. For each ` : d → d0 , Lemma 1.5.10
defines a unique morphism making the square
d

FGd



/d
`


FGd0



d 0


/ d0

commute. Since F is fully faithful, there is a unique morphism Gd → Gd0 with thisimage under F, which we define to be G`. This definition is arranged so that the chosen
isomorphisms assemble into the components of a natural transformation  : FG ⇒ 1D . It
remains to prove that the assignment of arrows ` 7→ G` is functorial and to define thenatural isomorphism η : 1C ⇒ GF.
Functoriality of G is another consequence of Lemma 1.5.10 and faithfulness of F. The
morphisms FG1d and F1Gd both make
FGd

d

/d



FG1d or F1Gd


FGd


/d


d

1d

commute, whence G1d = 1Gd . Similarly, given `0 : d0 → d00 , both F(G`0 · G`) and FG(`0 `)
make
d
/d
FGd


F(G` ·G`) or FG(` `)
0

0


FGd00


d00



`0 `

/ d00

commute, whence G`0 · G` = G(`0 `).
Finally, by full and faithfulness of F, we may define the isomorphisms ηc : c → GFc−1
by specifying isomorphisms Fηc : Fc → FGFc; see Exercise 1.5.iv. Define Fηc to be Fc.
0
For any f : c → c , the outer rectangle
Fc

Fηc

Fc

FGF f

Ff


Fc0

/ FGFcFηc/ FGFc0
0

/ Fc
Ff

Fc0


/ Fc0

commutes, both composites being F f . The right-hand square commutes by naturality of
. Because Fc0 is an isomorphism, this implies that the left-hand square commutes; see
Lemma 1.6.21. Faithfulness of F tells us that ηc0 · f = GF f · ηc , i.e., that η is a naturaltransformation.

The proof of Theorem 1.5.9 is an example of a proof by “diagram chasing,” a techniquethat is introduced more formally in §1.6. If some of the steps were hard to follow, we suggest
having a second look after that section is absorbed.

1.5. EQUIVALENCE OF CATEGORIES

33

Corollary 1.5.11 (an equivalence between abstract and concrete linear algebra). For any
field k, the categories Matk and Vectfd
k are equivalent.
Proof. Applying Theorem 1.5.9, it is easy to see that each of the functors
Matk  Vectbasis
 Vectfdkk

defines an equivalence of categories. For instance, the morphisms in the category Vectbasisk
are defined so that U : Vectbasis→ Vectfdkk is fully faithful.
A category is connected if any pair of objects can be connected by a finite zig-zag ofmorphisms.
Proposition 1.5.12. Any connected groupoid is equivalent, as a category, to the automorphism group of any of its objects.
Proof. Choose any object g of a connected groupoid G and let G = G(g, g) denote its
automorphism group. The inclusion BG ,→ G mapping the unique object of BG to g ∈ G
is full and faithful, by definition, and essentially surjective, since G was assumed to beconnected. Apply Theorem 1.5.9.

As a special case, we obtain the following result:Corollary 1.5.13. In a path-connected space X, any choice of basepoint x ∈ X yields an
isomorphic fundamental group π1 (X, x).
Proof. Recall from Example 1.1.12(ii) that any space X has a fundamental groupoid
Π1 (X) whose objects are points in X and whose morphisms are endpoint-preserving homotopy classes of paths in X. Picking any point x, the group of automorphisms of the objectx ∈ Π1 (X) is exactly the fundamental group π1 (X, x). Proposition 1.5.12 implies that any
pair of automorphism groups are equivalent, as categories, to the fundamental groupoid∼ ?_ ∼ /
π1 (X, x)
Π1 (X) oπ1 (X, x0 )and thus to each other. An equivalence between 1-object categories is an isomorphism.
Exercise 1.3.i reveals that an isomorphism of groups, regarded as 1-object categories, isexactly an isomorphism of groups in the usual sense (a bĳective homomorphism). Thus,
all of the fundamental groups defined by choosing a basepoint in a path-connected space
are isomorphic.
Remark 1.5.14. Frequently, one functor of an equivalence of categories can be defined
canonically, while the inverse equivalence requires the axiom of choice. In the case of theequivalence between the fundamental group and fundamental groupoid of a path-connected
space, a more precise statement is that one comparison equivalence is natural while thepcother, making use of the axiom of choice, is not. Write Top∗ for the category of pathconnected based topological spaces. We regard the fundamental group π1 and fundamentalgroupoid Π1 as a parallel pair of functors:pc

π1

π1 : Top∗ −→ Group ,→ Cat

and

pc

U

Π1

Π1 : Top∗ −→ Top −→ Groupoid ,→ Cat .

The inclusion of the fundamental group into the fundamental groupoid defines a naturaltransformation π1 ⇒ Π1 such that each component π1 (X, x) → Π1 (X), itself a functor,
is furthermore an equivalence of categories. The definition of the inverse equivalenceΠ1 (X) → π1 (X, x) requires the choice, for each point p ∈ X, of a path-connecting p to thebasepoint x. These (path homotopy classes of) chosen paths need not be preserved by maps

34

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS
pc

in Top∗ . Thus, the inverse equivalences Π1 (X) → π1 (X, x) do not assemble into a naturaltransformation.
The group of automorphisms of any object in a connected groupoid, considered in
Proposition 1.5.12, is one example of a skeleton of a category.
Definition 1.5.15. A category C is skeletal if it contains just one object in each isomorphism class. The skeleton skC of a category C is the unique (up to isomorphism) skeletal
category that is equivalent to C.
Remark 1.5.16. The category skC may be constructed by choosing one object in eachisomorphism class in C and defining skC to be the full subcategory on this collection of
objects. By Theorem 1.5.9, the inclusion skC ,→ C defines an equivalence of categories.
This construction, however, fails to define a functor sk(−) : CAT → CAT because there is no
reason that a functor F : C → D would necessarily restrict to define a functor between thechosen skeletal subcategories. It is possible to choose a functor skF : skC → skD whose
inclusion into D is naturally isomorphic to the restriction of F to skC, but these choices
will not be strictly functorial.39
Note than an equivalence between skeletal categories is necessarily an isomorphism of
categories. Thus, two categories are equivalent if and only if their skeletons are isomorphic.
For this reason, we feel free to speak of the skeleton of a category, even though its
construction is not canonical.
Example 1.5.17.
(i) The skeleton of a connected groupoid is the group of automorphisms of any of its
objects (see Proposition 1.5.12).
(ii) The skeleton of the category defined by a preorder, as described in Example 1.1.4(iii),
is a poset.
(iii) The skeleton of the category Vectfd
k is the category Matk .
(iv) The skeleton of the category Finiso is the category whose objects are positive integers
and with Hom(n, n) = Σn , the group of permutations of n elements. The hom-sets
between distinct natural numbers are all empty.
Example 1.5.18 (a categorification of the orbit-stabilizer theorem). Let X : BG → Set bea left G-set. Its translation groupoid TG X has elements of X as objects. A morphismg : x → y is an element g ∈ G so that g · x = y. The objects in the skeleton skTG X are theconnected components in the translation groupoid. These are precisely the orbits of thegroup action, which partition X in precisely this manner.
Consider x ∈ X as a representative of its orbit O x . Because the translation groupoid isequivalent to its skeleton, we must have
HomskTG X (O x , O x )  HomTG X (x, x) =: G x ,the set of automorphisms of x. This group consists of precisely those g ∈ G so thatg · x = x. In other words, the group HomTG X (x, x) is the stabilizer G x of x with respect to
the G-action. Note that this argument implies that any pair of elements in the same orbit
must have isomorphic stabilizers. As is always the case for a skeletal groupoid, there are no
morphisms between distinct objects. In summary, the skeleton of the translation groupoid,
as a category, is the disjoint union of the stabilizer groups, indexed by the orbits of theaction of G on X.
39There is, however, a pseudofunctor sk(−) : CAT → CAT, but a precise definition of this concept is beyond thescope of this text.

1.5. EQUIVALENCE OF CATEGORIES35

The set of morphisms in the translation groupoid with domain x is isomorphic to G.
This set may be expressed as a disjoint union of hom-sets HomTG X (x, y), where y ranges over
the orbit O x . Each of these hom-sets is isomorphic to HomTG X (x, x) = G x . In particular,|G| = |O x | · |G x |, proving the orbit-stabilizer theorem.
A guiding principle in category theory is that categorically-defined concepts shouldbe equivalence invariant. Some category theorists go so far as to call a definition “evil”
if it is not invariant under equivalence of categories. The only evil definitions that havebeen introduced thus far are smallness and discreteness. A category is essentially small if
it is equivalent to a small category or, equivalently, if its skeleton is a small category. A
category is essentially discrete if it is equivalent to a discrete category.
The following constructions and definitions are equivalence invariant:• If a category is locally small, any category equivalent to it is again locally small.
• If a category is a groupoid, any category equivalent to it is again a groupoid.
• If C ' D, then Cop ' Dop .
• The product of a pair of categories is equivalent to the product of any pair of equivalent
categories.
'• An arrow in C is an isomorphism if and only if its image under an equivalence C −→D
is an isomorphism.
The last of these properties can be generalized. By Theorem 1.5.9, a full and faithful
functor F : C → D defines an equivalence onto its essential image, the full subcategory ofobjects isomorphic to Fc for some c ∈ C. Fully faithful functors have a useful property
stated as Exercise 1.5.iv: if F is full and faithful and Fc and Fc0 are isomorphic in D, thenc and c0 are isomorphic in C. We will introduce what are easily the most important fully
faithful functors in category theory in Chapter 2: the covariant and contravariant Yoneda
embeddings.
Exercises.
Exercise 1.5.i. Prove Lemma 1.5.1.
Exercise 1.5.ii. Segal defined a category Γ in [Seg74] as follows:Γ is the category whose objects are all finite sets, and whose morphisms from S to T are the maps θ : S → P(T ) such that θ(α) andθ(β) are disjoint when α , β. The composite of θ : S → P(T ) andS
φ : T → P(U) is ψ : S → P(U), where ψ(α) =φ(β).
β∈θ(α)

Prove that Γ is equivalent to the opposite of the category Fin∗ of finite pointed sets. In
particular, the functors introduced in Example 1.3.2(xi) define presheaves on Γ.
Exercise 1.5.iii. Prove Lemma 1.5.10.
Exercise 1.5.iv. Show that a full and faithful functor F : C → D both reflects and createsisomorphisms. That is, show:(i) If f is a morphism in C so that F f is an isomorphism in D, then f is an isomorphism.
(ii) If x and y are objects in C so that F x and Fy are isomorphic in D, then x and y are
isomorphic in C.
By Lemma 1.3.8, the converses of these statements hold for any functor.
Exercise 1.5.v. Find an example to show that a faithful functor need not reflect isomorphisms.
Exercise 1.5.vi.

36

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS(i) Prove that the composite of a pair of full, faithful, or essentially surjective functors
again has the same properties.
(ii) Prove that if C ' D and D ' E, then C ' E. Conclude that equivalence of categories
is an equivalence relation.40
Exercise 1.5.vii. Let G be a connected groupoid and let G be the group of automorphisms
at any of its objects. The inclusion BG ,→ G defines an equivalence of categories. Construct
an inverse equivalence G → BG.
Exercise 1.5.viii. Klein’s Erlangen program studies groupoids of geometric spaces of various kinds. Prove that the groupoid Affine of affine planes is equivalent to the groupoid Proj|of projective planes with a distinguished line, called the “line at infinity.” The morphisms
in each groupoid are bĳections on both points and lines (preserving the distinguished linein the case of projective planes) that preserve and reflect the incidence relation. The functor
Proj| → Affine removes the line at infinity and the points it contains. Explicitly describe aninverse equivalence.
Exercise 1.5.ix. Show that any category that is equivalent to a locally small category is
locally small.
Exercise 1.5.x. Characterize the categories that are equivalent to discrete categories. A
category that is connected and essentially discrete is called chaotic.
Exercise 1.5.xi. Consider the functors Ab → Group (inclusion), Ring → Ab (forgetting
the multiplication), (−)× : Ring → Group (taking the group of units), Ring → Rng (inclusion), Field → Ring (inclusion), and ModR → Ab (forgetful). Determine which functors
are full, which are faithful, and which are essentially surjective. Do any define an equivalence of categories? (Warning: A few of these questions conceal research-level problems,
but they can be fun to think about even if full solutions are hard to come by.)

1.6. The art of the diagram chase
The diagrams incorporate a large amount of
information. Their use provides extensive savingsin space and in mental effort. In the case of many
theorems, the setting up of the correct diagram is
the major part of the proof. We therefore urge thatthe reader stop at the end of each theorem andattempt to construct for himself the relevant
diagram before examining the one which is givenin the text. Once this is done, the subsequent
demonstration can be followed more readily; infact, the reader can usually supply it himself.
Samuel Eilenberg and Norman Steenrod,
Foundations of Algebraic Topology [ES52]

The proof of Theorem 1.5.9 used a technique called “diagram chasing,” also called
abstract nonsense, that we now consider more formally. A diagram is typically presented
informally as a directed graph of morphisms in a category. In this informal presentation,
the diagram commutes if any two paths of composable arrows in the directed graph with

40A second, more direct proof of this result appears as Exercise 1.7.vi.

1.6. THE ART OF THE DIAGRAM CHASE

37

common source and target have the same composite. For example, a commutative triangleh•(1.6.1)

f



•

/
?•
g

asserts that the hypotenuse h equals the composite g f of the two legs. Commutative
diagrams in a category can be used to define more complicated mathematical objects. For
example:Definition 1.6.2. A monoid is an object M ∈ Set together with a pair of morphismsµ : M × M → M and η : 1 → M so that the following diagrams commute:
M×M×M

1 M ×µ

/ M×M

η×1 M

µ

µ×1 M


M×M

M

µ


/M

µ

/ M × M o 1M ×η M

1M

#  {
M

1M

The morphism µ : M × M → M defines a binary “multiplication” operation on M. The
morphism η : 1 → M, whose domain is a singleton set, identifies an element η ∈ M. The
three axioms demand that multiplication is associative and that multiplication on the leftor right by the element η acts as the identity. The advantage of the commutative diagrams
approach to this definition is that it readily generalizes to other categories. For example:
Definition 1.6.3. A topological monoid is an object M ∈ Top together with morphismsµ : M × M → M and η : 1 → M so that the following diagrams commute:
M×M×M

1 M ×µ

/ M×M

η×1 M

µ

µ×1 M


M×M

M

µ


/M

µ

/ M × M o 1M ×η M

1M

#  {
M

1M

A unital ring41 is an object R ∈ Ab together with morphisms µ : R ⊗Z R → R andη : Z → R so that the following diagrams commute:
R ⊗Z R ⊗Z R

1R ⊗Z µR

η⊗Z 1Rµµ⊗Z 1R
R ⊗Z R/ R ⊗Z Rµ/ R ⊗Z R o 1R ⊗Z η R
µ


/R

1R

"  |
R

1R

A k-algebra is an object R ∈ Vectk together with morphisms µ : R ⊗k R → R andη : k → R so that the following diagrams commute:R ⊗k R ⊗k R

1R ⊗ k µRη⊗k 1Rµµ⊗k 1R
R ⊗k R/ R ⊗k Rµ/R/ R ⊗k R o 1R ⊗k η R
µ

1R

"  |
R

1R

There are evident formal similarities in each of these four definitions; they are all
special cases of a general notion of a monoid in a monoidal category, as defined in §E.2.
41A not-necessarily unital ring may be defined by ignoring the morphism η and the pair of commutative
triangles.

38

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

The morphisms η : 1 → M in the case of topological monoids, η : Z → R in the case ofunital rings, and η : k → R in the case of k-algebras do no more and no less than specify
an element of M or R to serve as the multiplicative unit. We will introduce language todescribe the role played in each case by the topological space 1, the abelian group Z, andthe vector space k in Example 2.1.5(iii).
In the case of a topological monoid, the condition that µ : M × M → M is a morphism inTop demands that the multiplication function is continuous; for instance, the circle S 1 ⊂ C
defines a topological monoid with addition of angles. For unital rings, the morphismµ : R ⊗Z R → R represents a bilinear homomorphism of abelian groups from R × R to R; in
particular, multiplication distributes over addition in R. The role of the tensor product inthe definition of a k-algebra is similar.
Let us now give precise meaning to the term “diagram.”Definition 1.6.4. A diagram in a category C is a functor F : J → C whose domain, theindexing category, is a small category.
A diagram is typically depicted by drawing the objects and morphisms in its image,with the domain category left implicit, particularly in the case where the indexing category
J is a preorder, so that any two paths of composable arrows have a common composite.
Nonetheless, the indexing category J plays an important role. Functoriality requires thatany composition relations that hold in J must hold in the image of the diagram, which iswhat it means to say that the directed graph defined by the image of the diagram in C iscommutative. An immediate consequence of the form of our definition of a commutative
diagram is the following result.
Lemma 1.6.5. Functors preserve commutative diagrams.
Proof. A diagram in C is given by a functor F : J → C, whose domain is a small
category. Given any functor G : C → D, the composite GF : J → D defines the image ofthe diagram in D.
A few examples will help to illustrate the connection between commutative diagrams,
as formalized in Definition 1.6.4, and their informal directed graph presentations.
Example 1.6.6. Consider 2 × 2, the category with four objects and the displayed nonidentity morphisms/••/  •
•In 2×2, the diagonal morphism is the composite of both the top and right morphisms and theleft and bottom morphisms; in particular, these composites are equal. A diagram indexedby 2 × 2, typically drawn without the diagonal composite, is a commutative square.
Remark 1.6.7. In practice, one thinks of the indexing category as a directed graph, defining
the shape of the diagram, together with specified commutativity relations. For example,
to define a functor with domain 2 × 2, it suffices to specify the images of the four objects
together with four morphisms
a

f

g


c

k

/b

/d

h

1.6. THE ART OF THE DIAGRAM CHASE

39

subject to the relation that h f = kg. When indexing categories are represented in this way,
the commutativity relations become an essential part of the data. They distinguish between
the category 2 × 2 that indexes a commutative square and the category
•

/•


•

/  
•that indexes a not-necessarily commutative square; here the two diagonals represent distinct
composites of the two paths along the edges of the square.
Example 1.6.8. The category 4 has four objects and the six displayed non-identity morphisms
?•
/ •

•
?
 
•

This category is a preorder; each of the four displayed triangular “faces” commutes. These
commutativity relations in 4 imply that a diagram of shape 4 in a category C is given by asequence of three composable morphisms together with their composites

f

?•

hg

/ •

hg f

•

?

g
gf

 
•

h

Associativity of composition implies that the bottom and back faces commute in C. Example 4.1.13 introduces terminology to explain why a diagram of shape 4 is determined bya sequence of three composable morphisms: the category 4 is free on the directed graph• → • → • → •.
Example 1.6.9. Consider 2×3, the category with six objects and the displayed non-identity
morphisms
•

/•

/* •


•

/  •

'/4  •The long diagonal asserts that the outer triples of composable morphisms have a common
composite, i.e., that the outer rectangle commutes. The short diagonals assert, respectively,that the left-hand and right-hand squares commute. The inner parallelogram also commutes.
A diagram indexed by 2×3, typically drawn without any of the diagonals, is a commutative
rectangle.

40

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

Two commutative squares define a commutative rectangle: a collection of morphisms
with the indicated sources and targets
/b

f

a
g

(1.6.10)

/c

j

`

h


a0


/ b0

k


/ c0

m

define a 2 × 3-shaped diagram provided that h f = kg and ` j = mh. This is a special caseof the following more general result, which describes the induced relations in the “algebraof composition” encoded by the arrows in a category.
Lemma 1.6.11. Suppose f1 , . . . , fn is a composable sequence—a “path”—of morphisms
in a category. If the composite fk fk−1 · · · fi+1 fi equals gm · · · g1 , for another composable
sequence of morphisms g1 , . . . , gm , then fn · · · f1 = fn · · · fk+1 gm · · · g1 fi−1 · · · f1 .
Proof. Composition is well-defined: if the composites gm · · · g1 and fk fk−1 · · · fi+1 fi
define the same arrow, then the results of pre- or post-composing with other sequences ofarrows must also be the same.
This very simple result underlies most proofs by “diagram chasing.” When a diagram
is depicted by a simple (meaning there is at most one edge between any two vertices)
acyclic directed graph, the most common convention is to include commutativity relations
that assert that any two paths in the diagram with a common source and target commute,i.e., that the directed graph represents a poset category. For example, the category 2 × 2 × 2
indexes the commutative cube, which is typically depicted as follows:
/•

•


/ •

•


•

/ •
/  • •

In such cases, Lemma 1.6.11 and transitivity of equality implies that commutativity ofthe entire diagram may be checked by establishing commutativity of each minimal subdiagram in the directed graph. Here, a minimal subdiagram corresponds to a composition
relation hn · · · h1 = km · · · k1 that cannot be factored into a relation between shorter pathsof composable morphisms. The graph corresponding to a minimal relation is a “directed
polygon”
h1

?•

h2

/•

•

/•

hn−1


?•

•
k1



•

hn

k2

/•

•

/•

km

km−1

with a commutative triangle, as in (1.6.1), being the simplest case. This sort of argument is called “equational reasoning” in [Sim11a, 2.1], which provides an excellent short
introduction to diagram chasing.
The following results have simple proofs by diagram chasing.

1.6. THE ART OF THE DIAGRAM CHASE

41

Lemma 1.6.12. If the triangle displayed on the left commutes
•

h

•O
&

8•

f


•

f

h

&

8•

−1

g

g

•

and if f is an isomorphism, then the triangle displayed on the right also commutes.
Dually, for any triple of morphisms with domains and codomains as displayed with k
an isomorphism
8•
8 •O
j
j
•

k
`

& 
•

•

k−1
`

&

•

the left-hand triangle commutes if and only if the right-hand triangle commutes.
Proof. Pre-compose the composition relation h = g f with f −1 to yield h f −1 = g. Lemma 1.6.13. For any commutative square βα = δγ in which each of the morphisms is
an isomorphism, then the inverses define a commutative square α−1 β−1 = γ−1 δ−1 .
Proof. Apply Lemma 1.6.12 four times.



In certain special cases, commutativity of diagrams can be automatic. For instance, any
parallel sequences of composable morphisms in a preorder must have a common composite
precisely because any hom-set in a preorder has at most one element. Parallel sequencesof composable morphisms also necessarily have a common composite when the domain orcodomain objects have a certain special property:
Definition 1.6.14. An object i ∈ C is initial if for every c ∈ C there is a unique morphismi → c. Dually, an object t ∈ C is terminal if for every c ∈ C there is a unique morphismc → t.
Example 1.6.15. Many of the categories of our acquaintance have initial and terminal
objects.
(i) The empty set is an initial object in Set and any singleton set is terminal.
(ii) In Top, the empty and singleton spaces are, respectively, initial and terminal.
(iii) In Set∗ , any singleton set is both initial and terminal.
(iv) In ModR , the zero module is both initial and terminal. Similarly, the trivial group isboth initial and terminal in Group.
(v) The zero ring is the terminal ring. To identify an initial object, we must clarify whatsort of rings and ring homomorphisms are being considered. Recall that Ring denotes
the category of unital rings and ring homomorphisms that preserve the multiplicative
identity. This is a non-full subcategory of the larger category Rng of rings that do notnecessarily have a multiplicative identity and homomorphisms that need not preserve
one if it happens to exist. The integers define an initial object in Ring but not in Rng,in which the zero ring is initial.
(vi) The category Field of fields has neither initial nor terminal objects. Indeed, there areno homomorphisms between fields of different characteristic.
(vii) The empty category defines an initial object in Cat, and the category 1 is terminal.

42

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS(viii) An initial object in a preorder is a global minimal element, and a terminal object is a
global maximal element. These initial and terminal objects may or may not exist ineach particular instance.
Lemma 1.6.16. Let f1 , . . . , fn and g1 , . . . , gm be composable sequences of morphisms sothat the domain of f1 equals the domain of g1 and the codomain of fn equals the codomainof gm . If this common codomain is a terminal object, or if this common domain is an initial
object, then fn · · · f1 = gm · · · g1 .
Proof. The two dual statements are immediate consequences of the uniqueness partof Definition 1.6.14.

In certain cases, one can prove that a diagram commutes by appealing to “elements”
of the objects. For instance, this is possible in any concrete category.
Definition 1.6.17. A concrete category is a category C equipped with a faithful functor
U : C → Set.
The functor U typically carries an object of C to its “underlying set.” The faithfulness
condition asserts that any parallel pair of morphisms f, g : c ⇒ c0 that induce the samefunction U f = Ug between the underlying sets must be equal in C. The idea is that thequestion of whether a map between the underlying sets of objects in a concrete category is a
map in the category is a condition (e.g., continuity). By contrast, the functor U : C → Set
is not faithful if the maps in C have extra structure that is not visible at the level of theunderlying sets (e.g., homotopy classes of maps [Fre04]).
Example 1.6.18. Each category listed in Example 1.1.3 is concrete, although care must
be taken in the cases of Graph and ChR , whose objects are “multi-sorted” sets. The mostobvious forgetful functors, which send a graph to its set of vertices or its set of edges, are
not faithful. However, the functor V t E : Graph → Set considered in Example 1.3.2(ii)that sends a graph to the union of its set of vertices and edges is faithful.
Because faithful functors reflect identifications between parallel morphisms:
Lemma 1.6.19. If U : C → D is faithful, then any diagram in C whose image commutes inD also commutes in C.
Proof. Here a “diagram” in C is a directed graph of morphisms together with acollection of desired composition relations between composable paths of morphisms. If
f1 , . . . , fn and g1 , . . . , gm are parallel sequences of composable morphisms in C so thatU fn · · · U f1 = Ugm · · · Ug1
in D, then by faithfulness (and functoriality) of U, fn · · · f1 = gm · · · g1 in C.

In particular, to prove that a diagram in a concrete category commutes, it suffices toprove commutativity of the induced diagram of underlying sets. This amounts to showingthat certain composite functions between underlying sets are the same, and this can bechecked by considering the actions of these functions on the elements of their domain.
We close with a word of warning.
Remark 1.6.20 (on cancellability). We have seen that commutativity of a pair of adjacentsquares as in (1.6.10) implies commutativity of the exterior rectangle, but the converse
need not hold, as illustrated by the following diagram in Ab, in which the outer rectangle

1.6. THE ART OF THE DIAGRAM CHASE

43

commutes but neither square does:
Z

1Z

/Z

/0

1Z


/Z


0

1Z


/Z

Neither does commutativity of one of the two squares, plus the outer rectangle, imply
commutativity of the other in general. The issue is that a composition relation of the form
ghn · · · h1 f = gkm · · · k1 f
need not imply that hn · · · h1 = km · · · k1 unless f is “right cancellable” and g is “leftcancellable,” i.e., unless f is an epimorphism and g is a monomorphism; see Definition
1.2.7.
Lemma 1.6.21. Consider morphisms with the indicated sources and targets
a

f

g


a0

/b

j

`

h

k


/ b0

/c

m


/ c0

and suppose that the outer rectangle commutes. This data defines a commutative rectangle
if either:(i) the right-hand square commutes and m is a monomorphism; or(ii) the left-hand square commutes and f is an epimorphism.
Proof. The statements are dual. Assuming (i), mkg = ` j f = mh f by commutativityof the outer rectangle and right-hand square. Since m is a monomorphism, it follows thatkg = h f and thus that the diagram commutes.

Exercises.
Exercise 1.6.i. Show that any map from a terminal object in a category to an initial one is
an isomorphism. An object that is both initial and terminal is called a zero object.
Exercise 1.6.ii. Show that any two terminal objects in a category are connected by a unique
isomorphism.
Exercise 1.6.iii. Show that any faithful functor reflects monomorphisms. That is, if
F : C → D is faithful, prove that if F f is a monomorphism in D, then f is a monomorphism
in C. Argue by duality that faithful functors also reflect epimorphisms. Conclude thatin any concrete category, any morphism that defines an injection of underlying sets is
a monomorphism and any morphism that defines a surjection of underlying sets is an
epimorphism.
Exercise 1.6.iv. Find an example to show that a faithful functor need not preserve epimorphisms. Argue by duality, or by another counterexample, that a faithful functor need notpreserve monomorphisms.
Exercise 1.6.v. More specifically, find a concrete category that contains a monomorphism
whose underlying function is not injective. Find a concrete category that contains an
epimorphism whose underlying function is not surjective. Exercise 4.5.v explains why thelatter examples may seem less familiar than the former.

44

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

Exercise 1.6.vi. A coalgebra for an endofunctor T : C → C is an object C ∈ C equippedwith a map γ : C → TC. A morphism f : (C, γ) → (C 0 , γ0 ) of coalgebras is a map
f : C → C 0 so that the square
f

C
γ


TC

/ C0
γ0

Tf


/ TC 0commutes. Prove that if (C, γ) is a terminal coalgebra, that is a terminal object in the
category of coalgebras, then the map γ : C → TC is an isomorphism.
1.7. The 2-category of categories
A number of important facts about natural transformations are proven by diagramchasing. In this section, we define “vertical” and “horizontal” composition operations fornatural transformations. The upshot is that categories, functors, and natural transformations
assemble into a 2-dimensional categorical structure called a 2-category, a definition that isstated at the conclusion.
In French, a natural transformation is called a morphisme de foncteurs. Indeed, for any
fixed pair of categories C and D, there is a functor category DC whose objects are functors
C → D and whose morphisms are natural transformations. Given a functor F : C → D, its
identity natural transformation 1F : F ⇒ F is the natural transformation whose components(1F )c B 1Fc are identities. The following lemma defines the composition of morphisms in
DC .
Lemma 1.7.1 (vertical composition). Suppose α : F ⇒ G and β : G ⇒ H are naturaltransformations between parallel functors F, G, H : C → D. Then there is a naturaltransformation β · α : F ⇒ H whose components(β · α)c B βc · αc
are defined to be the composites of the components of α and β.
Proof. Naturality of α and β implies that for any f : c → c0 in the domain category,each square, and thus also the composite rectangle, commutes:
Fc

αc

Ff


Fc0

/ Gc

βc

Gf
αc0


/ Gc0

/ Hc
Hf

βc0


/ Hc0



Corollary 1.7.2. For any pair of categories C and D, the functors from C to D and naturaltransformations between them define a category DC .
Proof. It remains only to verify that the composition operation defined by Lemma
1.7.1 is associative and unital. It suffices to verify these properties componentwise, andthey follow immediately from the associativity and unitality of composition in D.
Remark 1.7.3 (sizes of functor categories). Care should be taken with size when discussing
functor categories. If C and D are small, then DC is again a small category, but if C and
D are locally small, then DC need not be. This is only guaranteed if D is locally small andC is small; see Exercise 1.7.i. In summary, the formation of functor categories defines a1.7. THE 2-CATEGORY OF CATEGORIES

45

bifunctor Catop × Cat → Cat or Catop × CAT → CAT, but the category of functors between
two non-locally small categories may be even larger than these categories are.
The composition operation defined in Lemma 1.7.1 is called vertical composition.
Drawing the parallel functors horizontally, a composable pair of natural transformations in
the category DC fits into a pasting diagram

C

F
⇓α 
G / DD
⇓β
H

F
=



C ⇓ β · α DD

H

As the terminology suggests, there is also a horizontal composition operation
F
C

⇓α

H


HF


⇓β

DD

G

DE



=

C ⇓ β ∗ α DE

K

KG

defined by the following lemma.
Lemma 1.7.4 (horizontal composition). Given a pair of natural transformations
F
C

H

⇓α



CD

G

⇓β



CE

K

there is a natural transformation β ∗ α : HF ⇒ KG whose component at c ∈ C is definedas the composite of the following commutative squareHFc
Hαc(1.7.5)


HGc

βFc

(β∗α)c
βGc

/ KFc
Kαc

# 
/ KGc

Proof. The square (1.7.5) commutes by naturality of β : H ⇒ K applied to the morphism αc : Fc → Gc in D. To prove that the components (β ∗ α)c : HFc → KGc so-defined
are natural, we must show that KG f · (β ∗ α)c = (β ∗ α)c0 · HF f for any f : c → c0 in C.
This relation holds on account of the commutative rectangle
HFc
HF f


HFc0

Hαc

/ HGc

βGc

HG f


/ HGc0
0

Hαc

/ KGc
KG f

βGc0


/ KGc046

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

The right-hand square commutes by naturality of β. The left-hand square commutes bynaturality of α and Lemma 1.6.5, which states that functors, in this case the functor H,
preserve commutative diagrams.42
Remark 1.7.6. The natural transformations
Hα : HF ⇒ HG,

Kα : KF ⇒ KG,

βF : HF ⇒ KF,

and

βG : HG ⇒ KG

appearing in Lemma 1.7.4 are defined by whiskering the natural transformations α andβ with the functors H and K or F and G, respectively. A precise definition of thisconstruction is given in Exercise 1.7.ii. The terminology is on account of the following
graphical depiction of the whiskered composite
H
LβF : LHF ⇒ LKF

FC/D ⇓β EE

L

/F

Kof the natural transformation β : H ⇒ K with the functors F and L. Exercise 1.7.iii explains
the particular interest in the case where either L or F is an identity.
Importantly, vertical and horizontal composition can be performed in either order,
satisfying the rule of middle four interchange:
Lemma 1.7.7 (middle four interchange). Given functors and natural transformations

C

F
⇓α 
G /D
E
⇓β
H

J
⇓γ 
K / EE⇓δL

the natural transformation JF ⇒ LH defined by first composing vertically and then composing horizontally equals the natural transformation defined by first composing horizontallyand then composing vertically:
J

F





C ⇓β·α DE ⇓ δ · γ EE

H

=

L

Proof. Exercise 1.7.iv.

C

JF
⇓γ∗α
KG
⇓δ∗β
LH

"
/E
<



Lemmas 1.7.1, 1.7.4, and 1.7.7 prove that categories, functors, and natural transformations assemble into a 2-category. Aside from this example, we will not meet any other
2-categories in this text. Nonetheless, the following definition is useful as an axiomatizationof the composition operations for natural transformations that are available. A succinct
introduction to 2-categories and pasting diagrams, which are used to display composite
natural transformations, can be found in [KS74].
Definition 1.7.8. A 2-category is comprised of:
42Naturality of β ∗ α could also be deduced from a second commutative rectangle that defines the component(β ∗ α)c as the top-right composite of (1.7.5). The point is that the squares (1.7.5) and the morphisms obtained byapplying the four functors HF, HG, KF, and KG to a morphism in C define a commutative cube.

1.7. THE 2-CATEGORY OF CATEGORIES

47• objects, for example the categories C,F• 1-morphisms between pairs of objects, for example, the functors C −→ D, and• 2-morphisms between parallel pairs of 1-morphisms, for example, the natural transforF%
mations C ⇓ α 9 D
G
so that:• The objects and 1-morphisms form a category, with identities 1C : C → C.
• For each fixed pair of objects C and D, the 1-morphisms F : C → D and 2-morphisms
between such form a category under an operation called vertical composition, asF%described in Lemma 1.7.1, with identities C ⇓ 1F 9 D .
F• There is also a category whose objects are the objects in which a morphism from C
F%
to D is a 2-cell C ⇓ α 9 D under an operation called horizontal composition, withG
1C%⇓
identities C 11C 9 C . The source and target 1-morphisms of a horizontal composition
1C
must have the form described in Lemma 1.7.4.
• The horizontal composite 1H ∗ 1F of identities for vertical composition must be theidentity 1HF for for the composite 1-morphisms.
• The law of middle four interchange described in Lemma 1.7.7 holds.
The reader who has taken the categorical philosophy to heart might ask: What is amorphism between 2-categories? 2-functors will make an appearance in §4.4.
Exercises.
Exercise 1.7.i. Prove that if C is small and D is locally small, then DC is locally small bydefining a monomorphism from the collection of natural transformations between a fixedpair of functors F, G : C ⇒ D into a set. (Hint: Think about the function that sends anatural transformation to its collection of components.)
Exercise 1.7.ii. Given a natural transformation β : H ⇒ K and functors F and L as
displayed in
H

L /
F /
D ⇓β EF
C
EKdefine a natural transformation LβF : LHF ⇒ LKF by (LβF)c = LβFc . This is thewhiskered composite of β with L and F. Prove that LβF is natural.
Exercise 1.7.iii. Redefine the horizontal composition of natural transformations introduced in Lemma 1.7.4 using vertical composition and whiskering.
Exercise 1.7.iv. Prove Lemma 1.7.7.

48

1. CATEGORIES, FUNCTORS, NATURAL TRANSFORMATIONS

Exercise 1.7.v. Show that for any category C, the collection of natural endomorphisms of
the identity functor 1C defines a commutative monoid, called the center of the category.
The proof of Proposition 1.4.4 demonstrates that the center of Abfg is the multiplicative
monoid (Z, ×, 1).
Exercise 1.7.vi. Suppose the functors and natural isomorphisms
Co
Do

F
G
F0
G0

/D

η : 1C  GF

 : FG  1D/E

η0 : 1D  G0 F 0

 0 : F 0 G 0  1E

define equivalences of categories C ' D and D ' E. Prove (again) that there is a composite
equivalence of categories C ' E by defining composite natural isomorphisms 1C  GG0 F 0 F
and F 0 FGG0  1E .
Exercise 1.7.vii. Prove that a bifunctor F : C × D → E determines and is uniquely determined by:(i) A functor F(c, −) : D → E for each c ∈ C.
(ii) A natural transformation F( f, −) : F(c, −) ⇒ F(c0 , −) for each f : c → c0 in C,
defined functorially in C.
In other words, prove that there is a bĳection between functors C × D → E and functors
C → ED . By symmetry of the product of categories, these classes of functors are also inbĳection with functors D → EC .

CHAPTER 2

Universal Properties, Representability, and the Yoneda
Lemma. . . a mathematical object X is best thought of in
the context of a category surrounding it, and isdetermined by the network of relations it enjoys
with all the objects of that category. Moreover, tounderstand X it might be more germane to deal
directly with the functor representing it.
Barry Mazur, “Thinking about Grothendieck”[Maz16]

The aim in this chapter is to explain what it means to say that the natural numbers is theuniversal discrete dynamical system, that the Sierpinski space is the universal space with
an open subset, or that the complete graph on n-vertices is the universal n-colored graph.
Universal properties expressed in this plain language manner might appear somewhat ad
hoc, but this is a false impression. By the chapter’s end, we will see several equivalent ways
in which the notion of universal property can be made precise. The key input to the theory
developed here is something that is a priori non-obvious: a functor valued in the categoryof sets. The category of sets plays a special role because traditional approaches to thefoundations of mathematics are based on set theory, with a wide variety of mathematicalobjects defined to be sets with additional structure.
To illustrate, consider the set of vertex colorings of a graph subject to the requirementthat adjacent vertices are assigned distinct colors. There is a contravariant functor fromthe category of graphs to the category of sets that takes a graph to the set of n-coloringsof its vertices. To explain the contravariance, note that an n-coloring of a graph G and agraph homomorphism G0 → G induce an n-coloring of G0 that colors each vertex of G0
to match the color of its image: as graph homomorphisms preserve the incidence relation
on vertices, any two adjacent vertices of G0 are assigned distinct colors. This defines theaction of the functor n-Color : Graphop → Set on morphisms.1
The graph with the fewest vertices and fewest edges that can be colored with no lessthan n colors is the complete graph on n vertices Kn . Indeed, the functor n-Color encodes
a universal property of the graph Kn in the sense that the graph Kn represents the functor
n-Color. Specifically, this means that there is a natural bĳection between the set n-Color(G)of n-colorings of a graph G and the set of graph homomorphisms G → Kn . In §2.1, we
introduce the notion of a representable functor along with a plethora of examples, intended
to convey the ubiquity of this abstract notion.
1The four color theorem states that if G is a simple planar graph, then the set 4-Color(G) is non-empty.
Functoriality implies that any graph admitting a graph homomorphism to a planar graph also admits a 4-coloring.
For instance, complete bipartite graphs, which are typically non-planar, will admit homomorphisms of this type.
49

50

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA

By the Yoneda lemma, which we prove in §2.2, this universal property uniquely
characterizes the graph Kn . The Yoneda lemma also establishes a correspondence between
natural endomorphisms of the functor n-Color—“color permutations”—and symmetries of
the graph Kn and proves that there is no uniform way to convert an m-coloring into an ncoloring if m > n, this result deduced from the fact that there are no graph homomorphisms
Km → Kn .
The Yoneda lemma is arguably the most important result in category theory, although
it takes some time to explore the depths of the consequences of this simple statement. In§2.3, we define the notion of universal element that witnesses a universal property of some
object in a locally small category. The universal element witnessing the universal propertyof the complete graph is an n-coloring of Kn , an element of the set n-Color(Kn ). In §2.4,
we use the Yoneda lemma to show that the pair comprised of an object characterized bya universal property and its universal element defines either an initial or a terminal object
in the category of elements of the functor that it represents. This gives precise meaning
to the term universal: it is a synonym for either “initial” or “terminal,” with context
disambiguating between the two cases. For instance, Kn is the terminal n-colored graph:
the terminal object in the category of n-colored graphs and graph homomorphisms that
preserve the coloring of vertices.
2.1. Representable functors
The further you go in mathematics, especiallypure mathematics, the more universal properties
you will meet.
Tom Leinster, Basic Category Theory [Lei14]

The most basic formulation of a universal property is to say that a particular object
defines an initial or terminal object in its ambient category. The problem with this paradigm
is that the most familiar categories—for instance, of sets, spaces, groups, modules, andso on—tend to have uninteresting initial and terminal objects. To express the universal
properties of more complicated objects, one has to cook up a less familiar category. For
example:Example 2.1.1. A set X with an endomorphism f : X → X and a distinguished element x0
is called a discrete dynamical system. This data allows one to consider the discrete-time
evolution of the initial element x0 , a sequence defined by xn+1 B f (xn ). The principle ofmathematical recursion asserts that the natural numbers N, the successor function s : N →N, and the element 0 ∈ N define the universal discrete dynamical system: which is tosay, there is a unique function r : N → X so that r(n) = xn for each n, i.e., so that r(0) = x0
and so that the diagramN(2.1.2)

r


X

s

f

/N

/X

r

commutes.
In this section, we introduce a vehicle for expressing universal properties that is lesscontrived than spontaneously generating an appropriate category in which the universal

2.1. REPRESENTABLE FUNCTORS

51

object is initial or terminal. The notion that we introduce can be understood as a generalization of the universal properties describing initial or terminal objects. In §2.4, we willshow that it is not a true generalization.
To say that an object c ∈ C is initial is to say that for all other objects x, the set C(c, x)
is a singleton. Dually, c ∈ C is terminal, if and only if for all x ∈ C the set C(x, c) is asingleton. These properties can be expressed more categorically as characterizations of theco- and contravariant functors C(c, −) and C(−, c) represented by c that were introduced in
Definition 1.3.11:
C

x
f


y

C(c,−)/ Set

Cop

7→ C(c, x)
7→

f∗


7
→
C(c, y)xfyC(−,c)/ Set

7→ C(x,O c)
7→

f∗

7→

C(y, c)Definition 2.1.3 (a representable characterization of initial or terminal objects).
(i) An object c in a category C is initial if and only if the functor C(c, −) : C → Set isnaturally isomorphic to the constant functor ∗ : C → Set that sends every object tothe singleton set.
(ii) An object c ∈ C is terminal if and only if the functor C(−, c) : Cop → Set is naturally
isomorphic to the constant functor ∗ : Cop → Set that sends every object to thesingleton set.
In terminology we now introduce, Definition 2.1.3 asserts that C has an initial object if
and only if the constant functor ∗ : C → Set is representable, and dually, C has a terminal
object if and only if ∗ : Cop → Set is representable.
Definition 2.1.4.
(i) A covariant or contravariant functor F from a locally small category C to Set isrepresentable if there is an object c ∈ C and a natural isomorphism between F andthe functor of appropriate variance2 represented by c, in which case one says that thefunctor F is represented by the object c.
(ii) A representation for a functor F is a choice of object c ∈ C together with a
specified natural isomorphism C(c, −)  F, if F is covariant, or C(−, c)  F, if F iscontravariant.
The domain C of a representable functor is required to be locally small so that thehom-functors C(c, −) and C(−, c) are valued in the category of sets.
As in the special case of Definition 2.1.3, a representable functor F : C → Set or
F : Cop → Set encodes a universal property of its representing object. Put colloquially,a universal property of an object X is a description of the covariant functor Hom(X, −) orof the contravariant functor Hom(−, X) associated to that object. Many examples occur “innature.”3
Example 2.1.5. The following covariant functors are representable.
(i) The identity functor 1Set : Set → Set is represented by the singleton set 1. That is,for any set X, there is a natural isomorphism Set(1, X)  X that defines a bĳection
2Some use the term “corepresentable” for covariant representable functors, reserving “representable” for thecontravariant case. We argue that this distinction is unnecessary since the variance of the functor F ought to beevident from its definition.
3If a friend or oracle is nearby, we suggest first encountering the examples listed in 2.1.5 and 2.1.6 as exercises.
Have the friend list the functors that are mentioned and challenge yourself to find the representing objects.

52

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA

between elements x ∈ X and functions x : 1 → X carrying the singleton element to
x. Naturality says that for any f : X → Y, the diagram
Set(1, X)f∗
Set(1, Y)



/X

/Y
x

f

f

commutes, i.e., that the composite function 1 →− X→− Y corresponds to the element
f (x) ∈ Y, as is evidently the case.
(ii) The forgetful functor U : Group → Set is represented by the group Z. That is, forany group G, there is a natural isomorphism Group(Z, G)  UG that associates, toevery element g ∈ UG, the unique homomorphism Z → G that maps the integer 1 tog. This defines a bĳection because every homomorphism Z → G is determined by
the image of the generator 1; that is to say, Z is the free group on a single generator.
gφThis bĳection is natural because the composite group homomorphism Z →− G→− Hcarries the integer 1 to φ(g) ∈ H.
(iii) For any unital ring R, the forgetful functor U : ModR → Set is represented by the
R-module R. That is, there is a natural bĳection between R-module homomorphisms
R → M and elements of the underlying set of M, in which m ∈ U M is associated tothe unique R-module homomorphism that carries the multiplicative identity of R tom; this is to say, R is the free R-module on a single generator. This explains theappearance of the abelian group Z and the vector space k in Definition 1.6.3, wheremaps with these domains were used to specify elements in the codomains.
(iv) The functor U : Ring → Set is represented by the unital ring Z[x], the polynomial
ring in one variable with integer coefficients. A unital ring homomorphism Z[x] → R
is uniquely determined by the image of x; put another way, Z[x] is the free unitalring on a single generator.
(v) The functor U(−)n : Group → Set that sends a group G to the set of n-tuples of
elements of G is represented by the free group Fn on n generators. Similarly, theLfunctor U(−)n : Ab → Set is represented by the free abelian group n Z on n
generators.
(vi) More generally, any group presentation, such as
DE
S 3 B s, t s2 = t2 = 1, sts = tst ,defines a functor Group → Set that carries a group G to the setno(g1 , g2 ) ∈ G2 g21 = g22 = e, g1 g2 g1 = g2 g1 g2 .
The functor is represented by the group admitting the given presentation, in thiscase by the symmetric group S 3 on three elements: the presentation tells us thathomomorphisms S 3 → G are classified by pairs of elements g1 , g2 ∈ G satisfying
the listed relations.
(vii) The functor (−)× : Ring → Set that sends a unital ring to its set of units is represented
by the ring Z[x, x−1 ] of Laurent polynomials in one variable. That is to say, aring homomorphism Z[x, x−1 ] → R may be defined by sending x to any unit of R
and is completely determined by this assignment, and moreover there are no ringhomomorphisms that carry x to a non-unit.

2.1. REPRESENTABLE FUNCTORS

53(viii) The forgetful functor U : Top → Set is represented by the singleton space: there is anatural bĳection between elements of a topological space and continuous functions
from the one-point space.
(ix) The functor ob : Cat → Set that takes a small category to its set of objects isrepresented by the terminal category 1: a functor 1 → C is no more and no less thana choice of object in C.
(x) The functor mor : Cat → Set that takes a small category to its set of morphisms isrepresented by the category 2: a functor 2 → C is no more and no less than a choiceof morphism in C. In this sense, the category 2 is the free or walking arrow.
(xi) The functor iso : Cat → Set that takes a small category to its set of isomorphisms(pointing in a specified direction) is represented by the category I, with two objects
and exactly one morphism in each hom-set. In this sense, the category I is the freeor walking isomorphism.
(xii) The functor comp : Cat → Set that takes a small category to the set of composable
pairs of morphisms in it is represented by the category 3. Generalizing, the ordinaln + 1 = 0 → 1 → · · · → n represents the functor that takes a small category to theset of paths of n composable morphisms in it.
(xiii) The forgetful functor U : Set∗ → Set is represented by the two-element based set:
based functions out of this set correspond naturally and bĳectively to elements ofthe target based set, the element in question being the image of the non-basepoint
element.
(xiv) The functor Path : Top → Set that carries a topological space to its set of paths andthe functor Loop : Top∗ → Set that carries a based space to its set of based loops
are each representable by definition, by the unit interval I and the based circle S 1 ,respectively. A path in X is a continuous function I → X while a (based) loop in X
is a based continuous function S 1 → X.
The adjective “free” is reserved for universal properties expressed by covariant represented functors. It could be applied to any of the objects listed in Example 2.1.5: 2 is thefree category with an arrow, S 1 is the free space containing a loop. The dual term “cofree,”for universal properties expressed by contravariant represented functors, is less commonly
used.
Example 2.1.6. The following contravariant functors are representable.
(i) The contravariant power set functor P : Setop → Set is represented by the set Ω ={>, ⊥} with two elements. The natural isomorphism Set(A, Ω)  PA is defined by
the bĳection that associates a function A → Ω with the subset that is the preimage of>; reversing perspectives, a subset A0 ⊂ A is identified with its classifying function
χA0 : A → Ω, which sends exactly the elements of A0 to the element >. The naturality
condition stipulates that for any function f : A → B, the diagram
Set(B, Ω)f∗
Set(A, Ω)

/ PB
f −1




/ PA

commutes. That is, naturality asserts that given a function χB0 : B → Ω classifyingfχB0

the subset B0 ⊂ B, the composite function A →− B −−→ Ω classifies the subset
f −1 (B0 ) ⊂ A.

54

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA(ii) The functor O : Topop → Set that sends a space to its set of open subsets is represented
by the Sierpinski space S , the topological space with two points, one closed andone open. The natural bĳection Top(X, S )  O(X) associates a continuous function
X → S to the preimage of the open point. This bĳection is natural because acomposite function Y → X → S classifies the preimage of the open subset of X
under the function Y → X.
(iii) The Sierpinski space also represents the functor C : Topop → Set that sends a space
to its set of closed subsets. Composing the natural isomorphisms O  Top(−, S )  C
we see that the closed set and open set functors are naturally isomorphic. The
composite natural isomorphism carries an open subset to its complement, which is
closed. This recovers the natural isomorphism described in Example 1.4.3(v).
(iv) The functor Hom(− × A, B) : Setop → Set that sends a set X to the set of functions
X × A → B is represented by the set BA of functions from A to B. That is, there is anatural bĳection between functions X × A → B and functions X → BA . This natural
isomorphism is referred to as currying in computer science; by fixing a variable ina two-variable function, one obtains a family of functions in a single variable.
op(v) The functor U(−)∗ : Vectk → Set that sends a vector space to the set of vectors in
its dual space is represented by the vector space k, i.e., linear maps V → k are, bydefinition, precisely the vectors in the dual space V ∗ .
(vi) For any fixed abelian group A and any n ≥ 0, singular cohomology with coefficientsin A defines a functor H n (−; A) : Topop → Ab. As in Example 1.3.2(iii), this functor
is a homotopy invariant, factoring through the quotient Top → Htpy to define afunctor H n (−; A) : Htpyop → Ab. Passing to underlying sets and restricting to a
subcategory of “nice” spaces, such as the CW complexes, the resulting functorop
H n (−; A) : HtpyCW → Set is represented by the Eilenberg–MacLane space K(A, n).
That is, for any CW complex X, homotopy classes of maps X → K(A, n) stand in
bĳection with elements of the nth singular cohomology group H n (X; A) of X withcoefficients in A.
(vii) A classifying space for a topological group G is a CW complex BG that represents
opthe functor HtpyCW → Set that takes a CW complex to the set of isomorphism classes
of principal G-bundles over it.4
Our language asserts that a representation encodes some sort of universal property of
its representing object, but many questions remain:• How unique are these universal properties? If two objects represent the same functor,
are they isomorphic?
• What data is involved in the construction of a natural isomorphism between a representable functor F and the functor represented by an object c?
• How do the universal properties expressed by representable functors relate to initial
and terminal objects—our first paradigm for “universality”?
Answers to all of these questions will make use of the Yoneda lemma, to which we now
turn.
Exercises.

4If G is a discrete group, the CW complex BG is built from a collection of combinatorial data that defines thenerve of the category BG (see Exercise 6.5.iv), hence our use of this notation for 1-object groupoids.

2.2. THE YONEDA LEMMA

55

Exercise 2.1.i. For each of the three functors
1o

0
!
1

/

/2

between the categories 1 and 2, describe the corresponding natural transformations between
the covariant functors Cat ⇒ Set represented by the categories 1 and 2.
Exercise 2.1.ii. Prove that if F : C → Set is representable, then F preserves monomorphisms, i.e., sends every monomorphism in C to an injective function. Use the contrapositive to find a covariant set-valued functor defined on your favorite concrete category that is
not representable.
Exercise 2.1.iii. Suppose F : C → Set is equivalent to G : D → Set in the sense that there
is an equivalence of categories H : C → D so that GH and F are naturally isomorphic.
(i) If G is representable, then is F representable?
(ii) If F is representable, then is G representable?
Exercise 2.1.iv. A functor F defines a subfunctor of G if there is a natural transformation
α : F ⇒ G whose components are monomorphisms. In the case of G : Cop → Set, asubfunctor is given by a collection of subsets Fc ⊂ Gc so that each G f : Gc → Gc0 restricts
to a function F f : Fc → Fc0 . Characterize those subsets that assemble into a subfunctorof the representable functor C(−, c).
Exercise 2.1.v. The functor of Example 2.1.5(xi) that sends a category to its collection of
isomorphisms is a subfunctor of the functor of Example 2.1.5(x) that sends a category to
its collection of morphisms. Define a functor between the representing categories I and 2
that induces the corresponding monic natural transformation between these representablefunctors.
2.2. The Yoneda lemma
Yoneda enjoyed relating the story of the origins of
this lemma, as follows . . . Yoneda spent a year in
France (apparently in 1954 and 1955). There hemet Saunders Mac Lane. Mac Lane, then visiting
Paris, was anxious to learn from Yoneda, andcommenced an interview with Yoneda in a café atGare du Nord. The interview was continued on
Yoneda’s train until its departure. In its course,
Mac Lane learned about the lemma andsubsequently baptized it.
Saunders Mac Lane, “The Yoneda lemma”[ML98b]

To gain a better understanding of the representations considered in Examples 2.1.5 and2.1.6 above, it would help to answer the following question: Given a functor F : C → Set,
what data is needed to define a natural isomorphism C(c, −)  F? More generally, what
data is needed to define a natural transformation C(c, −) ⇒ F?
Before addressing the general question, let us cut our teeth on a few warm-up examples.
Example 2.2.1. Consider diagrams indexed by the ordinal category ω. A functor F : ω →Set is given by a family of sets (Fn )n∈ω together with functions fn,n+1 : Fn → Fn+1 . The
functor ω(k, −) : ω → Set represented by the object k ∈ ω corresponds to a family whose

first k sets, indexed by objects n < k, are empty and whose remaining sets, indexed by

56

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMAn ≥ k are singletons. The data of a natural transformation α : ω(k, −) ⇒ F is given by its
components (αn : ω(k, n) → Fn )n∈ω , which must satisfy a naturality condition. Using thefact that the sets ω(k, n) are either empty or singletons, this data is displayed by the diagram:
/∅

∅
α0


F0

/ ···

αk−1

α1

f0,1


/ F1

/ ···

/∗

/∅

αk



/ Fk−1

fk−1,k


/ Fk

/∗
αk+1

fk,k+1

/ Fk+1

/ ···

/∗

/ ···

αn

/ ···


/ Fn

/ ···

Evidently, the components αn contain no information for n < k. For n ≥ k, the components
determine elements αn ∈ Fn (see Example 2.1.5(i)). The naturality condition demands that
αn+1 = fn,n+1 (αn ). By an inductive argument, the natural transformation α : ω(k, −) ⇒ F iscompletely and uniquely determined by the choice of the first element αk ∈ Fk , which is theimage of the identity morphism 1k ∈ ω(k, k) under the kth component map αk : ω(k, k) → Fk .
Moreover, any element of Fk may be chosen as this image; naturality places no further
restrictions.
As a second example, we consider diagrams whose indexing categories are groups.
Example 2.2.2. There is a single object in the category BG whose endomorphisms definea group G. Thus, there is a unique covariant represented functor BG → Set and aunique contravariant represented functor BGop → Set. Example 1.3.9 characterized thefunctors from BG to Set; in the covariant case, they correspond to left G-sets, and in
the contravariant case, they correspond to right G-sets. Recalling that the elements of G
define the set of automorphisms of the unique object in the category BG, we see that thecovariant represented functor is the G-set G, with its action by left multiplication, while thecontravariant represented functor is again G, but with its right action by right multiplication.
Now consider a G-set X : BG → Set. Example 1.4.3(iv) observed that a naturaltransformation φ : G ⇒ X is exactly a G-equivariant map φ : G → X. Here φ : G → X isthe unique component of the natural transformation, and equivariance of this map expresses
the naturality condition. To define φ we must specify elements φ(g) ∈ X for each g ∈ G.
Equivariance demands that φ(g · h) = g · φ(h). Taking h to be the identity element, we seethat φ(g) = g · φ(e). In other words, the choice of φ(e) ∈ X forces us to define φ(g) to be
g · φ(e). Moreover, any choice of φ(e) ∈ X is permitted, because the left action of G on G
is free.5
This argument proves:
Proposition 2.2.3. G-equivariant maps G → X correspond bĳectively to elements of X,
identified as the image of the identity element e ∈ G.
For diagrams indexed by the poset ω and by a group BG, Examples 2.2.1 and 2.2.2
reveal that natural transformations whose domain is a represented functor are determinedby the choice of a single element, which lives in the set defined by evaluating the codomainfunctor at the representing object, and moreover any choice is permitted. In each case,this element is the image of the identity morphism at the representing object.6 This is no5The action of a group G on a set X is free if every stabilizer group is trivial. In the context of the left action of
G on itself, if there were distinct elements k, h so that g = k · e = h · e, we might be forced to make contradictory
definitions of φ(g).
6Recall, the identity element in a group corresponds to the identity morphism in the one-object category.

2.2. THE YONEDA LEMMA

57

coincidence. Indeed, the same is true for diagrams of any shape by what is arguably themost important result in category theory: the Yoneda lemma.7
Theorem 2.2.4 (Yoneda lemma). For any functor F : C → Set, whose domain C is locallysmall and any object c ∈ C, there is a bĳection
Hom(C(c, −), F)  Fc
that associates a natural transformation α : C(c, −) ⇒ F to the element αc (1c ) ∈ Fc.
Moreover, this correspondence is natural in both c and F.
As C is locally small but not necessarily small, a priori the collection of naturaltransformations Hom(C(c, −), F) might be large. However, the bĳection in the Yoneda
lemma proves that this particular collection of natural transformations indeed forms a set.
The statement of the dual form of Theorem 2.2.4, for contravariant functors, is left to
Exercise 2.2.i. We divide the proof into two parts, first demonstrating the bĳection andthen turning our attention to the naturality statement.
Proof of the bijection. There is clearly a function Φ : Hom(C(c, −), F) → Fc thatmaps a natural transformation α : C(c, −) ⇒ F to the image of 1c under the component
function αc : C(c, c) → Fc, i.e.,Φ : Hom(C(c, −), F) → Fc

Φ(α) B αc (1c ) .

Our first aim is to define an inverse function Ψ : Fc → Hom(C(c, −), F) that constructs anatural transformation Ψ(x) : C(c, −) ⇒ F from any x ∈ Fc. To this end, we must definecomponents Ψ(x)d : C(c, d) → Fd so that naturality squares, such as displayed for f : c → d
in C, commute:
Ψ(x)c
/ Fc
C(c, c)Fff∗
C(c, d)

Ψ(x)d


/ Fd

The image of the identity element 1c ∈ C(c, c) under the left-bottom composite is Ψ(x)d ( f ) ∈Fd, the value of the component Ψ(x)d at the element f ∈ C(c, d). The image under the
top-right composite is F f (Ψ(x)c (1c )). For Ψ to define an inverse for Φ, we must define
Ψ(x)c (1c ) = x. Now, naturality forces us to define(2.2.5)

Ψ : Fc → Hom(C(c, −), F)

Ψ(x)d ( f ) B F f (x) .

This condition completely determines the components Ψ(x)d of Ψ(x).
It remains to verify that Ψ(x) is natural. To that end, we must show for a genericmorphism g : d → e in C (one whose domain is not necessarily the distinguished object c),that the squareC(c, d)

Ψ(x)d

g∗


C(c, e)

/ Fd
Fg

Ψ(x)e


/ Fe

commutes. The image of f ∈ C(c, d) along the left-bottom composite is Ψ(x)e (g f ) B
F(g f )(x). The image along the top-right composite is Fg(Ψ(x)d ( f )) B Fg(F f (x)). By
7For the oral history of the Yoneda lemma, which first appeared in print in a paper of Grothendieck [Gro60],
see [ML98b], quoted at the beginning of this section.

58

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA

functoriality of F, F(g f ) = Fg · F f , so these elements agree. Thus, the formula (2.2.5)
defines the function Ψ : Fc → Hom(C(c, −), F).
By construction, ΦΨ(x) = Ψ(x)c (1c ) = x, so Ψ is a right inverse to Φ. We wish to showthat ΨΦ(α) = α, i.e., that the natural transformation Ψ(αc (1c )) is α. It suffices to show thatthese natural transformations have the same components. By definition,
Ψ(αc (1c ))d ( f ) = F f (αc (1c )) .
By naturality of α, the squareC(c, c)(2.2.6)αcf∗C(c, d)

/ Fc
Ff

αd


/ Fd

commutes, from which we see that F f (αc (1c )) = αd ( f ). Thus, Ψ(αc (1c ))d = αd , provingthat Ψ is also a left inverse to Φ. The explicit bĳections Φ and Ψ prove that evaluation of anatural transformation at the identity of the representing object defines an isomorphism

Hom(C(c, −), F) −→ Fc ,
as claimed.

Proof of naturality. The naturality in the statement of the Yoneda lemma amounts
to the following pair of assertions. Naturality in the functor asserts that, given a naturaltransformation β : F ⇒ G, the element of Gc representing the composite natural transformation βα : C(c, −) ⇒ F ⇒ G is the image under βc : Fc → Gc of the element of Fc
representing α : C(c, −) ⇒ F, i.e., the diagram
Hom(C(c, −), F)

ΦF


β∗


Hom(C(c, −), G)

/ Fc
βc


ΦG


/ Gc

commutes in Set. By definition, ΦG (β · α) = (β · α)c (1c ), which is βc (αc (1c )) by thedefinition of vertical composition of natural transformations given in Lemma 1.7.1, andthis is βc (ΦF (α)).
Naturality in the object asserts that, given a morphism f : c → d in C, the element of
Fd representing the composite natural transformation α f ∗ : C(d, −) ⇒ C(c, −) ⇒ F is theimage under F f : Fc → Fd of the element of Fc representing α, i.e., the diagram
Hom(C(c, −), F)Φc( f ∗ )∗


Hom(C(d, −), F)

/ Fc
Ff


Φd


/ Fd

commutes. Here, the image of α ∈ Hom(C(c, −), F) along the top-right composite is
F f (αc (1c )), and the image along the left-bottom composite is (α· f ∗ )d (1d ). By the definitionof vertical composition, the dth component of the composite natural transformation α · f ∗2.2. THE YONEDA LEMMA

59

is the function
C(d, d)1df∗/ C(c, d)

αd

/ Fd

7→

f

7→

αd ( f )As demonstrated by the commutative square (2.2.6), αd ( f ) = F f (αc (1c )), which provesthat this second naturality square commutes.

Remark 2.2.7. Were it not for size issues, we could express Theorem 2.2.4 more concisely
as saying that the maps Φ define the components of a natural isomorphism between two
functors that we now introduce. The pair c and F in the statement of the Yoneda lemma
define an object in the product category C × SetC ; recall SetC is the category of functors
C → Set and natural transformations between them defined in Corollary 1.7.2. There is abifunctor ev : C × SetC → Set that maps (c, F) to the set Fc, i.e., to the set obtained whenevaluating the functor F at the object c. This functor defines the codomain of the natural
isomorphism Φ.
The definition of the domain of Φ makes use of a functor
Copf/ SetCyc

7→ C(c,O −)


d

7→

f∗

7→ C(d, −)This is the functor obtained by applying Exercise 1.7.vii to the bifunctor introduced in
Definition 1.3.13. Using Exercise 1.3.v to regard y as a functor C → (SetC )op , the domainof Φ is defined to be the composite
Hom(y(−), −) B C × SetC
(c, F)

y×1SetC/ (SetC )op × SetC

7→

(C(c, −), F)

Hom

/ SET

7→ Hom(C(c, −), F)

Here is where the size issues arise. If C is small, then SetC is locally small, and the hom
functor is valued in the category of sets, as in Definition 1.3.13. However, if C is only
locally small, SetC need not be locally small; see Remark 1.7.3. We write SET to indicatethat the collection of natural transformations between a pair of functors F, G : C ⇒ Set
might not be a set.
The composite functor Hom(y(−), −) : C × SetC → Set is valued in the usual categoryof sets by the proof of the bĳection in Theorem 2.2.4 just given. So the Yoneda lemma
asserts that evaluating at the representing object’s identity morphism defines a natural
isomorphism
Hom(y(−),−)C × SetC ⇓ Φ Set@ev

An easy application of the Yoneda lemma completely characterizes natural transformations between representable functors: any locally small category C is isomorphic to theopfull subcategory of SetC spanned by the contravariant represented functors, and Cop isisomorphic to the full subcategory of SetC spanned by the covariant represented functors,
via the canonically-defined covariant and contravariant Yoneda embeddings.

60

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA

Corollary 2.2.8 (Yoneda embedding). The functors
op y /

C
Cop
SetC

f

c

7→ C(−, c)


d

7→

f∗


7
→
C(−, d)fy/ SetCc

7→

C(c, −)


d

7→

f∗

7→

O

C(d, −)

define full and faithful embeddings.
Via Exercise 1.7.vii, the co- and contravariant Yoneda embeddings are really just two
different incarnations of a common bifunctor
C(−, −) : Cop × C → Set

defined for any locally small category C, namely the two-sided represented functor introduced in Definition 1.3.13. The natural transformations f∗ : C(−, c) ⇒ C(−, d) andf ∗ : C(d, −) ⇒ C(c, −) are defined in Example 1.4.7, by post- and pre-composition with themorphism f , respectively.
op

Proof. By Definition 1.5.7, the functors y : C ,→ SetC and y : Cop ,→ SetC are fully
faithful if they define local bĳections between hom-setsC(c, d) −→ Hom(C(−, c), C(−, d))C(c, d) −→ Hom(C(d, −), C(c, −)) .

From the construction of the functors y, it is easy to see that these maps are injections:distinct morphisms c ⇒ d induce distinct natural transformations (see Exercise 1.4.iv).
The Yoneda lemma implies that every natural transformation between represented functors
arises in this way. By Theorem 2.2.4, natural transformations
α : C(d, −) ⇒ C(c, −)correspond to elements of C(c, d), i.e., to morphisms f : c → d in C, where the element f
is αd (1d ). The natural transformation f ∗ : C(d, −) ⇒ C(c, −) defined by pre-composition
by f sends 1d to f . Thus, the bĳection implies that α = f ∗ .
Corollary 2.2.8, that natural transformations between represented functors correspondto morphisms between the representing objects, is an extraordinarily powerful result, as weshall gradually discover. Common convention refers to both Theorem 2.2.4 and Corollary
2.2.8 as “the Yoneda lemma.”
We close this section with two non-categorical applications of the fullness of the
Yoneda embedding. A third even more paradigmatic application appears as Proposition
2.3.9.
Corollary 2.2.9. Every row operation on matrices with n rows is defined by left multiplication by some n × n matrix, namely the matrix obtained by performing the row operationon the identity matrix.
Proof. Recall MatR is the category whose objects are positive integers and in which amorphism m → n is an n×m matrix, with n rows and m columns, whose entries are elementsof the unital ring R. The elements in the image of the represented functor Hom(−, n) arematrices with n rows. The row operations of elementary linear algebra—for instance,
replacing the ith row with the sum of the ith and jth row—define natural endomorphisms ofHom(−, n); naturality here follows from linearity of (right) matrix multiplication. Thus, by
Corollary 2.2.8, every row operation must be definable by left multiplication by a suitable2.2. THE YONEDA LEMMA

61

n × n matrix. Moreover, Theorem 2.2.4 allows us to identify this matrix: it is obtained byapplying the row operation in question to the n × n identity matrix.
Another corollary of the Yoneda lemma is a classical result known as Cayley’s theorem, stating that any abstract group may be realized as a subgroup of a permutationgroup.
Corollary 2.2.10. Any group is isomorphic to a subgroup of a permutation group.
Proof. Regarding a group G as a one-object category BG, Example 2.2.2 identifies theop
object in the image of the covariant Yoneda embedding BG ,→ SetBG as the right G-set G,with G acting by right multiplication. Corollary 2.2.8 tells us that the only G-equivariant
endomorphisms of the right G-set G are those maps defined by left multiplication witha fixed element of G. In particular, any G-equivariant endomorphism of G must be an
automorphism, a fact that is not otherwise obvious.
In this way, the Yoneda embedding defines an isomorphism between G and the autoop
morphism group of the right G-set G, an object in SetBG . Composing with the faithfulopforgetful functor SetBG → Set, we obtain an isomorphism between G and a subgroup of
the automorphism group Sym(G) of the set G.
Exercises.
Exercise 2.2.i. State and prove the dual to Theorem 2.2.4, characterizing natural transformations C(−, c) ⇒ F for a contravariant functor F : Cop → Set.
Exercise 2.2.ii. Explain why the Yoneda lemma does not dualize to classify natural transformations from an arbitrary set-valued functor to a represented functor.
Exercise 2.2.iii. As discussed in Section 2.2, diagrams of shape ω are determined bya countably infinite family of objects and a countable infinite sequence of morphisms.
op
Describe the Yoneda embedding y : ω ,→ Setω in this manner (as a family of ωop -indexed
functors and natural transformations). Prove directly, without appealing to the Yoneda
lemma, that y is full and faithful.
Exercise 2.2.iv. Prove the following strengthening of Lemma 1.2.3, demonstrating theequivalence between an isomorphism in a category and a representable isomorphism
between the corresponding co- or contravariant represented functors: the following are
equivalent:(i) f : x → y is an isomorphism in C.
(ii) f∗ : C(−, x) ⇒ C(−, y) is a natural isomorphism.
(iii) f ∗ : C(y, −) ⇒ C(x, −) is a natural isomorphism.
Exercise 2.2.v. By the Yoneda lemma, natural endomorphisms of the contravariant power
set functor P : Setop → Set correspond bĳectively to endomorphisms of its representing
object Ω = {⊥, >}. Describe the natural endomorphisms of P that correspond to each of
the four elements of Hom(Ω, Ω). Do these functions induce natural endomorphisms of thecovariant power set functor?
Exercise 2.2.vi. Do there exist any non-identity natural endomorphisms of the categoryof spaces? That is, does there exist any family of continuous maps X → X, defined for all
spaces X and not all of which are identities, that are natural in all maps in the category Top?
Exercise 2.2.vii. Use the Yoneda lemma to explain the connection between homeomorphisms of the standard unit interval I = [0, 1] ⊂ R and natural automorphisms of the pathfunctor Path : Top → Set of Example 2.1.5(xiv), which we call re-parameterizations.

62

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA

2.3. Universal properties and universal elements
The Yoneda lemma enables us to answer the questions posed at the end of §2.1. In
this section, we study the data associated to a representation and prove that a representablefunctor defines its representing object.
In a locally small category C, any pair of isomorphic objects x  y are representably
isomorphic, meaning that C(−, x)  C(−, y) and C(x, −)  C(y, −). This is an immediateopconsequence of the functoriality of the Yoneda embeddings C ,→ SetC and Cop ,→ SetC ;see Exercise 2.2.iv. The Yoneda lemma supplies the converse:
Proposition 2.3.1. Consider a pair of objects x and y in a locally small category C.
(i) If either the co- or contravariant functors represented by x and y are naturally
isomorphic, then x and y are isomorphic.
(ii) In particular, if x and y represent the same functor, then x and y are isomorphic.
op

Proof. The full and faithful Yoneda embeddings C ,→ SetC and Cop ,→ SetC create
isomorphisms (see Exercise 1.5.iv). So an isomorphism between represented functors is
induced by a unique isomorphism between their representing objects, which in particular
must be isomorphic, proving (i). Now (ii) is an immediate consequence: given a functor
represented by both x and y, the representing natural isomorphisms compose to demonstratethat x and y are representably isomorphic.

There may be many isomorphisms between the objects x and y appearing in the proofof Proposition 2.3.1, but there is a unique natural isomorphism commuting with the chosen
representations. On account of this, one typically refers to the representing object of arepresentable functor. Category theorists often use the definite article “the” in contexts
where the object in question is well-defined up to canonical isomorphism. For instance:
Corollary 2.3.2. The full subcategory of C spanned by its terminal objects is either empty
or is a contractible groupoid. In particular, any two terminal objects in C are uniquely
isomorphic.8
A contractible groupoid is a category that is equivalent to the terminal category 1.
Explicitly, a contractible groupoid is a non-empty category with a unique morphism in eachhom-set.
Proof. By Theorem 2.2.4, natural transformations C(−, t) ⇒ C(−, t0 ) between thefunctors represented by a pair of terminal objects t, t0 ∈ C stand in bĳection with elementsof the singleton set C(t, t0 ). So there can be at most one morphism between any pair ofterminal objects. Proposition 2.3.1 implies that this morphism is an isomorphism: theobjects t and t0 are terminal if and only if they represent the functor ∗ : Cop → Set that isconstant at the singleton set.
The Yoneda lemma also provides a more explicit characterization of the data encoded
by a representation.
Definition 2.3.3. A universal property of an object c ∈ C is expressed by a representablefunctor F together with a universal element x ∈ Fc that defines a natural isomorphism
C(c, −)  F or C(−, c)  F, as appropriate, via the Yoneda lemma.
8Corollary 2.3.2 proves that the so-called “universal cover” of a topological space X is not an initial object in the
category of connected covering spaces over X, unless X is simply connected. If it were, the Deck transformationgroup, which is isomorphic to the fundamental group π1 (X), would necessarily be trivial.

2.3. UNIVERSAL PROPERTIES AND UNIVERSAL ELEMENTS

63

Example 2.3.4. Recall from Example 2.1.5(iv) that the forgetful functor U : Ring → Set isrepresented by the ring Z[x]. The universal element, which defines the natural isomorphism(2.3.5)Ring(Z[x], R)  UR,

is the element x ∈ Z[x]. As in the proof of the Yoneda lemma, the bĳection (2.3.5) is
implemented by evaluating a ring homomorphism φ : Z[x] → R at the element x ∈ Z[x] toobtain an element φ(x) ∈ R.
Example 2.3.6. A functor E : BG → Set, i.e., a left G-set E, is representable if and only
if there is an isomorphism G  E of left G-sets. This implies that the action of G on E
is free (every stabilizer group is trivial) and transitive (the orbit of any point is the entireset), and that E is non-empty. Conversely, any non-empty free and transitive left G-set isrepresentable; a proof is given in Example 2.4.10. By the Yoneda lemma, the data of arepresentation for E, i.e., a specific isomorphism G  E, is determined by the choice ofan element e ∈ E, the universal element, serving as the image of the identity e ∈ G. In
other words, a representable left G-set E is a group that has forgotten its identity element.
A representable G-set is called a G-torsor.
For example, a torsor for the group Rn under addition is n-dimensional affine space,commonly denoted by An (or AnR to emphasize the ground field). We think of An as being
the collection of points in n-dimensional real space, but without a chosen origin. Thinkingof points in Rn as vectors, the free and transitive action of Rn on An is easy to describe: apoint in An is transported along the vector in question. This action is free, because every
non-zero vector acts on every point non-trivially, and transitive, because any two points inaffine space can be “subtracted” to define a unique vector in Rn that carries the one to theother. A choice of an origin point o in An determines coordinates for all of the other points,
hence an isomorphism Rn  An . The point o ∈ An is the universal element: the coordinates
for another point p ∈ An are defined to be the unique vector ~x ∈ Rn so that ~x · o = p.
Example 2.3.7. Fix k-vector spaces V and W and consider the functor
Bilin(V, W; −) : Vectk → Set
that sends a vector space U to the set of k-bilinear maps V × W → U. A bilinear map
f : V × W → U is a function of two variables so that for all v ∈ V, f (v, −) : W → U is alinear map and for all w ∈ W, f (−, w) : V → U is a linear map. Equivalently, by “currying,”
a bilinear map may be defined to be a linear map V → Hom(W, U) or W → Hom(V, U),
where the codomains are vector spaces of linear maps.
A representation for the functor Bilin(V, W; −) defines a vector space V ⊗k W, thetensor product of V and W. That is, the tensor product is defined by an isomorphism(2.3.8)Vectk (V ⊗k W, U)  Bilin(V, W; U),

between the set of linear maps V ⊗k W → U and the set of bilinear maps V × W → U thatis natural in U.9
Theorem 2.2.4 tells us that the natural isomorphism (2.3.8) is determined by a universal
element of Bilin(V, W; V ⊗k W), i.e., by a bilinear map ⊗ : V × W → V ⊗k W. The tensor
product V ⊗k W is the universal vector space equipped with a bilinear map from V × W.
The Yoneda lemma can be used to unpack what this means. The natural isomorphism(2.3.8) identifies any bilinear map f : V × W → U with a linear map f¯ : V ⊗k W → U. To
9This isomorphism is also natural in V and W; see Proposition 4.3.6.

64

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA

understand this identification, consider the naturality square induced by f¯.
Vectk (V ⊗k W, V ⊗k W)/ Bilin(V, W; V ⊗k W)

f¯∗

f¯∗


Vectk (V ⊗k W, U)/ Bilin(V, W; U)

Tracing 1V⊗k W around this commutative square reveals that the bilinear map f factors
uniquely through the bilinear map ⊗ along the linear map f¯.
V ×W

⊗

f

/ V ⊗k W
∃!

% 
U

f¯

In other words, the bĳection (2.3.8) is implemented by composing a linear map V ⊗k W → U
with the universal bilinear map ⊗. This universal property tells us that the bilinear map⊗ : V × W → V ⊗k W is initial in a category that will be described in Example 2.4.12(iii).
Moreover, the defining universal property of the tensor product gives a recipe forits construction as a vector space. Supposing the vector space V ⊗k W exists, consider
its quotient by the vector space spanned by10 the image of the bilinear map − ⊗ −. By
definition, the quotient map V ⊗k W → V ⊗k W/hv ⊗ wi restricts along − ⊗ − to yield thezero bilinear map. But the zero map V ⊗k W → V ⊗k W/hv ⊗ wi also has this property, soby the universal property of V ⊗ W, these linear maps must agree. Because the quotient
map is surjective, this implies that V ⊗k W is isomorphic to the span of the vectors v ⊗ w
for all v ∈ V and w ∈ W modulo the bilinearity relations satisfied by − ⊗ −, recovering theusual constructive definition.
The universal property of the tensor product of a pair of k-vector spaces, more thansimply characterizing the vector space up to isomorphism, allows one to prove usefulproperties about it without ever appealing to a specific basis.
Proposition 2.3.9. For any k-vector spaces V and W, V ⊗k W  W ⊗k V.
Proof. By Proposition 2.3.1, it suffices to show that the functors Bilin(V, W; −)and Bilin(W, V; −) are naturally isomorphic. The natural isomorphism Bilin(V, W; U) 
Bilin(W, V; U) sends a bilinear map f : V × W → U to the bilinear map f ] : W × V → U
defined by f ] (w, v) B f (v, w). Composing natural isomorphisms, the represented functors(2.3.10)

Vectk (V ⊗k W, −)  Bilin(V, W; −)  Bilin(W, V; −)  Vectk (W ⊗k V, −)are naturally isomorphic. By Proposition 2.3.1, a corollary of the Yoneda lemma, thisnatural isomorphism must arise from an isomorphism V ⊗k W  W ⊗k V in Vectk between
the representing objects.
Remark 2.3.11. Moreover, the Yoneda lemma provides an explicit isomorphism V ⊗k W W ⊗k V. The image of the identity linear transformation under the composite isomorphism
Vectk (V ⊗k W, V ⊗k W)  Bilin(V, W; V ⊗k W)

 Bilin(W, V; V ⊗k W)  Vectk (W ⊗k V, V ⊗k W)
defines an isomorphism φ : W ⊗k V −→ V ⊗k W so that the natural isomorphism (2.3.10)
is defined by precomposing with φ. In Example 2.3.7, we saw that the identity linear map
10The image of a bilinear map is not itself a sub-vector space, so closing under span is necessary.

2.3. UNIVERSAL PROPERTIES AND UNIVERSAL ELEMENTS

65

is sent under the bĳection Vectk (V ⊗k W, V ⊗k W)  Bilin(V, W; V ⊗k W) to the universal
bilinear map ⊗ : V × W → V ⊗k W. This in turn is sent to the bilinear map(w,v)7→(v,w)⊗

W × V −−−−−−−−→ V × W −→ V ⊗k W .
Appealing to the universal property of ⊗ : W × V → W ⊗k V, which defines the natural
isomorphism Bilin(W, V; V ⊗k W)  Vectk (W ⊗k V, V ⊗k W), this composite bilinear map
is sent to the unique linear map φ that makes the diagram
W ×V

⊗

∃! φ

(w,v)7→(v,w)

V ×W/ W ⊗k V⊗/ V ⊗k W

commute. This φ is the desired linear isomorphism.
Example 2.3.12. Fixing a group G, there is a functor
Group(G,(−)× )Ring −−−−−−−−−−→ Set

that sends a unital ring R to the set of group homomorphisms from G to the group of units
R× . This functor expresses the universal property of the group ring11 Z[G]. By the Yoneda
lemma, the natural isomorphism
Ring(Z[G], −)  Group(G, (−)× ) ,is encoded by a group homomorphism G → Z[G]× . As in the proof of the Yoneda
lemma, there is natural bĳection between ring homomorphisms Z[G] → R and grouphomomorphisms G → R× constructed as follows: a ring homomorphism Z[G] → R
induces a group homomorphism Z[G]× → R× and thus a group homomorphism G → R×by precomposing with G → Z[G]× .
Exercises.
Exercise 2.3.i. What are the universal elements for the representations:(i) defining the category 2 in Example 2.1.5(x)?
(ii) defining the Sierpinski space in Example 2.1.6(ii)?
(iii) defining the Sierpinski space in Example 2.1.6(iii)?
Exercise 2.3.ii. Use the defining universal property of the tensor product to prove that(i) k ⊗k V  V for any k-vector space V; and(ii) U ⊗k (V ⊗k W)  (U ⊗k V) ⊗k W for any k-vector spaces U, V, W.
Exercise 2.3.iii. The set BA of functions from a set A to a set B represents the contravariant
functor Set(− × A, B) : Setop → Set. The universal element for this representation is afunctionev : BA × A → B
called the evaluation map. Define the evaluation map and describe its universal property,
in analogy with the universal bilinear map ⊗ of Example 2.3.7.
11There is a more general notion of group ring in which Z is replaced by an arbitrary commutative unital ring
K. The universal property of K[G] can be described analogously by replacing the category Ring with the categoryof K-algebras.

66

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA

2.4. The category of elements
A universal property for an object c ∈ C is expressed either by a contravariant functor
F together with a representation C(−, c)  F or by a covariant functor F together witha representation C(c, −)  F. The representations define a natural characterization of
the maps into (in the contravariant case) or out of (in the covariant case) the object c.
Proposition 2.3.1 implies that a universal property characterizes the object c ∈ C up toisomorphism. More precisely, there is a unique isomorphism between c and any otherobject representing F that commutes with the chosen representations.
In such contexts, the phrase “c is the universal object in C with an x” asserts that x ∈ Fc
is a universal element in the sense of Definition 2.3.3, i.e., x is the element of Fc thatclassifies the natural isomorphism that defines the representation by the Yoneda lemma,
where F is the functor that carries c to the set of x’s associated to c. In this section, we provethat the term “universal” is being used in the precise sense alluded to at the beginning of thischapter: the universal element is either initial or terminal in an appropriate category. The
category in question, called the category of elements, can be constructed in a canonical way
from the data of the representable functor F. The main result of this section, Proposition
2.4.8, proves that any universal property can be understood as defining an initial or terminal
object, as variance dictates, completing the promise made in §2.1.
The category of elements is also used to answer another question inspired by consideration of representable functors: namely, how to determine whether a given set-valued
functor F is representable. And if this is the case, is there a way to identify when anelement x ∈ Fc determines a natural isomorphism C(−, c)  F, rather than simply a naturaltransformation C(−, c) ⇒ F?
R
Definition 2.4.1. The category of elements F of a covariant functor F : C → Set has• as objects, pairs (c, x) where c ∈ C and x ∈ Fc, and• a morphism (c, x) → (c0 , x0 ) is a morphism f : c → c0 in C so that F f (x) = x0 .
R
The category of elements has an evident forgetful functor Π : F → C.

(c, x)cf/ (c0 , x0 )7→

If F f (x) = x0 , then

Π

f

∈

R

F
Π

/ c0

∈


C

R
For a contravariant functor F : Cop → Set, the category of elements F is defined tobe the opposite of the category of elements of F, regarded as covariant functor of Cop . For
both co- and contravariant functors on C, ourR convention is that the category of elements
should have a Rcanonical forgetful functor F → C. This dictates the direction for themorphisms in F in both Definitions 2.4.1 and 2.4.2.
Explicitly:R
Definition 2.4.2. The category of elements F of a contravariant functor F : Cop → Set
has• as objects, pairs (c, x) where c ∈ C and x ∈ Fc, and• a morphism (c, x) → (c0 , x0 ) is a morphism f : c → c0 in C so that F f (x0 ) = x.

2.4. THE CATEGORY OF ELEMENTS

67

R
The category of elements has an evident forgetful functor Π : F → C.
(c, x)cf/ (c0 , x0 )7→

If F f (x0 ) = x, then

Π

R

F
Π

/ c0

f

∈

∈


C

Example 2.4.3. Recall the functor n-Color : Graphop → Set that carries a graph to the setof n-colorings of its vertices with the property that no adjacent vertices are assigned thesame color. An object in the category of elements of n-Color is a graph together with achosen n-coloring; that is, objects are n-colored graphs. A morphism φ : G → G0 between apair of n-colored graphs is a graph homomorphism φ : G → G0 so that the induced function
n-Color(φ) : n-Color(G0 ) → n-Color(G) carries the chosen coloring of G0 to the chosencoloring of G. In other words, the graph homomorphism φ preserves the chosen colorings,
0
inR the sense that each purple vertex of G is carried to a purple vertex of G . In summary,n-Color is the category of n-colored graphs and color-preserving graph homomorphisms.
Example 2.4.4. For a concrete category C, the objects in the category of elements for theforgetful functor U : C → Set are elements x ∈ Uc in the underlying set of a specified
object c ∈ C. Morphisms are maps in C whose underlying functions preserve the chosenelements. Extending previous
notation introduced in certain special cases, it would beRreasonable to write C∗ for U and refer to the category of elements as the category of based
objects in C.
Example 2.4.5. When C is a discrete category, a functor F : C → Set encodes an indexedfamily of sets (Fc )c∈C . The category of elements of F is again a discrete category whose``
set of objects is c∈C Fc . The canonical projection Π : c∈C Fc → C is defined so that the`fiber over c ∈ C is the set Fc . The set c∈C Fc is called the dependent sum of the indexedQfamily of sets (Fc )c∈C . The related dependent product c∈C Fc is the set of sections of the`functor Π : c∈C Fc → C.
Example 2.4.6. ObjectsRin the category of elements of C(c, −) are morphisms f : c → x
in C, i.e., the objects of C(c, −) are morphisms in C with domain c. A morphism fromf : c → x to g : c → y is a morphism h : x → y so that g = h f ; we say that h is a morphism
under c:
c

f

x

h

g

/ y

This category has another name: it is the slice category c/C under the object c ∈ C
introduced in Exercise 1.1.iii. The forgetful functor c/C → C sends a morphism f : c → x
to its codomain
R and a commutative triangle to the leg opposite the object c.
Dually, C(−, c) is the slice category C/c over the object c ∈ C. Objects are morphisms
f : x → c with codomain c, and a morphism from f to g : y → c is a morphism h : x → yso that gh = f ; h is a morphism over c:
h

x
f



c

/y
g

The forgetful functor C/c → C projects onto the domain.

68

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA

Note that C(−, c)  Cop (c, −) : Cop → Set. And indeed c/(Cop ), the category ofelements of C(−, c) regarded as a covariant functor of Cop , is isomorphic to (C/c)op , theopposite of the category of elements of the contravariant functor C(−, c); see Exercise 1.2.i.
The Yoneda lemma supplies an alternative
definition of the category of elements of F.
R
In the contravariant case, objects (c, x) ∈ F are in bĳection with natural transformations
Ψ(x) : C(−, c) ⇒ F whose domain is a represented functor and whose codomain is F. A
morphism from Ψ(x) to a second element Ψ(x0 ) : C(−, c0 ) ⇒ F is a natural transformation
C(−, c) ⇒ C(−, c0 )—i.e., by Corollary 2.2.8, is a morphism f : c → c0 —so that the triangleof natural transformationsf∗
C(−, c0 )

Ψ(x)

+3
8@ F

x ∈ FcO
7→

C(−, c)!

Ψ(x0 )

Ffx0 ∈ Fc0

commutes. This proves that the category of elements can be reconstructed as a comma
category, as defined in Exercise 1.3.vi.
Lemma 2.4.7. For F : Cop → Set, the category of elements is isomorphic to the comma
categoryRF  y↓F
op

op

defined relative to the Yoneda embedding y : C → SetC and the object F : 1 → SetC .
Similarly,
in the covariant case, the Yoneda lemma defines a bĳection between objects
R(c, x) ∈ F and natural transformations Ψ(x) : C(c, −) ⇒ F, and a morphism from Ψ(x)to Ψ(x0 ) : C(c0 , −) ⇒ F is a natural transformation C(c0 , −) ⇒ C(c, −), i.e., a morphismf : c → c0 , so that
C(c, −)KS

C(c0 , −)Ψ(x0 )+3@8 F

x ∈ Fc
!

7→

f∗

Ψ(x)

Ff


x0 ∈ Fc0

commutes.
Importantly, the category of elements can be used to show that all universal properties
can be encoded by saying that certain data, the universal element of Definition 2.3.3, defines
an initial or terminal object in an appropriate category, namely the category of elements of
the representable functor.
Proposition 2.4.8 (universal elements are universal elements). A covariant set-valued
functor is representable if and only if its category of elements has an initial object. Dually,a contravariant set-valued functor is representable if and only if its category of elements
has a terminal object.
It is easy to see that this conditionR is necessary.
A natural isomorphism C(c, −)  F
Rinduces an isomorphism of categories F  C(c, −)  c/C (see Exercise 2.4.vii), andR thelatter has an initial object: the identity 1c ∈ c/C. Thus, if F is representable,
R then F has
an initial object. The surprise is that the presence of an initial object in F is sufficientto establish representability of the functor F. The key to the proof, unsurprisingly, is the
Yoneda lemma in the form of Theorem 2.2.4.12
12In fact, now that the statement and proof strategy are known, we invite the reader to first try to come up withthe argument on their own.

2.4. THE CATEGORY OF ELEMENTS

69

R
Proof. Consider a functor F : C → Set and suppose (c, x) ∈ F is initial. We willshow that the natural transformation Ψ(x) : C(c, −) ⇒ F defined by the Yoneda lemma isa natural isomorphism. For any y ∈ Fd, initiality of x ∈ Fc says there exists a uniquemorphism (c, x) → (d, y), i.e., a unique morphism f : c → d in C so that F f (x) =y. As y runs through the elements of the set Fd, this says exactly that the component
Ψ(x)d : C(c, d) → Fd is an isomorphism: in the proof of the YonedaR lemma, Ψ(x)d ( f )was defined to be F f (x). Existence of the morphism (c, x) → (d, y) in F asserts that thefunction Ψ(x)d is surjective while uniqueness asserts that it is injective.
Reversing this argument, a natural isomorphism α : C(c, −)  F defines an object αc (1c ) ∈ Fc in the category of elements, which we prove is initial. The bĳec
tion αd : C(c, d) −→ Fd says that for each object y ∈ Fd,
R there is a unique morphismf : c → d so that F f (αc (1Rc )) = y. Thus, for each (d, y) ∈ F, there is a unique morphismf : (c, αc (1c )) → (d, y) in F, which says exactly that (c, αc (1c )) defines an initial object in
the category of elements.

Recall that a representation for a functor F : C → Set consists of an object c ∈ C
together with a natural isomorphism C(c,R −)  F. Equivalently by Proposition 2.4.8, arepresentation for F is an initial object in F. Representations are not strictly unique: if c0
is any object isomorphic to c, then a representation C(c, −)  F induces a representation
C(c0 , −)  C(c, −)  F defined by composing with the chosen isomorphism. However,representations are unique in an appropriate category-theoretic sense.
R
Proposition 2.4.9. For any functor F : C → Set, the full subcategory of F spanned by
its representations is either empty or a contractible groupoid.
Proof. If F is not representable, then it has no representations and this subcategory
is empty. R Otherwise, Proposition 2.4.8 implies that any representation defines an initial
object in F. By Corollary 2.3.2, the subcategory spanned by the initial objects is either
empty or is a contractible groupoid: there exists a unique (iso)morphism between any two
initial objects.

Example 2.4.10. The category of elements of a G-set X : BG → Set is the translationgroupoid TG X introduced in Example 1.5.18: its objects are elements x ∈ X and its
morphisms g : x → y are elements g ∈ G so that g · x = y. By Example 2.3.6, if X isrepresentable, then any element of X may be chosen asR a universal element. By Proposition
2.4.9, it follows that X is representable if and only if X  TG X is a contractible groupoid.
Assuming X is non-empty, this groupoid is contractible if and only if for each pair ofelements x, y ∈ X, there is a unique g ∈ G so that g · x = y. Existence tells us that the actionof G on X is transitive. Uniqueness, in the case x = y, tells us that it is free. This proves
the claim made in Example 2.3.6 that any free and transitive left G-set is representable.
Trivially, any category E arises as a category of elements, namely for the constantfunctor ∗ : E → Set valued at the singleton set, a representation for which defines an initial
object in E. More interesting is when E arises as the category of elements for a functor
indexed by a different category C, related by a forgetful functor E → C.13 For instance:
Example 2.4.11. The category of discrete dynamical systems, introduced in Example 2.1.1,
is the category of elements for the functor U : End → Set whose domain is the categoryof sets equipped with an endomorphism and whose maps are functions so that the diagram
13An elementary characterization of the functors E → C arising as categories of elements for some C → Set
is suggested by Exercise 2.4.viii.

70

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA

analogous to (2.1.2) commutes. This functor is represented by the object (N, s : N → N)
and the representation is given by the universal element 0 ∈ N = U(N, s). Proposition 2.4.8
then implies the universal property observed in Example 2.1.1: that (N, s : N → N, 0 ∈ N)
is the universal discrete dynamical system.
Example 2.4.12.
(i) The objects in the category of elements of the contravariant power set functor are
pairs A0 ⊂ A, a set with a chosen subset. A morphism f : (A0 ⊂ A) → (B0 ⊂ B)
is a function f : A → B so that f −1 (B0 ) = A0 . The terminal object, corresponding
to the representation described in Example 2.1.6(i), is the set Ω B {>, ⊥} of twoelements with the distinguished singleton subset {>} ⊂ Ω. This is the universal set
equipped with a subset.14 For any A0 ⊂ A, there is a unique function h : A → Ω sothat h−1 (>) = A0 . Note that even if we begin with a skeletal category of sets, {>} ⊂ Ω
is not the unique terminal object in the category of elements. It is isomorphic to{⊥} ⊂ Ω.
(ii) An object in the category of elements of the forgetful functor U : Vectk → Set isa vector v in some k-vector space V. A morphism (V, v) → (W, w) is a linear map
T : V → W that carries v to w. The vector space k, which represents U, together
with its universal element 1 ∈ k, defines an initial object in the category of elements
but it is not the only one. Any non-zero scalar c ∈ k defines a linear isomorphismc · − : k → k. Thus, the pairs (k, c) are also initial in the category of elements. Moregenerally, any 1-dimensional vector space V and non-zero vector v define an initial
object. Fixing such a pair (V, v), linear maps V → W are in bĳection with vectors
w ∈ W, taken to be the image of v.
(iii) Objects in the category of elements of Bilin(V, W; −) are bilinear maps f : V×W → U,
with target some k-vector space U. Morphisms are linear maps T : U → U 0 so thatthe diagram of functions
f

V ×W
f0

/U
T

# 
U0

commutes, i.e., so that the bilinear map f 0 is the composite of the bilinear map
f and the linear map T . The universal property of the universal bilinear map⊗ : V × W → V ⊗k W described in Example 2.3.7 says exactly that ⊗ is initial in thiscategory.
(iv) An object in the category of elements of the functor U(−)n : Group → Set consistsof a group G together with an n-tuple of elements g1 , . . . , gn ∈ G. A morphism(g1 , . . . , gn ∈ G) → (h1 , . . . , hn ∈ H) is a homomorphism φ : G → H so that φ(gi ) =hi for all i. The universal group with n elements, denoted by Fn , is the groupwith specified elements x1 , . . . , xn ∈ Fn with the universal property that for any
g1 , . . . , gn ∈ G there is a unique group homomorphism φ : Fn → G so that φ(xi ) = gi .
The free group on n generators x1 , . . . , xn has this property. Every element in Fn is a
product of the xi and their inverses, so a choice of n elements φ(xi ) ∈ G determines
the entire map φ : Fn → G. Moreover, there are no relations between the xi in Fn so
any choices are permitted.
14In the topos of sets, {>} ⊂ Ω is called the subobject classifier; see §E.4.

2.4. THE CATEGORY OF ELEMENTS71
op(v) An object in the category of elements of the functor U(−)∗ : Vectk → Set is a vector
space with a dual vector. That is, an object is simply a linear map f : V → k with
codomain k. A morphism from f : V → k to g : W → k is a linear map T : V → W
so that f = gT . Immediately from this definition it is clear that the category ofelements of U(−)∗ is the slice category Vectk /k. The identity on k is terminal, i.e.,
1 : k → k is the universal dual vector.
(vi) An object in the category of elements of U : Ring → Set is a unital ring R with anelement r ∈ R. Maps are ring homomorphisms preserving the chosen elements. The
initial object is x ∈ Z[x]. In the category of elements, the object x ∈ Z[x] has nonon-identity endomorphisms: initial objects never do. But the ring Z[x] has many
endomorphisms in the category of rings: the universal property Ring(Z[x], −)  Utells us that maps Z[x] → Z[x] are classified by polynomials with integer coefficients,
i.e., by elements p(x) ∈ Z[x]. By Corollary 2.2.8, all natural endomorphisms of
U : Ring → Set must have components R → R defined in this manner: i.e., thecomponent of any natural endomorphism of rings is a map p(x) : R → R defined by
r 7→ p(r), where p is an integer polynomial.
Definitions via universal properties emerge as an important theme in the coming
chapters. To whet the reader’s appetite, let us pose the following puzzle. Fixing two objects
A, B in a locally small category C, define a functor
C(A, −) × C(B, −) : C → Set

that carries an object X to the set C(A, X) × C(B, X) whose elements are pairs of mapsa : A → X and b : B → X in C. What would it mean for this functor to be representable?
Exercises.
R
Exercise 2.4.i. Given F : C → Set, show that F is isomorphic to the comma category∗ ↓ F of the singleton set ∗ : 1 → Set over the functor F : C → Set.
Exercise 2.4.ii. Characterize the terminal objects of C/c.
Exercise 2.4.iii. Use the principle of duality to convert the proof that a covariant functor
is representable if and only if its category of elements has an initial object into a proof thata contravariant functor is representable if and only if its category of elements has a terminal
object.
Exercise 2.4.iv. Explain the sense in which the Sierpinski space is the universal topologicalspace with an open subset.
Exercise 2.4.v. Define a contravariant functor F : Setop → Set that carries a set to the setof preorders on it. What is its category of elements? Is F representable?
Exercise 2.4.vi. For a locally small category C, regard the two-sided represented functor
Hom(−, −) : Cop × C → Set as a covariant functor of its domain Cop × C. The category ofelements of Hom is called the twisted arrow category. Justify this name by describing its
objects and morphisms.
Exercise 2.4.vii. Prove that the construction of the category of elements defines the action
on objects of a functor
R(−) : SetC → CAT/C .
R
R
Conclude that if F, G : C → Set are naturally isomorphic, then F  G over C.
R
Exercise 2.4.viii. Prove that for any F : C → Set, the canonical forgetful functor Π : F →C has the following property: for any morphism f : c → d in the “base category” C and72

2. UNIVERSAL PROPERTIES, REPRESENTABILITY, AND THE YONEDA LEMMA

anyR object (c, x) in the fiber over c, there is a unique lift of the morphism f to a morphismin F with domain (c, x) that projects along Π to f . A functor with this property is calleda discrete left fibration.
Exercise 2.4.ix. Formulate
R the dual definition of a discrete right fibration satisfied by
the canonical functor Π : F → C associated to a contravariant functor F : Cop → Set.
Exercise 2.4.x. Answer the question posed at the end of this section: fixing two objects
A, B in a locally small category C, we define a functor
C(A, −) × C(B, −) : C → Set

that carries an object X to the set C(A, X) × C(B, X) whose elements are pairs of mapsa : A → X and b : B → X in C. What would it mean for this functor to be representable?

CHAPTER 3

Limits and Colimits
Many properties of mathematical constructions
may be represented by universal properties ofdiagrams.
Saunders Mac Lane, Categories for the Working
Mathematician [ML98a]

From a very simple topological space, the real line R with its usual metric, one can build
a wide variety of new topological spaces. Taking products of R with itself, one defines
the Euclidean spaces Rn in finite or infinite dimensions. The space Rn has interesting
subspaces, defined to be the solutions to certain continuous polynomial functions Rn → R,
including the (n − 1)-sphere S n−1 , which bounds the closed n-disk Dn . From the sphere S n ,one can define real projective space RPn as a quotient by identifying each pair of antipodal
points. From spheres and disks one can build tori, the Möbius band, the Klein bottle,
and indeed any cell complex through a sequence of gluing constructions, in which disks
are attached to an existing space along their boundary spheres. In each case, the newly
constructed object is a particular set equipped with a specific topology. In this chapter, we
will explain how all of these topologies can be defined in a uniform way, via a universal
property that characterizes the newly constructed space either as a limit or a colimit of aparticular diagram in the category of topological spaces.
Limits and colimits can be defined in any category. Special cases include constructionsof the infimum and supremum, free products, cartesian products, direct sums, kernels,
cokernels, fiber products, amalgamated free products, inverse limits of sequences, unions,and even the category of elements, among many others. This chapter begins in §3.1 byintroducing the abstract notions of limits and colimits and the special terminology used
for particular diagram shapes. We then turn our focus to “computational” results, which
describe how more complicated limits and colimits can be built out of simpler ones. To
ground intuition for limits and colimits, these abstract constructions are first introducedin §3.2 in the special case of limits of diagrams valued in the category of sets. Indeed,
as a consequence of the Yoneda lemma, the set-theoretical constructions of limits suffice
to prove general formulae for limits and colimits in any category. To understand this, weconsider a variety of possible interactions between functors and limits in §3.3, presenting
functors that preserve, reflect, or create limits and colimits. Such functors can be used torecognize cases in which limits or colimits in one category can be constructed from limits
or colimits in another. This vocabulary is used in §3.4 to describe the representable natureof limits and colimits and extend the constructions of general limits from simple limits
from Set to any category.
In §3.5, we apply the computational theorems from §3.4 to prove that the categories ofsets, spaces, and categories are each complete and cocomplete, meaning that any diagram
indexed by a small category has both a limit and a colimit. Most of the “algebraic”
73

74

3. LIMITS AND COLIMITS

categories introduced in Example 1.1.3 are also complete and cocomplete—the categoryof fields being a notable exception—but we defer the proof of these facts to §5.6 where they
can be deduced simultaneously as special cases of a general theorem. In §3.6, we introduce
limit and colimit functors defined on categories of diagrams and use the functoriality of thelimit and the colimit to prove the result that motivated Eilenberg and Mac Lane to introduce
the concept of naturality: that naturally isomorphic diagrams have isomorphic limits.
In §3.7, we explain why few categories admit limits and colimits of diagrams ofunbounded size. In §3.8, we study the interactions between limits and colimits. Whilelimits commute with limits and colimits commute with colimits, in general, limits andcolimits fail to commute with each other, with a few notable exceptions that depend bothon the indexing shapes and the ambient category in which the diagrams are valued. To
illustrate, we prove the most famous of these: that finite limits commute with filteredcolimits in the category of sets.
3.1. Limits and colimits as universal cones
Recall that a diagram of shape J in a category C is a functor F : J → C. Our aim
in this section is to introduce the dual notions of limit and colimit of a diagram, which aredefined, respectively, to be the universal cones over and under the diagram. The first task
is to explain what is meant by a cone.
Definition 3.1.1. For any object c ∈ C and any category J, the constant functor c : J → C
sends every object of J to c and every morphism in J to the identity morphism 1c . The
constant functors define an embedding ∆ : C → CJ that sends an object c to the constantfunctor at c and a morphism f : c → c0 to the constant natural transformation, in which
each component is defined to be the morphism f .
Definition 3.1.2. A cone over a diagram F : J → C with summit or apex c ∈ C isa natural transformation λ : c ⇒ F whose domain is the constant functor at c. The
components (λ j : c → F j) j∈J of the natural transformation are called the legs of the cone.
Explicitly:• The data of a cone over F : J → C with summit c is a collection of morphisms
λ j : c → F j, indexed by the objects j ∈ J.
• A family of morphisms (λ j : c → F j ) j∈J defines a cone over F if and only if, for eachmorphism f : j → k in J, the following triangle commutes in C:
λj(3.1.3)

Fj

c

λk

~

/ Fk

Ff

Dually, a cone under F with nadir c is a natural transformation λ : F ⇒ c, whoselegs are the components (λ j : F j → c) j∈J . The naturality condition asserts that, for eachmorphism f : j → k of J, the triangle
Fj
λj

/ Fk

Ff

c

~

λk

commutes in C.
Cones under a diagram are also called cocones—a cone under F : J → C is precisely
a cone over F : Jop → Cop —but we find the terminology “under” and “over” to be more
evocative. To illustrate, if F is a diagram indexed by the poset category (Z, ≤), then a cone

3.1. LIMITS AND COLIMITS AS UNIVERSAL CONES

75

over F with summit c is comprised of a family of morphisms (λn : c → Fn)n∈Z so that, foreach n ≤ m, the triangle defined by the morphisms λn , λm , and Fn → Fm commutes:c(3.1.4)
λ−2

···

···

t


/ F(−1)v/ F(−2)

λ0

λ−1

λ1


/ F0

λ2

···



/ F1

&

/ F2

)/

···

A cone under F with nadir c is comprised of a family of morphisms (λn : Fn → c)n∈Z so
that, for each n ≤ m, the triangle defined by the morphisms λn , λm , and Fn → Fm commutes:/ F(−1)
/ F0
/ F1
/ F2
/ ···
/ F(−2)
···
···

λ−2

λ−1

λ0

λ1

λ2

···

(* c  ux
The limit of F is most simply described as the universal cone over the diagram F,
while the colimit is the universal cone under F. As discussed in Chapter 2, there are two
equivalent ways to understand this universal property. A limit may be defined to be a
representation for a particular contravariant functor or as a terminal object in its categoryof elements; dually, a colimit is a representation for a particular covariant functor and,equivalently, is an initial object in its category of elements. As is always the case forrepresentable definitions, limits and colimits can be contemplated for any diagram valued
in any category, but in any particular case these universal objects may or may not exist.
Definition 3.1.5 (limits and colimits I). For any diagram F : J → C, there is a functor
Cone(−, F) : Cop → Set
that sends c ∈ C to the set1 of cones over F with summit c; we leave a description of its
action on morphisms as Exercise 3.1.i. A limit of F is a representation for Cone(−, F). By
the Yoneda lemma, a limit consists of an object lim F ∈ C together with a universal cone
λ : lim F ⇒ F, called the limit cone, defining the natural isomorphism
C(−, lim F)  Cone(−, F) .

Dually, there is a functor
Cone(F, −) : C → Set
that sends c to the set of cones under F with nadir c. A colimit of F is a representation forCone(F, −). By the Yoneda lemma, a colimit consists of an object colim F ∈ C together
with a universal cone λ : F ⇒ colim F, called the colimit cone, defining the natural
isomorphism
C(colim F, −)  Cone(F, −) .
Applying Proposition 2.4.8, limits and colimits may also be defined to be terminal andinitial objects, respectively, in the appropriate categories of elements.
Definition 3.1.6 (limits and colimits II). For any diagram F : RJ → C, a limit is a terminal
object in the category of cones over F, i.e., in the category Cone(−, F). An object in
the category of cones over F is a cone over F, with any summit. In particular, the data
1To guarantee that there is a set of cones with fixed summit over the diagram F, it suffices to assume that
J is small and C is locally small, so that CJ is locally small. Until §3.7, all of the limits and colimits underconsideration are indexed by small categories.

76

3. LIMITS AND COLIMITS

of a terminal object in the category of cones consists of a limit object in C together witha specified limit cone. A morphism from a cone λ : c ⇒ F to a cone µ : d ⇒ F is amorphism f : c → d in C so that for each j ∈ J, µ j · f = λ j . In other words, a morphismof cones is a map between the summits so that each leg of the domain cone factors throughthe corresponding leg of the codomain cone along this map.
c
f
λ−2

λ−1

···

u
··· r

x
/ F(−2) t

···

µ−2

 y
/ F(−1)µ−1

 λ0 λ1
d
µ0


/ F0

µ1

λ2
···
µ2

$
/ F1

···

$)

/ F2

+/ ( · · ·R
The forgetful functor Cone(−, F) → C takes a cone to its summit.
Dually,R a colimit is an initial object in the category of cones under F, i.e., in the
category Cone(F, −). An object in the category of cones under F is a cone under F, withany nadir. In particular, the data of an initial object is comprised of the colimit object inC together with a specified colimit cone. A morphism from a cone λ : F ⇒ c to a cone
µ : F ⇒ d is a morphism f : c → d so that for each j ∈ J, µ j = f · λ j . In other words,
a morphism of cones is a map between the nadirs so that each leg of the codomain cone
factors throughthe corresponding leg of the domain cone along this map. The forgetfulRfunctor Cone(F, −) → C takes a cone to its nadir.
Limits and colimits of a diagram F of shape J might be denoted by limJ F or colimJ F
for emphasis, particularly when multiple diagram shapes are involved. On other occasions,no special notation is used: to say that a cone is a “limit cone” is to assert that its summit
is a limit, however it is denoted.
On account of the following proposition, it is common to use the definite article “the”
to refer to a limit or a colimit of a fixed diagram because these objects are well-defined up
to a unique isomorphism:
Proposition 3.1.7 (essential uniqueness of limits and colimits). Given any two limit cones
λ : ` ⇒ F and λ0 : `0 ⇒ F over a common diagram F, there is a unique isomorphism `  `0that commutes with the legs of the limit cones.
Note that this result does not prohibit limit objects from having non-trivial automorphisms. If λ : ` ⇒ F is a limit cone and σ : `  ` is any automorphism of `, then λσ : ` ⇒ F
is another limit cone. However, Proposition 3.1.7 implies that the only automorphism of `that commutes with the specified limit cone λ is the identity.
Proof. The limit cones λ : ` ⇒ F and λ0 : `0 ⇒ F each define terminal objects inthe category of cones over F. By Corollary 2.3.2, the terminal objects in any category
define a contractible groupoid. This means, in particular, that these objects are uniquely
isomorphic in the category of cones, i.e., that there is a unique isomorphism between ` and`0 commuting with the cone legs.

Remark 3.1.8 (limit and colimit diagrams). The data of a diagram, together with a limit
cone over it, is called a limit diagram and the data of a diagram, together with a colimit
cone under it, is called a colimit diagram. A J-indexed diagram and a cone over it—suchas displayed in (3.1.4)—combine to define a diagram indexed by the category J/ , which has
J as a full subcategory together with a freely-adjoined initial object. Dually, a J-indexed
diagram and a cone under it combine to define a diagram indexed by the category J. , which3.1. LIMITS AND COLIMITS AS UNIVERSAL CONES

77

is built from the category J together with a freely-adjoined terminal object. Exercise 3.5.iv
provides a more explicit construction of this category.
There are special names for limit and colimit diagrams of certain shapes.
Definition 3.1.9. A product is a limit of a diagram indexed by a discrete category, with
only identity morphisms. A diagram in C indexed by a discrete category J is simply acollection of objects F j ∈ C indexed by j ∈ J. A cone over this diagram is a J-indexedfamily of morphisms (λ j : c → F j ) j∈J , subject to no further constraints. The limit isQ
typically denoted by j∈J F j and the legs of the limit cone are maps


Y


πk :F j → Fk 
j∈J

k∈J

called (product) projections. The universal property asserts that composition with the
product projections defines a natural isomorphism
C(c,Q(πk )∗

j∈J F j )

/ Qk∈J C(c, Fk )  Cone(c, F).

Example 3.1.10. The product of a pair of spaces X and Y is a space X × Y equipped with
continuous projection functions
πX

Xo

X×Y

πY

/Y

satisfying the following universal property: for any other space A with continuous maps
f : A → X and g : A → Y, there is a unique continuous function h : A → X × Y so that the
diagram
A
f

Xo

g
∃! h

|
πX


X×Y

πY

"/

Y

commutes.
Taking A to be the point, which represents the underlying set functor U : Top → Set,
the bĳectionTop(∗, X × Y)  Top(∗, X) × Top(∗, Y)
tells us that the points of X×Y correspond to points in the cartesian product of the underlying
sets of X and Y.
It remains to define the topology on the set X ×Y. Taking A to be the set X ×Y equippedwith various topologies, this universal property forces X × Y to be defined to be the coarsest
topology on the cartesian product of the underlying sets of X and Y so that the projection
functions πX and πY are continuous.2
Similarly, the product of an arbitrary (possibly infinite) family of spaces (X j ) j∈J is
defined by assigning the product of the underlying sets the coarsest topology so that each ofQ
the projection functions πk :j X j → Xk is continuous. The more intuitive “box topology”
Qon the set j X j , whose basic open sets are products of basic opens in each X j , coincides
with the product topology for finite indexing sets, but fails to satisfy the universal propertyof Definition 3.1.9 when the indexing set J is infinite.
2Coarsest means “having the fewest open and closed sets”; the antonym is finest, meaning “having the mostopen and closed sets.”

78

3. LIMITS AND COLIMITSDefinition 3.1.11. A terminal object is often regarded as a trivial special case of a
product, where the indexing category is empty. A cone over an empty diagram is just an
object in the target category, its summit, together with no additional data. Any morphism
between the summits defines a map of empty cones. Thus, the category of cones in thisspecial case is just the ambient category itself and so the limit, a terminal object in thiscategory, is just a terminal object in the sense of Definition 1.6.14.
Example 3.1.12. The category 1 with one object and only its identity morphism is theterminal category, the terminal object in Cat or in CAT.
Definition 3.1.13. An equalizer is a limit of a diagram indexed by the parallel pair, thecategory • ⇒ • with two objects and two parallel non-identity morphisms. A diagram ofthis shape is simply a parallel pair of morphisms f, g : A ⇒ B in the target category C.
A cone over this diagram with summit C consists of a pair of morphisms a : C → A andb : C → B so that f a = b and ga = b; these two equations correspond to the naturality
conditions (3.1.3) imposed by each of the two non-identity morphisms in the indexing
category. Together, they assert that f a = ga; the morphism b is necessarily equal to thiscommon composite. Thus, a cone over a parallel pair f, g : A ⇒ B is represented by asingle morphism a : C → A so that f a = ga.
The equalizer h : E → A is the universal arrow with this property. The limit diagram
E

/A

h

f
g

// B

is called an equalizer diagram. The universal property asserts that given any a : C → A
so that f a = ga, there exists a unique factorization k : C → E of a through h.
C
a

k ∃!


E


h

/A

f
g

// B

Example 3.1.14. For instance, the equalizer of a group homomorphism φ : G → H andthe trivial homomorphism e : G → H, sending every element of G to the identity in H, isthe kernel of φ, and the leg of the limit cone is the inclusion ker φ ,→ G.
The equalizer of a pair of homomorphisms φ, ψ : G ⇒ H is the subgroup of elementsg ∈ G so that φ(g) = ψ(g). If H is abelian, this is the kernel of the homomorphism
φ − ψ : G → H.
The fact that the equalizer defines a subgroup of G is no coincidence: the map from anequalizer into the domain of the parallel pair that it equalizes is always a monomorphism;
see Exercise 3.1.vi.
Definition 3.1.15. A pullback is a limit of a diagram indexed by the poset category•→•←•comprised of two non-identity morphisms with common codomain. Writing f and g forthe morphisms defining the image of a diagram of this shape in a category C, a cone withsummit D consists of a triple of morphisms, one for each object in the indexing category,3.1. LIMITS AND COLIMITS AS UNIVERSAL CONES

79

so that both triangles in the diagram

b

/C

c

D

(3.1.16)

a


B

g

 
/A

f

commute; the two triangles represent the two naturality conditions (3.1.3) imposed by the
non-identity morphisms in the indexing category. The leg a asserts that gc and f b have afgcommon composite. Thus, the data of a cone over B →− A←− C may be described morebc

simply as a pair of morphisms B ←− D→− C defining a commutative square.
The pullback, the universal cone over f and g, is a commutative square f h = gk withthe following universal property: given any commutative square (3.1.16), there is a uniquefactorization of its legs through the summit of the pullback cone:
D
c

∃!


(3.1.17)

P

b

"
/C

k

y

g

h

 
B


/A

f

The symbol “y” indicates that the square gk = f h is a pullback, i.e., a limit diagram, andnot simply a commutative square. The pullback P is also called the fiber product and isfrequently denoted by B ×A C. The precise relationship between pullbacks and products is
explored in §3.2 and §3.4.
Particularly when the map f : 1 → A represents an “element” of the object A, such as
when its domain is an object 1 ∈ C that represents an “underlying set” functor C → Set,
the pullback P
/C
P
y

g


1

f


/A

defines the fiber of the map g : C → A over f .
Example 3.1.18. For instance, consider the continuous quotient map ρ : R → S 1 in the
category of spaces that sends a real number t to the point e2πit of S 1 , thought of as the unit
circle in the complex plane. The pullback of the diagram
Z

∗

/R

y



1

ρ

/ S1

is the fiber of ρ over the point 1 ∈ S 1 . This is the discrete subspace Z ⊂ R.
The fact that the fiber defines a subspace of the domain of the map being pulled backis no coincidence: the pullback of a monomorphism B  A along any map C → A defines
a monomorphism B ×A C  C; see Exercise 3.1.vii.

80

3. LIMITS AND COLIMITS

Example 3.1.19. For another example of this flavor, recall that maps Z → Z of abeliangroups correspond to elements n ∈ Z because Z represents the forgetful functor U : Ab →Set, i.e., Hom(Z, A)  UA for any abelian group A. Elements of the pullback of
Z
n

Z

m


/Z

are pairs of integers x and y so that nx = my. From this description, assuming m and n are
not both zero, it follows that the pullback is isomorphic to the abelian group Z and the legsof the pullback cone
Z

b

y

n

a

(3.1.20)

/Z


Z

m


/Z

are defined to be the unique3 integers so that ma = nb is the least common multiple of nand m.
Definition 3.1.21. The limit of a diagram indexed by the category ωop is called an inverse
limit of a tower or a sequence of morphisms. On account of this example, the term “inverse
limit” is sometimes used to refer to limits of any shape. A diagram indexed by ωop consistsof a sequence of objects and morphisms
/ F3

···

/ F2

/ F1

/ F0

together with composites and identities, which are not displayed. A cone over this diagram
is an extension of this data to a diagram of shape (ω + 1)op . Explicitly, a cone consistsof a new object “all the way to the left” together with morphisms making every triangle
commute:
c
···


···

λ3

λ2

/ F3

λ0

λ1

(/

F2

/* F1

/+ F 0The inverse limit, frequently denoted by lim Fn , is the terminal cone. Similar remarks apply←−−
with any limit ordinal4 α in place of ω.
Example 3.1.22. The p-adic integers are defined to be the inverse limit of the following
diagram of rings:
Z p = lim Z/pn←−−// ···/ / Z/p4/ / Z/p3/ / Z/p2/ / Z/pwhere the maps in the diagram are the canonical quotient maps.
Definition 3.1.23. Definitions 3.1.9, 3.1.11, 3.1.13, 3.1.15, and 3.1.21 dualize to define:
3More accurately, the universal property described here only defines the pair of integers (a, b) up to a compatible
choice of sign; see Exercise 3.1.x.
4Exercise 3.1.ix explains the limited interest of limits of diagrams indexed by opposites of successor ordinals.

3.1. LIMITS AND COLIMITS AS UNIVERSAL CONES81`• A coproduct j∈J A j is the colimit of a diagram (A j ) j∈J indexed by a discrete category`
J. The legs of the colimit cone ι j0 : A j0 →j∈J A j are referred to as coproductinjections, though in pathological cases these maps might not be monomorphisms(see Exercise 3.1.xi).
• An initial object is the colimit of the empty diagram.
• A coequalizer is a colimit of a diagram indexed by the parallel pair category • ⇒ •.
The coequalizer of a parallel pair of maps f, g : A ⇒ B is the universal map h : B → C
with the property that h f = hg. The colimit cone
f

A

g

// B

// C

h

is called a coequalizer diagram. Diagrams of this shape are also called forks.
• A pushout is a colimit of a diagram indexed by the poset category • ← • → •.
Dualizing the convention introduced in Definition 3.1.15, the symbol “p” indicates thata commutative square
A

f

g


C

/B
k


/P

p
h

is a pushout, i.e., is a colimit diagram. The pushout is the universal commutativesquare under the maps f and g.
• The colimit of a diagram indexed by the ordinal ω is called a sequential colimit ordirect limit. The colimit of a diagram
F0

/ F1

/ F2

/ F3

/ ···

frequently denoted by lim Fn , defines a diagram of shape ω + 1. The term “direct limit”−−→
is sometimes also used to refer to colimits of any shape.
Each of the colimits introduced in Definition 3.1.23 is indexed by the opposite of thecategory used to index the corresponding limit notion. Exercise 3.1.ix explains why thecategory ω and the category • ← • → • are used to index colimit diagrams, but not limit
diagrams.
Example 3.1.24. The figure eight or the wedge of two circles is a space S 1 ∨ S 1 that canbe defined to be the pushout of the diagram S 1 ← ∗ → S 1 of unbased spaces or as the
coproduct of S 1 with itself in the category of based spaces. A further pushout diagram inTopS1

aba−1 b−1 /S1 ∨ S1

i


D2

p 
/T

defines the torus T  S 1 × S 1 . Here i is the inclusion of the circle as the boundary of thedisk and the map aba−1 b−1 is the loop in S 1 ∨ S 1 that wraps once around one circle, thenonce around the other, then again around the first but in the reversed orientation, and thenagain around the second but in the reversed orientation.

82

3. LIMITS AND COLIMITS

Example 3.1.25. The coequalizer of a group homomorphism φ : G → H paired with thetrivial homomorphism e : G → H defines the cokernel, the quotient of H by the normal
subgroup generated by the image of φ.
Example 3.1.26. The colimit of a sequence of sets and inclusions
X0 ,→ X1 ,→ X2 ,→ · · ·S
can be understood to be their union n≥0 Xn . For instance, a CW complex is a colimit of its
n-skeleta, the subspaces built by attaching disks of dimension at most n.
The forms of the universal properties that characterize limits and colimits are quite
useful when one is asked to define a map into a limit object or out of a colimit object.
Moreover, the uniqueness statements in these universal properties can be used to provethe commutativity of a diagram whose initial vertex is a colimit object or whose terminal
vertex is a limit object. We illustrate this in the simplest case, for products and coproducts,
but these remarks apply more generally.
Remark 3.1.27 (the universal properties of the product and the coproduct). In any category`
C with coproducts, maps f :i∈I Ai → X correspond naturally and bĳectively to I-indexedfamilies of maps ( fi : Ai → X)i∈I . The i0 -indexed component fi0 of the map f is defined`by restricting along the inclusion ιi0 : Ai0 → i∈I Ai that defines the i0 -indexed leg of thecolimit cone.
` / Q
C( i∈I Ai , X)i∈I C(Ai , X)f
− Xi∈I Ai →

`

fi

(Ai −
→ X)i∈I
Q
Dually, in any category with products, maps g : X → j∈J B j correspond naturally
and bĳectively to J-indexed families of maps (g j : X → B j ) j∈J .
C(X,
g

X→
−

Q

j∈J B j )Q

j∈J B j!



/ Q j∈J C(X, B j )!

(X −→ B j ) j∈Jgj

The j0 -indexed component g j0 of the map g is defined by composing with the projectionQ0
π j0 :
j∈J B j → B j0 that defines the j -indexed leg of the limit cone.
Combining these universal properties, to define a map from a coproduct to a product,`i∈IO Ai

f

π j0

ιi0

Ai0

/ Q j∈J B jf(i0 , j0 )
/ B j0it suffices, for each ordered pair (i0 , j0 ) ∈ I × J, to define a map f(i0 , j0 ) : Ai0 → B j0 . That is,a single map from a coproduct to a product is determined by a matrix of component maps
indexed by the product of the indexing sets.5
In certain categories, this matrix representation can be pushed further. The categories
Ab, ModR , or ChR have a notion of a zero homomorphism between any pair of objects.
5In the future, we adopt the same notation for the bounded variable that indexes a product or coproductand the variable that indexes a generic component so that we can avoid cluttering the text with primes: e.g.,`Qf : i∈I Ai → j∈J B j is given by component maps f(i, j) = π j · f · ιi : Ai → B j .

3.1. LIMITS AND COLIMITS AS UNIVERSAL CONES83

Given any finite collection of objects Ai , it makes sense then to consider the map· · · 0 
. 
..
. .. 
0


.. . .
..
.
. 0 
.

a
0 ···
0 1  YAi
Ai −−−−−−−−−−−−−−−−−−−→












1

0
..
.



i∈I

i∈I

encoded by the “identity matrix,” and in each of these categories, this map turns out tobe an isomorphism.6 In such contexts, the isomorphic finite coproducts and products
L`Qi∈I Ai i∈I Ai are written asi∈I Ai and referred to as direct sums.
Combining the universal properties of the product and the coproduct, all maps between
finite direct sums can be represented as matrices of component maps, and, as one mighthope, the composite matrix is defined by “matrix multiplication”:
Li∈I Ai

Lj( f(i, j) )

/L



% 
L

g( j,k) f(i, j)j∈J B j(g( j,k) )k∈K C kThe examples mentioned above serve to illustrate the pervasiveness of limits andcolimits among interesting mathematical objects, but how does one prove that a particular
diagram has a limit or colimit and construct one if it exists? There are two general strategies.
The first is to guess and check. A conjectured limit or colimit cone, such as displayed in(3.1.20), can often be shown to have the appropriate universal property via an elementaryargument.7
A second strategy is to construct a limit of a more complicated diagram out of limitsof simpler diagrams, whose constructions are easier to understand. In the next section, we
begin to explore this viewpoint in the category of sets.
Exercises.
Exercise 3.1.i. For a fixed diagram F ∈ CJ , describe the actions of the cone functors
Cone(−, F) : Cop → Set and Cone(F, −) : C → Set on morphisms in C.
Exercise 3.1.ii. For a fixed diagram F ∈ CJ , show that the cone functor Cone(−, F) isnaturally isomorphic to Hom(∆(−), F), the restriction of the hom functor for the category
CJ along the constant functor embedding defined in 3.1.1.
Exercise 3.1.iii. Prove that the category of cones over F ∈ CJ is isomorphic to the comma
category ∆ ↓ F formed from the constant functor ∆ : C → CJ and the functor F : 1 → CJ .
Argue by duality that the category of cones under F is the comma category F ↓ ∆.
Exercise 3.1.iv. Give a second proof of Proposition 3.1.7 by using the universal propertiesof each of a pair of limit cones λ : ` ⇒ F and λ0 : `0 ⇒ F to directly construct the unique
isomorphism `  `0 between their summits.
6The “identity matrix” defines an isomorphism between the finite coproduct and the finite product in any
abelian category; see Definition E.5.1.
7In the author’s experience, most proofs that a desired universal property is fulfilled in a particular category
proceed in this manner, by guess and check. The first step is to intuit that a particular construction might define
the sought-for object; the next is to verify that the universal property holds.

84

3. LIMITS AND COLIMITS

Exercise 3.1.v. Consider a diagram F : J → P valued in a poset (P, ≤). Use order-theoretic
language to characterize the limit and the colimit.
Exercise 3.1.vi. Prove that if
// B

f

/A

h

E

g

is an equalizer diagram, then h is a monomorphism.
Exercise 3.1.vii. Prove that if
P

/C

k

y

g

h


B /


/A

f

is a pullback square and f is a monomorphism, then k is a monomorphism.
Exercise 3.1.viii. Consider a commutative rectangle
•

/•


•

/ •

y

/•
/ •

whose right-hand square is a pullback. Show that the left-hand square is a pullback if andonly if the composite rectangle is a pullback.
Exercise 3.1.ix. Show that if J has an initial object, then the limit of any functor indexed
by J is the value of that functor at an initial object. Apply the dual of this result to describe
the colimit of a diagram indexed by a successor ordinal.
Exercise 3.1.x. If (a, b) are positive integers satisfying the universal property of (3.1.20),
show that the pair (−a, −b) also satisfies the same universal property. Explain why thisobservation does not imply that the pullback is ill-defined.
Exercise 3.1.xi. Use the universal property of the coproduct to prove that in any category
with at least one morphism between each ordered pair of objects, the legs of a coproductcone are split monomorphisms.
'
Exercise 3.1.xii. Suppose E : I −→J defines an equivalence between small categories
and consider a diagram F : J → C. Show that the category of J-shaped cones over F isequivalent to the category of I-shaped cones over FE, and use this equivalence to describe
the relationship between limits of F and limits of FE.

Exercise 3.1.xiii. What is the coproduct in the category of commutative rings?
3.2. Limits in the category of sets
While a limit can be defined for any diagram of any shape valued in any category,
the ability to state the definition is no guarantee that an object with that universal property
exists. In this section, we restrict consideration to diagrams valued in the category of sets
and give a constructive proof that a limit exists for any set-valued diagram that satisfies
an important size constraint. Special cases of this general construction give computationsof products, equalizers, pullbacks, and inverse limits of sequences of sets, the formulae
for which provide intuition for general results that facilitate the computation of limits andcolimits in more exotic categories.

3.2. LIMITS IN THE CATEGORY OF SETS

85

Definition 3.2.1. A diagram is small if its indexing category is a small category. A
category C is complete if it admits limits of all small diagrams valued in C and is cocomplete
if it admits colimits of all small diagrams valued in C.
A complete category is a category that “has all small limits.” To prove that Set iscomplete, consider a small diagram F : J → Set. A limit is a representation
Set(X, lim F)  Cone(X, F)of the functor that sends a set X to the set of cones over F with summit X. Specializing toa fixed singleton set 1, which represents the identity functor Set → Set,
(3.2.2)

lim F  Set(1, lim F)  Cone(1, F) .

The idea is to take (3.2.2) as a definition of the limit of the diagram F:
Definition 3.2.3. For any small diagram F : J → Set, define
lim F B Cone(1, F)
to be the set of cones over F with summit 1. Define the legs of the limit cone over F to be
the functions λ j : lim F → F j, indexed by the objects j ∈ J, that are defined by
λj

lim F
(3.2.4)µ: 1 ⇒ F/ Fj

7→ µ j : 1 → F j

That is, λ j is the function that carries a cone µ with summit 1 to its jth leg, regarded as anelement of the set F j .
Remark 3.2.5. The size hypothesis in Definition 3.2.3 is essential. If J is small, then SetJ
is locally small and, in particular, Cone(1, F), the collection of natural transformations fromthe constant diagram at 1 to F, is a set.
To prove that Set has all small limits, it remains to show that the data of Definition
3.2.3 has the desired universal property. The proof makes repeated use of the fact that
1 represents the identity functor on Set, allowing us to conflate elements x ∈ X andmorphisms x : 1 → X.
Theorem 3.2.6. The category Set is complete, admitting all small limits.
Proof. Our first task is to show that the functions (3.2.4) define a cone over F,
demonstrating that for each morphism f : j → k in J, the diagram
λj

Fj

lim F

{

Ff

λk

#

/ Fk

commutes. For any element µ : 1 ⇒ F of the set lim F,
F f (λ j (µ)) = F f (µ j ) = µk = λk (µ) ,where the middle equality holds because µ : 1 ⇒ F defines a cone. This proves that thefunctions (λ j : lim F → F j) j∈J define a cone over F.
To prove that λ : lim F ⇒ F is the universal cone, consider a cone ζ : X ⇒ F withgeneric summit. We must show that ζ factors uniquely through λ : lim F ⇒ F along afunction r : X → lim F. For each element x ∈ X, thought of as a function x : 1 → X,
there is a cone ζ x : 1 ⇒ F defined by restricting the cone ζ along the map x. Define

86

3. LIMITS AND COLIMITSr(x) ∈ lim F = Cone(1, F) to be this cone ζ x. By the definition of the legs of the limit coneλ,λ j (r(x)) = λ j (ζ x) = (ζ x) j = ζ j x ,so the cone ζ indeed factors along r through the cone λ. This calculation also reveals thatour definition r(x) = ζ x is necessary, proving uniqueness of the factorization and thereby
verifying that lim F indeed defines a limit of F.

We now apply Definition 3.2.3 to compute the limits of various diagrams of sets.
Example 3.2.7. By Theorem 3.2.6, the product of sets A j indexed by elements of some set
j ∈ J is the set of cones over this collection of sets with summit 1. A cone over a J-indexed
discrete diagram of sets with summit 1 is just a J-tuple of elements in the sets. Thus, thecategorical product in Set coincides with the usual cartesian productY
A j B {(a j ∈ A j ) j∈J } .
j∈J

Example 3.2.8. The terminal object in Set is the set of cones over the empty diagram withsummit 1. There is a unique such cone, proving that the terminal object is again a singleton
set.
Example 3.2.9. Given a parallel pair of functions f, g : X ⇒ Y, their equalizer is the set ofxmaps 1 →− X so that f x = gx. In more familiar notation, the equalizer is the set E definedby
E B {x ∈ X f (x) = g(x)} .
Example 3.2.10. Elements of the limit of a diagram F : ωop → Set are cones
1
···


···

x3

x2

/ F3

x0

x1

f32

(/

F2

f21

/* F 1

f10/+ F 0The data of such a cone is given by a tuple of elements (xn ∈ Fn )n∈ω making each triangle
commute. Thus, we see thatY
lim F B (xn )n ∈Fn fn,n−1 (xn ) = xn−1 .
nfg

Example 3.2.11. Elements of the pullback of a pair of functions B →− A←− C are cones
over this diagram with summit 1, i.e., are commutative squares
1
b


B

c

/C
g

f


/A

The data of a cone of this form consists of a pair of elements b ∈ B and c ∈ C so thatf (b) = g(c). Thus, the pullback is defined to be the set
B ×A C B {(b, c) ∈ B × C f (b) = g(c)} .

3.2. LIMITS IN THE CATEGORY OF SETS

87

Example 3.2.12. The limit of a left G-set X : BG → Set is the set of cones with summit
1. A map x : 1 → X defines a cone over X if and only if the diagram
x

X

1

x



g∗

/X

commutes for all g ∈ G, i.e., if and only if g · x = x for all g in G. Thus, lim X  XG , theset of G-fixed points of X.
Comparing Example 3.2.11 with Examples 3.2.7 and 3.2.9, it looks like the pullback
may be constructed as an equalizer of a diagram involving a product. Indeed, given a pairof functions f : B → A and g : C → A, the pullback B ×A C is the equalizer of the diagram
B ×A C /(b,c)7→ f (b)/ B×C(b,c)7→g(c)// A.

Similarly, Example 3.2.10 defines the limit of a tower to be a subset of a product. These
are special cases of a general theorem, which constructs the limit of any small diagram ofsets in an analogous manner.
Theorem 3.2.13. Any small limit in Set may be expressed as an equalizer of a pair of maps
between products. Explicitly, for any small F : J → Set, there is an equalizer diagram(3.2.14)

limJ F // Q Fjcj∈ob Jd//Q

F(cod f ).

f ∈mor J

Proof. By Theorem 3.2.6, elements of the limit of F : J → Set correspond to cones
with summit 1 over F. The data of such a cone consists of an element λ j in each set F j,indexed by the objects j ∈ J. The conditions that make the family of elements (λ j ) j∈J into
a cone over F are indexed by the (non-identity) morphisms in J: for each morphism f , wedemand that the following triangle commutes:
λdom f1yF(dom f )Ffλcod f%/ F(cod f )i.e.,

F f (λdom f ) = λcod f .

The data just described determines the domain of the equalizer diagram (3.2.14): (λ j ) j∈JQ
is an element of the product j∈ob J F j. The conditions determine the codomain and theparallel pair of maps. Note that the equality expressing the cone condition identifies a pairQof elements in f ∈mor J F(cod f ).
It remains to define the parallel pair of morphisms c and d of (3.2.14). An element
Q(λ j ) j ∈ j∈ob J F j—thought of as the legs of a cone with summit 1 over F—is sent by c toQ
the element (λcod f ) f ∈ f ∈mor J F(cod f ) and is sent by d to the element (F f (λdom f )) f ∈QQf ∈mor J F(cod f ). The equalizer is the subset ofj∈ob J F j for which these elements areequal, which is precisely the set of tuples of maps (λ j : 1 → F j) j∈J that satisfy the cone
compatibility conditions. This, together with our explicit descriptions of products andequalizers in Set given in Examples 3.2.7 and 3.2.9, completes the proof that limJ F is theequalizer of c and d.
Remark 3.2.15. The maps in the equalizer diagram of Theorem 3.2.13 can also be definedQ
categorically, using the universal property of the product f ∈mor J F(cod f ). By Remark
3.1.27, to define c and d it is necessary and sufficient to define each component function,

88

3. LIMITS AND COLIMITS

by which we mean the composites of c and of d with the projection π f . The components ofc are themselves projections, as displayed in the top triangle:
F(cod f )8 O

πcod flimJ F // Q Fjcj∈ob J

d

//

πf

Q

F(cod f )f ∈mor J

πdom fπfF(dom f )
/ F(cod f )FfThat is, the component of the map c at the indexing element f ∈ mor J is the projection
Qfrom the product j F j onto the component indexed by the object cod f ∈ J.
The component at f of the map d, displayed in the bottom square, is defined byQ
projecting from the product j F j onto the component indexed by the object dom f ∈ J
and then composing with the map F f : F(dom f ) → F(cod f ). Considering the action ofthese functions on elements, one can verify that the categorical descriptions agree with theexplicit ones described in the proof of Theorem 3.2.13.
Remark 3.2.15 is used to generalize the statement of Theorem 3.2.13 to categories
other than Set. By the principle of duality, this generalization will provide a formula forconstructing colimits as well; see Theorem 3.4.12.
Example 3.2.16. An idempotent is an endomorphism e : A → A of some object so thate · e = e. The limit of an idempotent in Set is the set of cones with summit 1, i.e., the setof a ∈ A so that ea = a. This is the set of fixed points for the idempotent e, often denotedby Ae .
Alternatively, applying Theorem 3.2.13 in the simplified form of Exercise 3.2.ii, thelimit Ae is constructed as the equalizer
Ae /

/A

s

1
e

// A

The universal property of the equalizer implies that e factors through s along a unique map
r.
A
e

r


Ae /

/A

s

1
e

// A

The factorization e = sr is said to split the idempotent. Now srs = es = s implies that rs
and 1Ae both define factorizations of the diagram
Ae
rs

s

1Ae

 
Ae /

/A

s

1
e

// A

Uniqueness implies rs = 1Ae so Ae is a retract of A. Conversely, any retract diagram
B /

s

/A

r

// B

rs = 1B

gives rise to an idempotent sr on A, which is split by B.

3.2. LIMITS IN THE CATEGORY OF SETS

89

Exercises.
Exercise 3.2.i. A small category can be redefined to be a particular diagram in Set. The
data is given by a pair of suggestively-named sets with functions
mor C o

dom
id
cod

/
/ ob C

together with a “composition function” yet to be defined. Use a pullback to define the set ofcomposable pairs of morphisms, which serves as the domain for the composition function,and formulate the axioms for a category using commutative diagrams in Set. When Set isreplaced by a category E with pullbacks, this defines a category internal to E.
Exercise 3.2.ii. Show that for any small diagram F : J → Set, the equalizer diagram(3.2.14) can be modified to yield a slightly smaller equalizer diagram:limJ F // Q Fjcj∈ob Jd//Q

F(cod f )f ∈mor J\ob J

in which the second product is indexed only by non-identity morphisms.
Exercise 3.2.iii. For any pair of morphisms f : a → b, g : c → d in a locally small category
C, construct the set of commutative squares

/c 
a











 f
g 
Sq( f, g) B 










 b
/d 
from f to g as a pullback in Set.
Exercise 3.2.iv. Generalize Exercise 3.2.iii to show that for any small category J, any
locally small category C, and any parallel pair of functors F, G : J ⇒ C, the set Hom(F, G)of natural transformations from F to G can be defined as a small limit in Set. (Hint: The
diagram whose limit is Hom(F, G) is indexed by a category J§ whose objects are morphisms
in J and which has morphisms 1 x → f and 1y → f for every f : x → y in J.)
Exercise 3.2.v. Show that for any small category J, any locally small category C, and any
parallel pair of functors F, G : J ⇒ C, there is an equalizer diagram
C(F j0 , G j0 )

F f∗

O

/ C(F j, G j0 )O

π j0Hom(F, G) // Q C(F j, G j)

πf//j∈ob JπjC(F j, G j)

Q

C(F j, G j0 )f : j→ j0 ∈mor J
πf

G f∗


/ C(F j, G j0 )Note that is not the equalizer diagram obtained by applying Theorem 3.2.13 to the diagram constructed in Exercise 3.2.iv. Rather, this construction gives a second formula for
Hom(F, G) as a limit in Set.
Exercise 3.2.vi. Proveto theR that the limit of any small functor F : C → Set is isomorphicR
set of functors C → F that define a section to the canonical projection Π : F → C fromthe category of elements of F. Using this description of the limit, define the limit cone.

90

3. LIMITS AND COLIMITS

3.3. Preservation, reflection, and creation of limits and colimits
Theorem 3.2.13 would be of somewhat limited interest if it only provided formulae forlimits of diagrams of sets. In §3.4, we prove the analogous formulae for limits or colimitsof diagrams valued in any category by reducing the proofs of these general results to thecase of limits in Set. This strategy succeeds because limits and colimits in a general locallysmall category are defined representably in terms of limits in the category of sets.
The precise meaning of the slogan “limits and colimits are defined representably using
limits of sets” is explained in the next section. First, we introduce language to describe avariety of possible ways in which a functor can mediate between the limits and colimits in
its domain and codomain categories.
Definition 3.3.1. For any class of diagrams K : J → C valued in C, a functor F : C → D• preserves those limits if for any diagram K : J → C and limit cone over K, the imageof this cone defines a limit cone over the composite diagram FK : J → D;
• reflects those limits if any cone over a diagram K : J → C, whose image upon applying
F is a limit cone for the diagram FK : J → D, is a limit cone over K; and• creates those limits if whenever FK : J → D has a limit in D, there is some limit coneover FK that can be lifted to a limit cone over K, and moreover F reflects the limits inthe class of diagrams.
Definition 3.3.1 dualizes to describe what it means for a functor to preserve, reflect,or create a class of colimits. The terms “preserves,” “reflects,” and “creates” are used
more generally. For instance, Exercise 1.5.iv shows that a fully faithful functor preserves,
reflects, and creates isomorphisms.
Remark 3.3.2. To ground intuition for these terms, the following metaphor may prove
helpful: a functor F : C → D maps from “upstairs” (the category C) to “downstairs” (thecategory D). An upstairs diagram (a functor K : J → C) sits above a downstairs diagram(by composing with F).
Now F preserves limits if any upstairs limit cone maps to a downstairs limit cone. Freflects limits if any upstairs cone that maps to a downstairs limit cone is also an upstairs
limit cone. Finally, F creates limits if the mere presence of a downstairs limit cone implies
the existence of an upstairs limit cone, and if any cone sitting above a limit cone is a limit
cone. The following diagram illustrates this transfer of information in the case of a pullback
diagram c0 → c ← c00 in C:
p

y

/ c00

p

/ c


c0

y


c0

F

preserves/ Fc00
Fp

Fp


Fc0


Fc0


D 3

f

C 3

y


/ Fc

/ c00

p

/ c


c0

reflects

/ c00
/ c

creates
/ Fc

d


/ Fc


Fc0

00

y

y

y

/ Fc00/ Fc

The context in which it is of greatest interest to have a functor F : C → D that creates
limits or colimits is when the codomain category D is already known to have the (co)limits
in question. In such cases, a functor that creates (co)limits both preserves and reflects them:3.3. PRESERVATION, REFLECTION, AND CREATION OF LIMITS AND COLIMITS

91

Proposition 3.3.3. If F : C → D creates limits for a particular class of diagrams in C andD has limits of those diagrams, then C admits those limits and F preserves them.
Proof. For any diagram K : J → C in the class, the hypothesis asserts that there is alimit cone µ : d ⇒ FK in D. As F creates these limits, there must be a limit cone λ : c ⇒ K
in C whose image under F is isomorphic to µ (because any two limit cones over a common
diagram are isomorphic; see Exercise 3.1.iv). This shows that C admits this class of limits.
To see that F preserves them, consider another limit cone λ0 : c0 ⇒ K. By Proposition
3.1.7, the two limit cones in C are isomorphic and by composing isomorphisms we see thatthe cone Fλ0 : Fc0 ⇒ FK is isomorphic to the limit cone µ : d ⇒ FK. This implies that
Fλ0 : Fc0 ⇒ FK is again a limit cone, proving that F preserves these limits.
Important families of limit-preserving functors appear in the next section and again
in the next chapter. For now, let us remark that limit-preservation can be an important
hypothesis:
Definition 3.3.4. Let X be a topological space and write O(X) for the poset of open
subsets, ordered by inclusion. An I-indexed family of open subsets Ui ⊂ U is said tocover U if the full diagram comprised of the sets Ui and the inclusions of their pairwiseintersections Ui ∩ U j has colimit U.8 A presheaf F : O(X)op → Set is a sheaf if it preserves
these colimits, sending them to limits in Set. Applying Theorem 3.2.13, the hypothesis isthat for any open cover {Ui }i∈I of U, the following is an equalizer diagram:
F(U) /

F(Ui ,→U)/ Q F(Ui )i∈I

F(Ui ∩U j ,→Ui )·πi
F(Ui ∩U j ,→U j )·π j// Q F(U ∩ U )i

ji, j∈IThe following lemmas introduce general classes of examples of functors that reflect or
create limits and colimits.
Lemma 3.3.5. Any full and faithful functor reflects any limits and colimits that are present
in its codomain.
Proof. Exercise 3.3.ii.

Lemma 3.3.6. Any equivalence of categories preserves, reflects, and creates any limits andcolimits that are present in its domain or codomain.
Proof. Exercise 3.3.iii.

With the exception of the equivalences, the functors that one meets in practice thatcreate certain limits and colimits tend to do so strictly, in the sense of the following
definition.
Definition 3.3.7. A functor F : C → D strictly creates limits for a given class of diagrams
if for any diagram K : J → C in the class and limit cone over FK : J → D,
• there exists a unique lift of that cone to a cone over K, and• moreover, this lift defines a limit cone in C.
A category J is connected if it is non-empty and if, in its underlying directed graph,every pair of objects is connected by a finite zig-zag of morphisms. A diagram is connected
if it is indexed by a connected category.
Proposition 3.3.8. For any object c ∈ C, the forgetful functor Π : c/C → C strictly creates`
8U is also isomorphic to the coproduct i∈I Ui in O(X), but to state the sheaf condition, it is necessary todescribe U as a colimit of the diagram that includes the pairwise intersections U j ←- Ui ∩ U j ,→ Ui .

92

3. LIMITS AND COLIMITS(i) all limits that C admits, and(ii) all connected colimits that C admits.
A generalization of this result appears as Exercise 3.4.iii.
Proof. To define a diagram (K, κ) : J → c/C is to define a functor K : J → C together
with a cone κ : c ⇒ K with summit c. We will show that if
K: J

Π

/ c/C

(K,κ)

/C

admits a limit cone in C, then this cone lifts to define a unique cone over (K, κ) in c/C andmoreover this cone is a limit cone.
Given a limit cone λ : ` ⇒ K in C, there is a unique factorization of the cone κ : c ⇒ Kthrough the limit cone along a map t : c → `. In the case where J is the parallel paircategory
f

a

g

// b,

this process is illustrated below:
`

∈ c/C
κa

c
κb

/ Ka
Kf

λa


Ka
Kf

Kg

 
Kb

∈C

κa

c

Kg

κb

 
Kb

>`


t
∃!

λa


/ Ka

Kf

∈ C.

Kg

 
Kb

It is straightforward to verify that the object t : c → ` is a limit for the original diagram in
c/C. By construction, Π : c/C → C preserves this limit.
Now suppose that J is connected and that K admits a colimit cone µ : K ⇒ p. We
will show that this data lifts to define a unique cone under (K, κ) in c/C that is moreover a
colimit cone. The first task is to lift the colimit object p along the functor Π : c/C → C.
For this, choose any object j ∈ J and define the lifted object to be the composite
c

κj

/ Kj

µj

/p

of the leg of the cone κ : c ⇒ K and the leg of the colimit cone µ : K ⇒ p. Because thecategory J is connected, the composite map µ j · κ j : c → p is independent of the choice ofj ∈ J. Now, given a cone under (K, κ) : J → c/C, the underlying cone in C factors uniquely
through p, and this factorization lifts to a unique factorization in c/C because the object c is“on the other side of the diagram” K and does not interact with the commutative condition
required to define the cone factorization. This proves that Π : c/C → C strictly creates
connected colimits.
A final result shows that functor categories inherit limits and colimits, defined “objectwise” in the target category: that is, given a J-indexed diagram in CA whose objects arefunctors F j : A → C, the value of the limit functor lim j∈J F j : A → C at an object a ∈ A
is the limit of the J-indexed diagram in C whose objects are the objects F j (a) ∈ C. This
result is analogous to familiar operations on functions: e.g., the sum of a finite collectionof abelian group homomorphisms f j : A → C is the homomorphism defined at a ∈ A byP
forming the sum j f j (a) in the abelian group C.
To state this result precisely, we consider the objects of a small indexing category A asa maximal discrete subcategory ob A ,→ A.

3.4. THE REPRESENTABLE NATURE OF LIMITS AND COLIMITS

93

Proposition 3.3.9. If A is small, then the forgetful functor CA → Cob A strictly creates all
limits and colimits that exist in C. These limits are defined objectwise, meaning that foreach a ∈ A, the evaluation functor eva : CA → C preserves all limits and colimits existingin C.
Proof. The functor category Cob A is isomorphic to the ob A-indexed product of thecategory C with itself, defined in 1.3.12; as the name would suggest, categorical products
define products in CAT. By the universal property of products, a diagram of shape J in aQ
product category ob A C is an ob A-indexed family of diagrams J → C. It is easy to seethat a limit of each of these component diagrams assembles into a limit for the diagram in
Q
Cob A  ob A C. In particular, Cob A has all limits or colimits that C does, and these arepreserved by the evaluation functors eva : Cob A → C.
To show that CA → Cob A strictly creates all limits and colimits, we must show that for
any F : J → CA the ob A-indexed family of objects lim j∈J F j (a) extends to a functor on A
valued in C. The universal property of the limit construction is used to define the actionof this functor on morphisms in A. The uniqueness statement in this universal property
then implies that this construction is functorial. The remaining details are left as Exercise
3.3.vi.

Exercises.
Exercise 3.3.i. For any diagram K : J → C and any functor F : C → D:(i) Define a canonical map colim FK → F colim K, assuming both colimits exist.
(ii) Show that the functor F preserves the colimit of K just when this map is an isomorphism.
Exercise 3.3.ii. Prove Lemma 3.3.5, that a full and faithful functor reflects both limits andcolimits.
Exercise 3.3.iii. Prove Lemma 3.3.6, that an equivalence of categories preserves, reflects,
and creates any limits and colimits that are present in either its domain or codomain.
Exercise 3.3.iv. Prove that F : C → D creates limits for a particular class of diagrams if
both of the following hold:(i) C has those limits and F preserves them.
(ii) F : C → D reflects isomorphisms.
Exercise 3.3.v. Show that the forgetful functors U : Set∗ → Set and U : Top∗ → Top failto preserve coproducts and explain why this result demonstrates that the connectedness
hypothesis in Proposition 3.3.8(ii) is necessary.
Exercise 3.3.vi. Prove that for any small category A, the functor category CA again has any
limits or colimits that C does, constructed objectwise. That is, given a diagram F : J → CA ,with J small, show that whenever the limits of the diagrams
J

F

/ CA

eva

/C

exist in C for all a ∈ A, then these values define the action on objects of lim F ∈ CA , a limitof the diagram F. (Hint: See Proposition 3.6.1.)
3.4. The representable nature of limits and colimits
The aim in this section is to explore the meaning of the remark made at the beginningof the preceding one, that all limits and colimits are defined representably in terms of limits
in the category of sets.

94

3. LIMITS AND COLIMITSFix a small diagram F : J → C in a locally small category C and an object X ∈ C, andconsider the composite functor
C(X, F−) B JF/ C C(X,−) / Seta diagram of shape J in Set. Theorem 3.2.6 tells us that the limit of C(X, F−) exists andTheorem 3.2.13 tells us how to construct it. An element in the set limJ C(X, F−) is anQelement of the product j∈J C(X, F j), i.e., a tuple of morphisms (λ j : X → F j) j∈J , subject
to some compatibility conditions. There is one condition imposed by each non-identity
morphism f : j → k in J, namely that the diagram
λj

Fj

X

~

Ff

λk

/ Fk

commutes. In summary, an element of limJ C(X, F−) is precisely a cone over F withsummit X; hence(3.4.1)

limJ C(X, F−)  Cone(X, F).9

Exercise 3.4.i demonstrates that this isomorphism is natural in X. Since the limit of F isdefined to be an object that represents the functor Cone(−, F), we conclude that:
Theorem 3.4.2. For any diagram F : J → C whose limit exists there is a natural isomorphism(3.4.3)C(X, limJ F)  limJ C(X, F−) .

The natural isomorphism (3.4.3) expresses the representable universal propertyof the limit: the limit of a J-indexed diagram in a locally small category C is defined
representably as the limit of J-indexed diagrams valued in Set.
Example 3.4.4. Iterated products of an object A ∈ C are called powers or cotensors. For aQ
set I, the I-indexed power of A is denoted I A or AI . The representable universal property
is expressed by the natural isomorphism
C(X, AI )  C(X, A)I ,i.e., a map h : X → AI is determined by an I-indexed family of maps hi : X → A defined bycomposing with each product projection i : AI → A. In the case of C = Set, this notation isconsistent with the exponential notation introduced previously: the power AI is isomorphic
to the set of functions from I to A, and the map i : AI → A evaluates each function at i ∈ I.
There are a number of interpretations of Theorem 3.4.2 that are worth stating explicitly:
Proposition 3.4.5. Covariant representable functors C(X, −) : C → Set preserve any
limits that exist in a locally small category C, sending them to limits in Set.
Proof. Theorem 3.4.2 asserts that the image under C(X, −) of a limit of a diagram Fof shape J in C is a limit in Set of the composite diagramJF/ C C(X,−) / Set .

9The size hypotheses are not essential for this isomorphism: for any diagram F : J → C, an element oflimJ C(X, F−) is precisely a cone with summit X over F. This limit is related to the limits considered in Exercises
3.2.iv and 3.2.v with some simplifications arising from the fact that a cone is a natural transformation involving a
constant functor.

3.4. THE REPRESENTABLE NATURE OF LIMITS AND COLIMITS

95

Moreover, the functor C(X, −) preserves the legs of the limit cone because, by construction,
the natural isomorphism of Theorem 3.4.2 commutes with the natural maps to the product:limJ C(X, F−) 

Cone(X, F)  C(X, limJ F)
Q &



x

j∈J C(X, F j)

The components of the left-hand diagonal map are the legs of the limit cone for the diagram
C(X, F−). The components of the right-hand diagonal are the images of the legs of thelimit cone for F under the functor C(X, −).
A second interpretation of Theorem 3.4.2 is that the contravariant functor C(−, lim F)
oprepresented by the limit of F : J → C, when considered as an object of the category SetC ,is the limit of the composite diagram
J

/C

F

/ SetCopywhose objects are the representable functors C(−, F j). Put more concisely, the Yoneda
opembedding y : C ,→ SetC preserves all limits that exist in C.
Of course, by the Yoneda lemma, the Yoneda embedding is full and faithful, so Lemma
op
3.3.5 implies that y : C ,→ SetC also reflects all limits. In summary:
Theorem 3.4.6. Let C be any locally small category.
(i) Covariant representable functors C(X, −) preserve all limits that exist in C.
op(ii) The covariant Yoneda embedding y : C ,→ SetC both preserves and reflects limits,i.e., a cone over a diagram in C is a limit cone if and only if its image defines a limit
opcone in SetC .
A representable characterization of colimits is obtained by dualizing the preceding
discussion. For fixed F : J → C and X ∈ C, consider the composite functor
C(F−, X) B Jop/ Cop C(−,X) / Set

F

Again, Theorem 3.2.6 tells us that a limit exists and Theorem 3.2.13 provides a construction.
Q
An element in the set limJop C(F−, X) is an element of the product j∈Jop C(F j, X), i.e., atuple of morphisms (λ j : F j → X) j∈J , subject to some conditions. There is one condition
imposed by each non-identity morphism f : j → k in J, namely that the diagram
Fj
λj

/ Fk

Ff

X

~

λk

commutes, so we see that an element of limJop C(F−, X) is precisely a cone under F withnadir X. The isomorphismlim
C(F−, X)  Cone(F, X)
opJ

is again natural in X proving:
Theorem 3.4.7. For any diagram F : J → C whose colimit exists, there is a natural
isomorphism
C(colimJ F, X)  limJop C(F−, X) .

96

3. LIMITS AND COLIMITS

Theorem 3.4.7 expresses the representable universal property of the colimit: thecolimit of a J-indexed diagram in a category C is defined representably as the limit ofJop -indexed diagrams valued in Set. For instance:
Example 3.4.8. For any triple of objects X, Y, Z in a locally small category C, the representable universal property of the coproduct takes the form of a natural isomorphism
C(X t Y, Z)  C(X, Z) × C(Y, Z) ,in which the set of maps out of a coproduct is expressed as the product of a pair of hom-sets.
This answers the puzzle posed in Exercise 2.4.x at the end of Chapter 2.
Example 3.4.9. Iterated coproducts of an object A ∈ C are called copowers or tensors.
`
For a set I, the I-indexed copower of A is denoted I A or I · A. The representable universal
property is expressed by the natural isomorphism`
C( I A, X)  C(A, X)I ,`i.e., a map h : I A → X is determined by an I-indexed family of maps hi : A → X defined`by composing with each coproduct inclusion ιi : A → I A. In the case of C = Set, the`copower I A is isomorphic to the set I × A.
Example 3.4.10. The representable universal property of a pushout
U
g


W

/V

f

h

k

p/ 
P

asserts that for any X ∈ C, the hom-set C(P, X) is defined by the pullback
C(P, X)

C(h,X)yC( f,X)

C(k,X)

C(W, X)/ C(V, X)

C(g,X)/ C(U, X)

Theorem 3.4.6 dualizes to:
Theorem 3.4.11. Let C be any locally small category.
(i) Contravariant representable functors C(−, X) carry colimits in C to limits in Set.
(ii) The contravariant Yoneda embedding y : Cop ,→ SetC both preserves and reflects
limits in Cop , i.e., a cone under a diagram in C is a colimit cone if and only if its
image defines a limit cone in SetC .
We now apply Theorems 3.4.6 and 3.4.11 to generalize Theorem 3.2.13, which constructs a generic limit in Set as an equalizer of a pair of maps between products, to arbitrary
categories and hence, by the principle of duality, also to colimits.
Theorem 3.4.12. The colimit of any small diagram F : J → C may be expressed as acoequalizer of a pair of maps between coproducts`f ∈mor J

F(dom f )dc// ` F j/ / colimJ F

j∈ob J

In particular, any category with coproducts and coequalizers is cocomplete. Dually, thelimit of any small diagram may be expressed as an equalizer (3.2.14) of a pair of maps
between products, and any category with products and equalizers is complete.

3.4. THE REPRESENTABLE NATURE OF LIMITS AND COLIMITS

97

Our proof will tacitly assume that the category C is locally small, but the argument
could be extended to large categories by “passing to a higher set-theoretical universe.”
Proof. Consider a diagram F : J → C. Dualizing the construction of Remark 3.2.15,
define maps d and cF(dom f )ιdom fx`
F j ooj∈ob JO

C oo(3.4.13)ιf
`F(dom f )f ∈mor J
O

d
c

ιf

ιcod fF(cod f ) oFf

F(dom f )between the coproducts indexed by the morphisms and objects of J, respectively: thecomponent of d at f ∈ mor J is defined to be the coproduct inclusion ιdom f , while thecomponent of c at f ∈ mor J is defined to be the composite of F f with the coproductinclusion ιcod f . By hypothesis, their coequalizer C exists in C. Our aim is to show that C
defines a colimit of F.
By Theorem 3.4.11(ii), the Yoneda embedding y : Cop → SetC carries the coequalizer
diagram (3.4.13) to an equalizer diagram in SetC
C(F(dom f ), X)

6

C(ιdom f ,X)

C(C, X) // C( ` F j, X)

C(d,X)
C(c,X)

j∈ob J

O

C(ι f ,X)// C (`

F(dom f ), X)f ∈mor J

C(ιcod f ,X)C(ι f ,X)C(F(cod f ), X)

C(F f,X)/ C(F(dom f ), X)

comprised of functors and natural transformations in the variable X. Applying Theorem
3.4.11(i) to each object in this equalizer diagram, we obtain an isomorphic diagram
C(F(cod f ), X)

5

πcod f(3.4.14)C(C, X) //Q

C(F j, X)j∈ob Jop

c
d

//

πf

Q

C(F(cod f ), X)f ∈mor Jop

πdom fC(F(dom f ), X)O

πf

C(F f,X)/ C(F(cod f ), X)

Note that C(−, X) carries the coproduct inclusion maps to product projection maps. Whenf is regarded as a morphism in Jop , its domain and codomain are exchanged, so we now
write “c” for the map C(d, X) and “d” for the map C(c, X).
When X is fixed, applying Proposition 3.3.9 to the evaluation functor evX : SetC → Set
tells us that (3.4.14) defines an equalizer diagram in Set. Applying Theorem 3.2.13 to the98

3. LIMITS AND COLIMITS

functor C(F−, X) : Jop → Set, we recognize (3.4.14) as the diagram whose equalizer
defines the limit limJop C(F−, X). Thus, we conclude that
limJop C(F−, X)  C(C, X)
for each X ∈ C. These isomorphisms, defined for each X ∈ C, assemble into an isomorphismin Setob C , from which point we apply Proposition 3.3.9 to the forgetful functor SetC →Setob C to conclude that C(C, −) is the limit of the Jop -indexed diagram of covariant functors
C(F j, −). Now Theorem 3.4.7 tells us that the coequalizer C is the colimit of F : J → C. 
Remark 3.4.15 (on generalized elements). A direct proof of Theorem 3.4.12 simply constructs the diagram (3.4.13) in C and checks that the coequalizer of c and d has the universal
property that defines colimJ F. There is a sense in which this direct proof is quite similar toour strategy—to first verify the result in Set and then apply representability—by somethingcalled the philosophy of generalized elements, first articulated by Lawvere [Law05].
Elements of a set A stand in bĳection with morphisms a : 1 → A in the category ofsets. By composition, morphisms f : A → B in the category of sets act on elements: thecomposite f a encodes the element f (a).10 In any category C, a generalized element ofan object A is a morphism a : X → A with codomain A. Morphisms f : A → B in C act
on X-shaped elements by composition: the composite f a encodes an X-shaped generalized
element of B. In this terminology, the contravariant represented functor
Hom(−, A) : Cop → Set
sends an object X to the set of X-shaped generalized elements of A. Since the Yoneda
opembedding C ,→ SetC is fully faithful, no information about A is lost by considering
instead the functor Hom(−, A) of generalized elements of A.
In particular, to show that a parallel pair of morphisms f, g : A ⇒ B are equal, it suffices
to show that f and g induce the same functions on generalized elements. Or to show thata cone with summit A is a limit cone, it suffices to show, for generalized elements of any
shape, that the induced functions define a limit cone in Set: this argument establishes therepresentable universal property of the limit. See [Lei08] for an application of this principleof “doing without diagrams” in more sophisticated contexts.
We conclude this section with an informal illustration of the philosophy of generalized
elements by showing that finite products and equalizers can be constructed out of a terminal
object 1 and (iterated) pullbacks.
Lemma 3.4.16. In any category with a terminal object 1, the pullback diagram
A×B
y
πA


A

πB

!

/B

/1

!

defines the product A × B of A and B.
Proof. It is straightforward to use the universal property of the displayed pullback to
πA
πB
verify that the diagram A ←−− A × B −−→ B has the universal property that defines a product.
The details are left as Exercise 3.4.iv.
Hence any category with pullbacks and a terminal object also has all binary, and by
iteration finite, products.
10The standard function notation motivates the usual “composition order.”3.5. COMPLETE AND COCOMPLETE CATEGORIES

99

Lemma 3.4.17. In any category with binary products, the pullback diagram
E
`


B

/A

e

y

E /

( f,g)(1B ,1B )


/ B×B

e

/A

f
g

// B

defines an equalizer of the parallel pair f, g : A ⇒ B.
Proof. The representable universal property of the product
Hom(X, B × B)  Hom(X, B) × Hom(X, B)
and of the pullback, in the form of Theorem 3.4.6(i), supply a pullback diagrame∗

Hom(X, E)y( f∗ ,g∗ )`∗
Hom(X, B)/ Hom(X, A)(1∗ ,1∗ )/ Hom(X, B) × Hom(X, B)

By the description of pullbacks in the category of sets given in Example 3.2.11, composition
with e and ` defines a bĳection between X-shaped generalized elements of E and pairs ofab

Hom(X, E) /e∗generalized elements (X →− A, X →− B) so that f a = b = ga. Thus, composing with edefines an isomorphism between Hom(X, E) and the subset of Hom(X, A) consisting of
those generalized elements a so that f a = ga. By Example 3.2.9, this tells us that
/ Hom(X, A)f∗g∗// Hom(X, B)

is an equalizer diagram. The conclusion follows from Theorem 3.4.6(ii).

A diagram is said to be finite if its indexing category contains only finitely manymorphisms. Lemmas 3.4.16 and 3.4.17 prove:
Theorem 3.4.18. Any category with pullbacks and a terminal object has all finite limits.
Dually, any category with pushouts and an initial object has all finite colimits.
Exercises.
Exercise 3.4.i. Show that the isomorphism (3.4.1) is natural in X.
Exercise 3.4.ii. Explain in your own words why the Yoneda embedding C ,→ SetC
preserves and reflects but does not create limits.

op

Exercise 3.4.iii. Generalize
Proposition 3.3.8 to show that for any F : C → Set, theRprojection functor Π : F → C:(i) strictly creates all limits that C admits and that F preserves, and(ii) strictly creates all connected colimits that C admits.
Exercise 3.4.iv. Prove Lemma 3.4.16.
3.5. Complete and cocomplete categories
Theorems, such as 3.4.12 and 3.4.18, that reduce the construction of general limits andcolimits to a handful of simple ones are particularly useful when it comes to proving thata category is complete or cocomplete—that is, has all small limits or colimits—or thata functor is continuous or cocontinuous—that is, preserves all small limits or colimits.

100

3. LIMITS AND COLIMITSThe proofs reduce to checking a few cases. In this section, we describe the constructionsof limits and colimits in various categories, in most cases leaving the proofs that these
constructions have the claimed universal properties to the reader.
Many of the concrete categories of our acquaintance are complete and cocomplete
including Set, Top, Set∗ , Top∗ , Cat or CAT, Group, Ab, Ring, ModR , among others. Here,
we restrict consideration to the first six cases. A result from categorical universal algebra
specializes to prove that the latter categories are complete and cocomplete (see Theorem
5.6.12).
In the first example, half the work is done already: Theorem 3.2.6 demonstrates that
Set is complete.
Proposition 3.5.1. The category Set is cocomplete.
Proof. By Theorem 3.4.12, it suffices to show that Set has coproducts and coequalizers. The former are given by disjoint unions, while the coequalizer of a parallel pair offunctions f, g : A ⇒ B is the quotient of the set B by the equivalence relation generated by
the relations f (a) ∼ g(a) for each a ∈ A.

Proposition 3.5.2. The category Top of spaces is complete and cocomplete.
Proof. The underlying set functor U : Top → Set is represented by the singleton
space ∗. Hence, Proposition 3.4.5 implies that if Top has any limits, they must be formed
by topologizing the limit of the diagram of underlying sets, as was argued in the case of thecartesian product in Example 3.1.10.
As the name suggests, the product of a family of spaces Xα is formed by givingQ
the cartesian product α Xα the product topology, which is the coarsest topology so thatQ
each of the projection functions πβ : α Xα → Xβ is continuous. The equalizer of apair of continuous functions f, g : X ⇒ Y is given by assigning the equalizer E ⊂ X of
the underlying functions the subspace topology, which also may be characterized as thecoarsest topology on E so that the inclusion E ,→ X is continuous.
Dually, we might guess that colimits in Top are formed by topologizing the colimit ofthe diagram of underlying sets; Theorem 4.5.3 implies that the colimit must be constructed`in this manner. The coproduct is formed by assigning the disjoint union α Xα the finest`
topology so that each of the coproduct-inclusions ιβ : Xβ → α Xα is continuous. The`
disjoint union of any bases for the spaces Xα defines a basis for α Xα . The coequalizer ofa pair of continuous functions f, g : X ⇒ Y is constructed by first forming the set-theoretic
quotient Y/∼ of Y under the equivalence relation generated by the relations f (x) ∼ g(x) for
each point x ∈ X. The set Y/∼ is then given the quotient topology, under which a subset inY/∼ is open if and only if its preimage in Y is open; this is the finest topology so that thequotient map Y  Y/∼ is continuous.
Theorem 3.4.12 and its dual now imply that Top has all small limits and colimits. The
constructions described here demonstrate moreover that the forgetful functor U : Top → Set
preserves them.

Example 3.5.3. Any limit or colimit in Top can be constructed in the way prescribed in
the proof of Proposition 3.5.2: by forming the limit or colimit of the diagram of underlying
sets and then topologizing the resulting space with the coarsest (in the case of limits) orfinest (in the case of colimits) topologies so that the legs of the limit or colimit cones arecontinuous.
For instance, consider the diagram ωop → Top whose objects are circles S 1 and inwhich each generating map is the “pth power map,” the covering map that wraps the domain

3.5. COMPLETE AND COCOMPLETE CATEGORIES

101

circle uniformly p times around the codomain circle:
···

/ S1

p

/ S1

p

/ S1

p

/ S1

The inverse limit defines the p-adic solenoid.
Example 3.5.4. The space of (unordered) configurations of n points in a space X is the
topological space whose points are collections of n distinct points in X and whose topology
considers two configurations to be “close” if there is a bĳection between their points so thateach pair of points is “close” to each other.
More formally, the space of ordered configurations of n points is a subspace of the
product space X n , namely the complement of the “fat diagonal”
PConfn (X) B (x1 , . . . , xn ) ∈ X n xi , x j ∀i , j .
The symmetric group Σn acts on X n by permuting the coordinates, and this action restricts
to the subspace PConfn (X) ⊂ X n . The colimit of the diagram PConfn (X) : BΣn → Top
defines the space of configurations of n points
PConfn (X)Confn (X) B colim BΣn −−−−−−−→ Top .
Exercise 3.5.i describes its underlying set.
Completeness and cocompleteness of Set and of Top implies the same for Set∗ andTop∗ as a special case11 of a general result:

Proposition 3.5.5. If C is complete and cocomplete, then so are the slice categories c/C
and C/c for any c ∈ C.
Proof. The statements are dual, so it suffices to prove the result for c/C. By Proposition
3.3.8, if C is complete and cocomplete, then c/C has all limits and all connected colimits,created by the forgetful functor Π : c/C → C. Thus, c/C is complete and to prove
cocompleteness it remains only to show that c/C has coproducts. By the construction ofLemma 3.4.16, generalized to higher arity coproducts, it suffices to show that c/C has an
initial object, which it does: the identity 1c is initial.
Note that this proof also tells us how the coproduct is constructed: the coproduct of afamily of objects c → xα is the map from c to the colimit of the diagram defined by thesemorphisms in C (an “asterisk” with source c and all the morphisms pointing out).12
Next we consider limits of diagrams of categories. Examples 2.1.5(ix) and (x) revealthat the functors ob, mor : Cat ⇒ Set are represented by the ordinal categories 1 and 2. By
Theorem 3.4.6(i), both functors preserve limits, which means that if the limit of a diagramof categories exists, then its set of objects must be the limit of the underlying diagramsof object-sets and its set of morphisms must be the limit of the underlying diagrams ofmorphism-sets. Furthermore, domains, codomains, and identities are also preserved, as
each of these maps is expressible, by Exercise 2.1.i, as a natural transformation between
representable functors. A similar argument involving the category 3 proves that composition
is preserved as well. These observations suggest candidates for the product of any categories
and for the equalizer of any parallel pair of functors, allowing us to prove that:
Proposition 3.5.6. The categories Cat and CAT are complete.
11Exercise 3.3.v explains that Set∗ and Top∗ are slice categories.
R
12Particularly when C is concrete and c represents the underlying set functor U : C → Set, so that c/C  U
is a category of “based objects,” the coproduct is typically written with the symbol “∨” in place of the usual “t”;
see Example 3.1.24 for instance.

102

3. LIMITS AND COLIMITSProof. Higher arity versions of the product, introduced in Definition 1.3.12, define
products of categories. The equalizer E of a pair of parallel functors
E /

/C

// D

F
G

is the subcategory of C consisting of those objects c ∈ C so that Fc = Gc and thosemorphisms f so that F f = G f . Applying Theorem 3.4.12, it follows that Cat and CAT
admit all small limits.

Example 3.5.7. For any functor F : C → Set, the pullback in CAT ofC/ Set o

F

U

Set∗

is a category whose objects are pairs c ∈ C and (X, x) ∈ Set∗ so that Fc = X. Morphisms
are pairs f : c → c0 and g : (X, x) → (X 0 , x0 ) so that F f = Ug. Expressing this data more
efficiently, we see that the pullbackR/ Set∗
F
y

C

U

F


/ Set

R

is the category F of elements of F.13
Both Cat and CAT are also cocomplete. Coproducts in each case are straightforward:
any category is uniquely expressible as a coproduct of its connected components. By
Lemma 3.4.17, it suffices to construct pushouts. But these can be rather more complicatedthan was the case for the corresponding limit notions.
Example 3.5.8. To calculate the pushout,
1t1 /

/2


1

p/ 
P

it helps to reflect on its universal property. A functor P → C corresponds to a cone under
the diagram 1 ← 1 t 1 → 2 with nadir C. This data defines an endomorphism f in C,a morphism 2 → C so that its domain and codomain define the same object 1 → C. In
defining P, we must take care not to impose any relations on the composite endomorphisms
f · f · · · f . Thus, P must be the free category with one object and one non-identity morphism.
Recalling that one-object categories can be identified with monoids, we find that P is quite
familiar: it is the monoid N of natural numbers under addition.
Similarly, the group Z of integers under addition can be constructed as a pushout
/I
1t1 /

1

p/ 
BZ

in which the walking arrow 2 is replaced by the walking isomorphism I, the category with
two objects and two non-identity morphisms pointing in opposite directions, which are
inverse isomorphisms.
R
13The pullback of a contravariant functor F : Cop → Set along U : Set∗ → Set defines the opposite of F.

3.5. COMPLETE AND COCOMPLETE CATEGORIES103

The proof of cocompleteness of Cat is deferred to Corollary 4.5.16, where it is deduced
as a special case of a general result that gives an explicit construction of the colimit of any
diagram of categories. The rough idea is to embed the category of categories into a larger
category in which colimits are more easily understood and then “reflect” the colimits fromthe larger category back into Cat.
Before considering a final class of examples, note that completeness and cocompleteness of a category are equivalence-invariant. In particular, a preorder is complete andcocomplete if and only if its skeleton, a poset, is so.
Proposition 3.5.9. A poset P is complete and cocomplete as a category if and only if it is
a complete lattice, that is, if and only if every subset A ⊂ P has both an infimum (greatest
lower bound) and a supremum (least upper bound).
Proof. In any poset (or preorder), a limit of a diagram is an infimum of its objects,
while a colimit of a diagram is a supremum of its objects. Whether or not there are anymorphisms in the diagram makes no difference because all diagrams in a preorder commute.
In particular, any collection of morphisms with common domain defines a cone over any
diagram whose objects are indexed by the codomains.
With this work behind us, it is reasonable to ask: What are all these limits and colimits
good for? One use of a handful of simple limit constructions is to define a notion ofequivalence relation internally to a general category. Other significant applications willappear in §4.6, where limits and colimits are used to define functors.
Example 3.5.10. In any category with finite limits, the kernel pair14 of a morphism
f : X → Y is the pullback of f along itself:
R

/X

t

y

s


X


/Y

f

f

These maps define a monomorphism (s, t) : R  X × X, so the object R is always asubobject of the product X × X.15 In Set, a subset R ⊂ X × X defines a relation on
X. Indeed, subobjects defined by kernel pairs are always equivalence relations, in thefollowing categorical sense:(i) There is a reflexivity map ρ defined by
X

1X

ρ


1X

R

t

y

s

 
X

f

"

/X

/Y

f

14In the absence of a zero object—an object that is both initial and terminal, which can be used to define azero map between any pair of objects—kernel pairs are the best analog of kernels. Barry Fawcett acknowledges
the pervasiveness of this sort of construction, writing “Category theory tells us: to learn about A, study pairs ofmaps out of [or into] A” [Faw86].
15A subobject of an object c is a monomorphism with codomain c; see Definition 4.6.8.

104

3. LIMITS AND COLIMITS

that is a section of both s and t, i.e., that defines a factorization of the diagonal(1X ,1X )

X

%

ρ

7

R

/ X×X
7
(s,t)

(ii) There is a symmetry map σ defined by
R
s

σ



R

t

"
/X

t

y

s

 
X


/Y

f

f

so that tσ = s and sσ = t.
(iii) There is a transitivity map τ whose domain is the pullback of t along s/Rt˜

R ×X R
y

t

/X

s

s̃


R


/X

t

s


X

This diagram defines a cone over the pullback defining R and thus induces a map
R ×X R

tt˜

τ

"
s s̃

R

t

y

s

 
X

f

/# X

/Y

f

so that sτ = s s̃ and tτ = tt˜.
An equivalence relation in a category C with finite limits is a subobject (s, t) : R  X × X
equipped with maps ρ, σ, and τ commuting with the morphisms s and t in the mannerdisplayed in the diagrams of (i), (ii), and (iii). When it exists, the coequalizer of the mapss, t : R ⇒ X of an equivalence relation defines a quotient object e : X  X/R . In Set, X/R
is the set of R-equivalence classes of elements of X. For equivalence relations arising as
kernel pairs, there is a unique factorization
/Y
>

f

X
e

m

X/R

3.5. COMPLETE AND COCOMPLETE CATEGORIES

105

In good situations, such as when C is a Grothendieck topos (see §E.4), the map m is amonomorphism and this factorization defines the image factorization of the map f : themonomorphism X/R  Y identifies the image X/R as a subobject of Y.
Exercises.
Exercise 3.5.i. Let G be a group regarded as 1-object category BG. Describe the colimitof a diagram BG → Set in group-theoretic terms, as was done for the limit in Example
3.2.12.
Exercise 3.5.ii.
R Prove that the colimit of any small functor F : C → Set is isomorphic
to the set π0 ( F) of connected components of the category of elements of F. What is thecolimit cone?
Exercise 3.5.iii. Prove that the category DirGraph of directed graphs is complete andcocomplete and explain how to construct its limits and colimits. (Hint: Use Proposition
3.3.9.)
Exercise 3.5.iv. For a small category J, define a functor i0 : J → J × 2 so that the pushout
J

!

/1
s

i0

p/  /J
J×2

in Cat defines the cone over J, with the functor s : 1 → J/ picking out the summit object.
Remark 3.1.8 gives an informal description of this category, which is used to index thediagram formed by a cone over a diagram of shape J.
Exercise 3.5.v. Describe the limits and colimits in the poset of natural numbers with theorder relation k ≤ n if and only if k divides n.
op

Exercise 3.5.vi. Define a contravariant functor Finmono → Top from the category of finite
sets and injections to the category of topological spaces that sends a set with n elements
to the space PConfn (X) constructed in Example 3.5.4. Explain why this functor does not
induce a similar functor sending an n-element set to the space Confn (X).
Exercise 3.5.vii. Following [Gro58], define a fiber space p : E → B to be a morphismin Top. A map of fiber spaces is a commutative square. Thus, the category of fiber
spaces is isomorphic to the diagram category Top2 . We are also interested in the non-fullsubcategory Top/B ⊂ Top2 of fiber spaces over B and maps whose codomain component
is the identity. Prove the following:(i) A map
E0

g

p0


B0

f

/E

/B

p

of fiber spaces induces a canonical map from the fiber over a point b ∈ B0 to the fiber
over its image f (b) ∈ B.
(ii) The fiber of a product of fiber spaces is the product of the fibers.
A projection B × F → B defines a trivial fiber space over B, a definition that makes sense
for any space F.
(iii) Show that the fiber of a trivial fiber space B × F → B is isomorphic to F.

106

3. LIMITS AND COLIMITS(iv) Characterize the isomorphisms in Top/B between two trivial fiber spaces (with apriori distinct fibers) over B.
(v) Prove that the assignment of the set of continuous sections of a fiber space over B
defines a functor Sect : Top/B → Set.
(vi) Consider the non-full subcategory Top2pb of fiber spaces in which the morphisms are
the pullback squares. Prove that the assignment of the set of continuous sections toa fiber space defines a functor Sect : (Top2pb )op → Set.
(vii) Describe the compatibility between the actions of the “sections” functors just introduced with respect to the map g of fiber spaces p and q over B and their restrictions
along f : B0 → B.
/E

E 0y
p

0

 
B0

g



/F

F 0y
p

q0
f

q

 
/B

3.6. Functoriality of limits and colimits
Proposition 3.1.7 demonstrates that limits and colimits are each well-defined up tounique isomorphism when they exist. In this section, we establish the functoriality of theconstruction of limits or colimits of diagrams of a fixed shape, and explore its consequences.
An important point is that functoriality of the limit is not the same as canonicity. In mostcases, there are many isomorphic objects that define the limit, and there is seldom areason to prefer any one to the others. To illustrate this point, we present an example that
demonstrates that even when limit objects exist uniquely, as is the case for instance in askeletal category, it does not necessarily follow that canonical isomorphisms induced by
the universal properties of these objects are identities.
Proposition 3.6.1. If C has all J-shaped limits, then a choice of a limit for each diagram
defines the action on objects of a functor limJ : CJ → C.
Note that this functor is not canonically defined but rather requires an arbitrary choiceof a limit to serve as “the” limit for each diagram.
Proof. Choose a limit and a limit cone for each diagram F ∈ CJ . This defines theaction of limJ : CJ → C on objects. It remains only to define the action of limJ : CJ → C
on morphisms. The vertical composite of a natural transformation α : F ⇒ G and the limit
cone λ : limJ F ⇒ F for F defines a conelimJ F

λ

+3 F

α

+3 G

with summit limJ F over G. This cone factors uniquely through the chosen limit conelimJ G ⇒ G for the diagram G via a map between their summits, which we take to be
limJ α : limJ F → limJ G. To illustrate, a morphism in C•→•←• is a pair of solid-arrow3.6. FUNCTORIALITY OF LIMITS AND COLIMITS

107

commutative squares as displayed:
/•

•y


•

/•

•y

 
•

/ •
/ • 

Choosing pullbacks for the top and bottom diagrams, with the legs of the pullback cones
displayed in dashes, there is a unique induced dotted arrow morphism between their summits, which defines the action of the functor C•→•←• → C on this morphism. Uniqueness of
the universal properties implies that this construction is functorial; see Exercise 3.6.i. 
Example 3.6.2. Recall the functor Path : Top → Set that carries a space X to the set
Path(X) B Top(I, X), where I is the standard unit interval. Precomposing with the endpoint
inclusions 0, 1 : ∗ ⇒ I defines a functor P : Top → Set•⇒• that carries a space X to theparallel pair of functions
ev0

Path(X)  Top(I, X)

ev1// Top(∗, X)  Point(X)that evaluate a path at its endpoints. Their coequalizer// Point(X)ev0

Path(X)ev1/ / π0 X

defines the set of path components of X, the quotient of the set of points in X by the
relation that identifies any pair of points connected by a path. Proposition 3.6.1 tells us that
this coequalizer defines a functor, the path components functor
π0 B TopP/ Set•⇒•

colim /

Set.

More generally, the construction of Proposition 3.6.1 implies that a natural transformation between diagrams gives rise to a morphism between their limits or between theircolimits, whenever these exist. Whether the codomain category has all limits or colimitsof that shape is irrelevant. Moreover, whenever the natural transformation is a natural
isomorphism, the induced map between the limits or colimits is an isomorphism.
Corollary 3.6.3. A natural isomorphism between diagrams induces a naturally-defined
isomorphism between their limits or colimits, whenever these exist.
Proof. The proof of Proposition 3.6.1 defines a limit functor from the full subcategoryof CJ spanned by those functors admitting limits in C. This functor, like all functors,preserves isomorphisms (Lemma 1.3.8).
The main computational motivation for Eilenberg and Mac Lane’s formulation of theconcept of naturality was to be able to carefully and correctly formulate this result. While
the general theory of limits and colimits had not yet been developed, [EM45, 21.2] proves
Corollary 3.6.3 in the special case of directed diagrams. Indeed, naturality is a necessary
condition in Corollary 3.6.3:
Example 3.6.4. For any group G, a pair of G-sets X, Y : BG ⇒ Set are objectwise isomorphic just when their underlying sets are isomorphic. The functors X and Y are naturallyisomorphic just when there exists an equivariant isomorphism, that is, a bĳection X −
→Y

108

3. LIMITS AND COLIMITS

that commutes with the actions by each g ∈ G. This second condition is stronger than thefirst.
For instance, consider the group Z/2 and the Z/2-sets 2, the two-element set with atrivial Z/2-action, and 20 , the two-element set where the generator of Z/2 acts by exchanging
the two elements. We may regard 2 and 20 as functors BZ/2 → Set from which perspective
they are objectwise isomorphic (because both sets have two elements) but not naturally
isomorphic (because there is no isomorphism that commutes with the Z/2-actions).
The limit of a functor X : BG → Set is the set of G-fixed points (see Example 3.2.12)
and the colimit is the set of G-orbits (by Exercise 3.5.i). In the case of our pair of Z/2-sets,lim 2 has two elements while lim 20 is empty, and colim 2 has two elements while colim 20
is a singleton.
Example 3.6.5. To a finite set X, we can associate the set Sym(X) of its permutations or
the set Ord(X) of its total orderings. Both sets have |X|! elements. These constructions arefunctorial, not with respect to all maps of finite sets, but with respect to the bĳections: thefunctor Sym : Finiso → Fin acts by conjugation and the functor Ord : Finiso → Fin acts bytranslation. However, the objectwise isomorphism Sym(X)  Ord(X) is not natural, andindeed limits or colimits of restrictions of these diagrams need not be isomorphic.
Combinatorialists refer to a functor F : Finiso → Fin as a species.16 The image F(n)of the n-element set n is the set of labeled F-structures on n. The set of unlabeled
F-structures on n is defined by restricting Finiso to the full subcategory spanned by then-element set, i.e., to the symmetric group Σn regarded as the group of automorphisms of
the object n ∈ Finiso , and forming the colimit of the diagramF

BΣn ,→ Finiso −→ Fin .

Because Ord and Sym are objectwise isomorphic, the sets of labeled Sym-structures andlabeled Ord-structures are isomorphic. However, the set of unlabeled Sym-structures on nis the set of conjugacy classes of permutations of n-elements, while the set of unlabeled
Ord-structures on n is trivial: all linear orders on n are isomorphic. See [Joy81] for more.
A discussion of the functoriality of chosen limits and colimits should come with a few
warnings. Limits, when they exist, are unique up to a unique isomorphism commuting withthe maps in the limit cone. But this is not the same thing as saying that limits are uniqueon the nose: even if there is a unique object satisfying the universal property of the limit,
each of its automorphisms gives rise to a distinct limit cone. Moreover, choices of limitsof diagrams of fixed shape, as required to define the limit functor of Proposition 3.6.1,
can seldom be made compatibly. For instance, Freyd has shown that in any category withpullbacks, it is possible to choose canonical pullbacks so that the “horizontal” composite
of canonical pullback squares, in the sense of Exercise 3.1.viii, is the canonical pullbackof the composite rectangle; however, it is not possible to also arrange so that the “vertical”
composites of canonical pullback squares are canonical pullbacks [FS90, §1.4].
Even when the chosen limit objects are equal, the various natural isomorphisms associated to the limit constructions might not be identities. To illustrate this, first observe:
Lemma 3.6.6. For any triple of objects X, Y, Z in a category with binary products, there isa unique natural isomorphism X × (Y × Z)  (X × Y) × Z commuting with the projections
to X, Y, and Z.
Proof. Exercise 3.6.ii.
16In this context, “species” is singular.

3.7. SIZE MATTERS

109

Lemma 3.6.6 asserts that the product is naturally associative. It follows that any
iteration of binary products can be used to define n-ary products. However, even in askeletal category, in which the objects X × (Y × Z) and (X × Y) × Z are necessarily equal,
the natural isomorphism may not be the identity. The following example, from [ML98a,
p. 164], is due to Isbell.
Example 3.6.7. Consider sk(Set), a skeletal category of sets. Since sk(Set) is equivalent
to a complete category it has all limits and, in particular, has products. Let C denote thecountably infinite set. Its product C × C = C and the product projections π1 , π2 : C → C
are both epimorphisms. Suppose the component of the natural isomorphism C × (C × C) 
(C × C) × C were the identity. Naturality would then imply that for any triple of maps
f, g, h : C → C that f × (g × h) = ( f × g) × h, on account of commutativity of the square(C × C) × C

C × (C × C)

( f ×g)×h

f ×(g×h)(C × C) × C


C × (C × C)

Now a pair of maps between products are equal if and only if their projections onto
components are equal.
// C
O

g×h

CO

h

π2

π2

// C × C

f ×(g×h)

C ×C( f ×g)×h
π1


C

f
f ×g


// C

π1

Because π1 and π2 are epimorphisms, this implies that f = f × g and g × h = h. So weconclude that any pair of maps f, g : C ⇒ C must be equal, which is absurd. It follows thatthe component of the natural associativity isomorphism C × (C × C)  (C × C) × C is not
the identity.
Exercises.
Exercise 3.6.i. In a category C with pullbacks, prove that the mapping lim : C•→•←• → C
defined in the proof of Proposition 3.6.1 is functorial.
Exercise 3.6.ii. Prove Lemma 3.6.6.
3.7. Size matters
Recall a category is complete if it admits a limit for any diagram whose domain isa small category. In this section, we explain the reason for this size restriction. As awarm-up, the following lemma highlights a commonly existing large limit. Somewhat
peculiarly, an initial object, which is defined to be the colimit of the empty diagram, mayalso be characterized as the limit of the identity functor:
Lemma 3.7.1. For any category C, the identity functor 1C : C → C admits a limit if andonly if C has an initial object.
Proof. Consider a limit cone (λc : ` → c)c∈C . The legs of the limit cone demonstratethat ` is a weakly initial object, admitting at least one morphism to every object c ∈ C. To
show that λc is the only morphism from ` to c, observe that for any morphism f : ` → c,

110

3. LIMITS AND COLIMITS

commutativity of the limit cone implies that f λ` = λc . So to prove that ` is initial, it suffices
to show that the cone leg λ` : ` → ` is the identity.
Considering λc as a morphism in the diagram 1C , the cone condition tells us thatλc λ` = λc . From this equation, we see that λ` defines a factorization of the cone λ : ` ⇒ 1C
through itself. By uniqueness of such factorizations, we conclude that λ` = 1` , as desired.
The verification that an initial object defines a limit of 1C is left as Exercise 3.7.i. 
By the size restrictions inherent in the notion of completeness, a complete (large)
category need not have an initial object. As it turns out, complete small categories are hard
to come by. To explain this, we require a precise definition of the cardinality of a small
category.
Definition 3.7.2. The cardinality of a small category is the cardinality of the set of its
morphisms. A category whose cardinality is less than κ is called κ-small.
A κ-small diagram is one whose indexing category is κ-small. The following result
explains why categories tend only to have limits or colimits of diagrams of smaller size.
Proposition 3.7.3 (Freyd). Any κ-small category that admits all κ-small limits or all
κ-small colimits is a preorder.
This result remains true under less restrictive completeness hypotheses, which thereader will have no trouble formulating.
Proof. Let λ be the cardinality of the set of morphisms in a κ-small category C, andsuppose there exists a parallel pair f, g : B ⇒ A of morphisms with f , g. By the universal
property of the λ-indexed power (see Example 3.4.4), there are 2λ distinct morphisms
B → Aλ whose components are either f or g. By Cantor’s diagonalization argument,|C(B, Aλ )| ≥ 2λ > λ = | mor C| ,contradicting the fact that C(B, Aλ ) is a subset of mor C. If C instead has κ-small copowers,
then a dual construction yields a similar contradiction.
Proposition 3.5.9 reveals that it is possible for a preorder to have all limits or colimits
of unrestricted size: a poset P has this property as a category if and only if it is a complete
lattice.
Exercises.
Exercise 3.7.i. Complete the proof of Lemma 3.7.1 by showing that an initial object
defines a limit of the identity functor 1C : C → C.
3.8. Interactions between limits and colimits
Given a functor of several variables, it is natural to consider limits and colimits formed
separately in each variable. Assuming these objects exist, it follows from their universal
properties that limits commute with limits and colimits commute with colimits. Because
the universal properties of limits and colimits have opposite handedness, in the few cases
where limits and colimits commute with each other, this tends not to follow from formal
abstract nonsense, but instead most likely has something to do with how these objects areconstructed in the particular category in question.
To explore these issues, consider a functor F : I × J → C of two variables, which, by
Exercise 1.7.vii, may also be regarded as a functor F : I → CJ or as F : J → CI . If foreach i ∈ I, lim j∈J F(i, j) exists, then by Proposition 3.3.9 these objects define the values3.8. INTERACTIONS BETWEEN LIMITS AND COLIMITS

111

of a diagram lim j∈J F(−, j) : I → C. The following theorem proves that the limit of this
I-indexed diagram is isomorphic to the object obtained by exchanging the roles of I and J.
Theorem 3.8.1. If the limits limi∈I lim j∈J F(i, j) and lim j∈J limi∈I F(i, j) associated to a
diagram F : I × J → C exist in C, they are isomorphic and define the limit limI×J F.
To illustrate, suppose I = • ⇒ • is the category that indexes equalizer diagrams andJ = • → • ← • is the category that indexes pullbacks. Then Theorem 3.8.1 says that thelimit of a diagram indexed by the category
I×J=

•


•

/ •

•


/  
••may be formed either by first forming the two pullbacks and then taking the equalizer ofthe induced maps between them, as displayed below-left
/•
•
•y



•y



/•


/ 

•y


•


•

•

/ •

/ •



•




•

/  •

•/  
••/  
••or by taking the equalizers of the three parallel pairs, and then forming the pullback of theinduced maps between them, as displayed above-right.
Proof of Theorem 3.8.1. By the Yoneda lemma, it suffices to prove that
C(X, lim lim F(i, j))  C(X, lim F(i, j))  C(X, lim lim F(i, j)) .
i∈I(i, j)∈I×J

j∈J

j∈J

i∈I

By Theorem 3.4.6(i), covariant representable functors preserve limits:
C(X, lim lim F(i, j))  lim C(X, lim F(i, j))  lim lim C(X, F(i, j)) .
i∈I

j∈J

i∈I

j∈J

i∈I

j∈J

Arguing similarly for the other limits under consideration, the general statement reduces
to the case of proving that for any set-valued functor H : I × J → Set, in particular for thefunctors C(X, F−), the limits
lim lim H(i, j)  lim H(i, j)  lim lim H(i, j)
i∈I

j∈J

(i, j)∈I×J

j∈J

i∈I

are isomorphic. On account of the isomorphism of categories I × J  J × I, it suffices toprove the left-hand isomorphism.
By Definition 3.2.3, the set lim(i, j)∈I×J H(i, j) is the set of cones with summit 1 over the
I × J-indexed diagram H. The set limi∈I lim j∈J H(i, j) is the set of cones with summit 1 overthe I-indexed diagram lim j∈J H(−, j). Such a cone is comprised of legs!
λi
1 −→ lim H(i, j)
j∈J

i∈I

112

3. LIMITS AND COLIMITS

that commute with the maps of limits lim j∈J H(i, j) → lim j∈J H(i0 , j) determined by eachmorphism i → i0 ∈ I. The map of limits is defined, as in Proposition 3.6.1, by the mapof J-indexed diagrams whose components H(i, j) → H(i0 , j) are defined by applying thefunctor H(−, j) to the morphism in I.
By the universal property of the J-indexed limits, each cone leg λi : 1 → lim j∈J H(i, j)
is itself determined by legs!
λi, j

1 −−→ H(i, j)

,
j∈J
0

which must commute with the maps H(i, j) → H(i, j ) induced by each j → j0 . Thus, thetotality of this data is precisely a cone!
λi, j1 −−→ H(i, j)(i, j)∈I×J

over the I × J-indexed diagram H. Having demonstrated that their elements coincide, we
conclude that limi∈I lim j∈J H(i, j)  lim(i, j)∈I×J H(i, j).

By Theorem 3.8.1 and its dual, limits commute with limits, and colimits commute withcolimits up to isomorphism. By the uniqueness statements in the universal properties forlimits and colimits, it follows that there exist natural isomorphisms(3.8.2)

CI×J
limI


CJ

limJ


limJ

/ CI

colimJ

CI×J

limI

colimI

colimI




CJ


/C

/ CI

colimJ/C

for any choices of limit and colimit functors.
The relationship between the construction of limits in one variable and colimits inanother is more subtle. These operations seldom commute. However, there is always acanonical comparison morphism:
Lemma 3.8.3. For any bifunctor F : I × J → C so that the displayed limits and colimits
exist, there is a canonical map
κ : colim lim F(i, j) → lim colim F(i, j) .
i∈I

j∈J

j∈J

i∈I

To remember the direction of the canonical map, recall that colimits are defined via
a “mapping out” universal property, while limits are defined via a “mapping in” universal
property.
Proof. By the universal property of the colimit, the map κ may be defined by specifying
components!
κilim F(i, j) −→ lim colim
F(i0 , j)
0
j∈J

j∈J

i ∈I

i∈I

that define a cone under the I-indexed diagram lim j∈J F(−, j). By the universal property of
the limit, each component κi in turn may be defined by specifying components
!
κi, j
0
0
lim
F(i, j ) −−→ colimF(i , j)
0
0
j ∈J

i ∈I

j∈J

that define a cone over the J-indexed diagram colimi∈I F(i, −). Define κi, j to be the composite
πi, j

ιi, j

κi, j : lim
F(i, j0 ) −−→ F(i, j) −−→ colim
F(i0 , j)
0
0
j ∈J

i ∈I

3.8. INTERACTIONS BETWEEN LIMITS AND COLIMITS

113

of the leg πi, j of the limit cone for F(i, −) and the leg ιi, j of the colimit cone for F(−, j).
Because the maps in the diagrams lim j∈J F(−, j) and colimi∈I F(i, −) are induced by themaps F(i, j) → F(i0 , j0 ) obtained by applying F to morphisms i → i0 ∈ I and j → j0 ∈ J,
the κi, j assemble into a cone, defining κi , and the κi assemble into a cone defining κ.
Specializing to the case where C is the poset category (R, ≤), we have the following
immediate corollary:
Corollary 3.8.4. For any pair of sets X and Y and any function f : X × Y → R
sup inf f (x, y) ≤ inf sup f (x, y)
x∈X y∈Yy∈Y x∈X

whenever these infima and suprema exist.
Proof. The presence of a map supX inf Y f → inf Y supX f in the poset category implies
the displayed inequality.
Remark 3.8.5. A direct proof of Corollary 3.8.4, as might be presented in a real analysis
course, is in fact the categorical proof used for Lemma 3.8.3. To show that supX inf Y f
is bounded above by some quantity it suffices—by what might be called the universal
property of the supremum—to show this is the case for each inf Y f (x, −). Dually, toshow that inf Y supX f is bounded below, it suffices to demonstrate the inequality for eachsupX f (−, y). So it suffices to prove that inf Y f (x, −) ≤ supX f (−, y), which follows fromtransitivity, i.e., composition, of the inequalities
inf f (x, −) ≤ f (x, y) ≤ sup f (−, y) .
Y

X

Example 3.8.6. Consider the extended real line R = R∪{−∞, ∞} with the obvious extended
ordering −∞ ≤ x ≤ ∞. As a category, R is complete and cocomplete: the limit of a function
f : X → R is its infimum and the colimit is its supremum. These are ordinary real numbersin the case where the range of f is bounded. The advantage of working in the extended
category R is that boundedness hypotheses can be ignored and all limits and colimits exist.
A function x : N → R defines a sequence (xn )n∈N of real numbers. The lim inf and lim+x
sup is defined by regarding the sequence as a bifunctor N × N −→N→− R indexed by thediscrete category N × N:
lim inf xn = sup inf xm = sup inf xn+m = colim lim xn+mn→∞n≥0 m≥nn

n≥0 m≥0m

lim sup xn = inf sup xm = inf sup xn+m = lim colim xn+mn→∞n≥0 m≥nn≥0 m≥0nm

Having translated these analytic notions into categorical ones, Lemma 3.8.3 applies in theform of an inequality:
lim inf xn ≤ lim sup xn .
n→∞

n→∞

The limit of this sequence exists if and only if this inequality is an equality.
In Set and related categories, there are certain classes of limits and colimits for which
the map defined in Lemma 3.8.3 is invertible. Theorem 3.8.9 describes the most famous ofthese results. A comprehensive survey can be found in [BJLS15].
Definition 3.8.7. A category J is filtered if there is a cone under every finite17 diagram
in J.
17This definition can be generalized, replacing the smallest infinite cardinal ω by a regular cardinal κ: a categoryis κ-filtered if there is a cone under any diagram with fewer than κ morphisms.

114

3. LIMITS AND COLIMITS

Example 3.8.8. The category ω (or, more generally, any ordinal category) and the category
with one object and one non-identity idempotent are filtered. The parallel pair category,the category • ← • → •, and discrete categories with more than one object are not.
Theorem 3.8.9. Filtered colimits commute with finite limits in Set.
The central observation used in this proof is the following. If F : J → Set is a diagram
indexed by a small filtered category, then colim j∈J F j is the quotient of the coproduct`j∈J F j under the equivalence relation that identifies x ∈ F j with y ∈ Fk if and only ifthere are maps f : j → t and g : k → t in J so that F f (x) = Fg(y). More generally, any`
finite collection of elements in j∈J F j that are identified in colim j∈J F j map to a common
element in some Ft.
Proof. Suppose I is a finite category and J is a small filtered category. Our task is toshow that for any functor F : I × J → Set the canonical map
κ : colim lim F(i, j) → lim colim F(i, j)
j∈J

i∈I

i∈I

j∈J

is a bĳection. By Definition 3.2.3, an element on the right-hand side is a cone λ withsummit 1 over colim j∈J F(i, j). As I is finite, such a cone is given by a finite collectionof elements λi ∈ colim j∈J F(i, j) satisfying a finite number of compatibility conditions.
Since J is filtered, there is some “sufficiently large” t ∈ J so that for each i ∈ I the λi are
represented by elements λ0i ∈ F(i, t) and moreover these elements assemble into a cone!
λ0i
1 −→ F(i, t).
i∈IThis cone defines an element in limi∈I F(i, t) that represents an element on the left-hand side
mapping to our chosen element under κ. This proves that κ is surjective.
Now consider a pair of elements in the left-hand set, represented by cones

 β

 α
i
i
and
1−
→ F(i, k)
1 −→ F(i, j)
i∈I

i∈I

for some j, k ∈ J. If these elements have a common image under κ, then for each i ∈ I, there
must be some ti ∈ J so that αi and βi have a common image in F(i, ti ). As I is finite and J isfiltered, there is some t ∈ J so that for all i αi and βi have a common image in F(i, t). But
this says that α and β represent the same element on the left-hand side, proving injectivityof κ.
The proof of Theorem 3.8.9 is not categorical; the argument relies upon an explicit
description of filtered colimits in the category of sets. However, for a large class of
categories that will be introduced in Chapter 5, the categories of models for an algebraic
theory, limits and filtered colimits are created in the category of sets (see Theorem 5.6.5).
Hence finite limits and filtered colimits commute in these contexts as well.
Exercises.
Exercise 3.8.i. Dualize the construction of Example 3.2.16 to express the splitting of an
idempotent as a coequalizer. Explain why these colimits (or limits) are preserved by any
functor and conclude that splittings of idempotents commute with both limits and colimitsof any shape.
Exercise 3.8.ii. Show that if G and H are groups whose orders are coprime, then BGindexed limits commute with BH-indexed colimits in Set. See [BJLS15] for more examplesof this flavor.

CHAPTER 4

Adjunctions
It appears . . . that there exists a kind of duality
between the tensor product and the . . . functor
Hom . . .
Daniel Kan, “Adjoint functors” [Kan58]

Vector spaces are sets equipped with a binary operation v + w (addition of vectors) anda unary multiplication c · v for each scalar c in the ground field k, satisfying certain axioms.
The passage from a vector space to its underlying set of “vectors” is functorial, which is to
say there is a forgetful functor U : Vectk → Set that carries a linear transformation to its
underlying function.
A more interesting challenge is to reverse the direction of this translation and build avector space from a set S . Cardinality restrictions might prevent the direct use of S as theset of vectors for a k-vector space, and even without this obstruction there is no natural way
to define vector addition in S . Instead, a more natural way to build a vector space is to let
the elements of S serve as a basis: vectors are then finite formal sums1 k1 s1 + · · · + kn sn
with ki ∈ k, si ∈ S , and n ≥ 0. This defines a vector space k[S ] whose dimension is equal
to the cardinality of S , called the free vector space on the set S . Moreover, as our useof the adjective “natural” would suggest, this construction is functorial, defining a functork[−] : Set → Vectk : a function f : S → T induces a linear map k[ f ] : k[S ] → k[T ] definedon the basis elements in the evident way.
There are many instances of free constructions in mathematics. One can define thefree (abelian) group on a set, the free ring on a set or on an abelian group, the free module
on an abelian group, the free category on a directed graph, and so on. Sometimes there arecompeting notions of universal constructions: Does the free graph on a set have a single
edge between every pair of vertices or none? Does the free topological space on a set haveas many open sets as possible or as few? Other times there are none: For instance, there
is no “free field.” All of these constructions, and many others besides, are clarified by theconcept of an adjunction, introduced by Kan [Kan58].
The universal property of the free vector space functor k[−] : Set → Vectk is expressedby saying that it is left adjoint to the underlying set functor U : Vectk → Set: linear maps
k[S ] → V correspond naturally to functions S → U(V), which specify the image of thebasis vectors S ⊂ k[S ]. By the Yoneda lemma, this universal property can be used todefine the action of the free vector space functor k[−] on maps. The forgetful functor
U : Vectk → Set has no right adjoint, so in this setting there are no competing notions offree construction.
1A precise way to define what is meant by this informal phrase is to say that a finite formal sum is a finitely
P
supported function φ : S → k, corresponding to the expression s∈S φ(s)s. In particular, two finite sums are
considered to be identical when they differ only up to reordering of terms, up to consolidating repeated instancesof the same term by adding their coefficients, or up to inclusion or deletion of terms whose coefficients are zero.
115

116

4. ADJUNCTIONSIn this chapter, we explore the general theory of adjoint functors, both in the abstract andaided by a plethora of examples. Two equivalent definitions of an adjunction are presented
in §4.1 and §4.2, both of which offer useful perspectives on the adjoint correspondence
between certain linear transformations and certain functions just mentioned. §4.3 explores
the forms of adjointness that arise between pairs of contravariant functors or between(n + 1)-tuples of functors with n-variables. For instance, a two-variable adjunction encodes
the duality between the tensor product and hom bifunctors observed by Kan.
The basic calculus of adjunctions is developed in §4.4, with each result proven twice,once using the Yoneda lemma and again via the techniques of “formal category theory,”
to illustrate two important methods of categorical reasoning. In §4.5, we prove what isperhaps the most frequently applied result in category theory: that right adjoint functors
preserve limits while left adjoint functors preserve colimits. This explains why limits tend
to be easier to construct than the formally dual colimit notions.
In particular, continuity is a necessary condition for a functor to admit a left adjoint,and cocontinuity is necessary for the existence of a right adjoint. In §4.6, we develop
sufficient conditions for a functor to have an adjoint, which can be used to construct themissing adjoint functor in certain contexts.

4.1. Adjoint functors
Adjoint functors arise everywhere.
Saunders Mac Lane, Categories for the Working
Mathematician [ML98a]

An adjunction consists of an opposing pair of functors F : C  D : G that enjoy aspecial relationship to one another.
Definition 4.1.1 (adjunctions I). An adjunction consists of a pair of functors F : C → D
and G : D → C together with an isomorphism
D(Fc, d)  C(c, Gd)(4.1.2)for each c ∈ C and d ∈ D that is natural in both variables. Here F is left adjoint to G and
G is right adjoint to F. The morphisms
Fc

f]

/d

!

c

f[

/ Gd

corresponding under the bĳection (4.1.2) are adjunct or are transposes of each other.2
When C and D are locally small, the naturality statement in Definition 4.1.1 assertsthat the isomorphisms (4.1.2) assemble into a natural isomorphism between the functors
D(F−,−)opC(× D ⇓ 6 Set .
C(−,G−)

2For now, we use “(−)] ” and “(−)[ ” to decorate a pair of adjunct arrows, but in practice it tends to be mostˆ to signal any adjoint transpose, with no preference as toconvenient to use some symbol, such as “(−)† ” or “(−)”
which of the adjunct pair is decorated in this way.

4.1. ADJOINT FUNCTORS

117

More explicitly, naturality in D says that for any morphism k : d → d0 , the left-hand diagram
displayed below commutes in Set:
/ C(c, Gd)

D(Fc, d)f
D(Fc, d0 )/ C(c, Gd0 )



/ Gd

∀Fc −→ d

Gk∗

k∗

f[

c

]

Gk


Gd0

(k· f ] )[which amounts to the assertion that for any f ] : Fc → d and k : d → d0 , the transpose ofk · f ] : Fc → d0 is equal to the composite of f [ : c → Gd with Gk : Gd → Gd0 .
Dually, naturality in C says that for any morphism h : c0 → c, the left-hand diagram
displayed below commutes in Set:

Fh∗

/ C(c, Gd)

D(Fc, d)c0f

/ C(c0 , Gd)





h

∀Fc −→ dh∗D(Fc0 , d)

( f ] ·Fh)[

]


c

/ Gd

f[

which amounts to the assertion that the transpose of f ] · Fh : Fc0 → d is the composite ofh : c0 → c with f [ : c → Gd.
The following lemma, whose proof is left as Exercise 4.1.i, provides an equivalent
expression of the naturality of a collection of isomorphisms (4.1.2) in a form that tends tobe convenient to use in proofs.
Lemma 4.1.3. Consider a pair of functors F : C  D : G equipped with isomorphisms
D(Fc, d)  C(c, Gd) for all c ∈ C and d ∈ D. Naturality of this collection of isomorphisms
is equivalent to the assertion that for any morphisms with domains and codomains as
displayed below
Fc

f]

Fh

/d

(4.1.4)


Fc0

c
!

k

g]


/ d0

f[

Gk

h


c0

/ Gd

g[


/ Gd0

the left-hand square commutes in D if and only if the right-hand transposed square commutes
in C.
Notation 4.1.5. A turnstyle “a” is used to designate that an opposing pair of functors areadjoints: the expressions F a G and G ` F and the diagrams
/

D

C

o

G
⊥

F
⊥
G

F

/D

D

o

F
⊥
G

G

/C

Do

⊥

Co

/

C

F

all assert that F : C → D is left adjoint to G : D → C.
The utility of this versatile notation is most apparent in contexts involving several
adjoint functors.
Example 4.1.6. The forgetful functor U : Top → Set admits both left and right adjoints.
The left adjoint constructs a topological space from a set S in such a way that continuous
maps from this space to another space T correspond naturally and bĳectively to functions
S → U(T ). The discrete topology on S has this universal property: writing D(S ) for thisdiscrete space, any function S → U(T ) defines a continuous map D(S ) → T .

118

4. ADJUNCTIONS

Similarly, the right adjoint constructs a topological space from a set S in such a waythat continuous maps from T to this space correspond naturally and bĳectively to functions
U(T ) → S . The indiscrete topology on S has this universal property: writing I(S ) for thisindiscrete space, any function U(T ) → S defines a continuous function T → I(S ). The
natural isomorphisms
Top(D(S ), T )  Set(S , U(T ))and Set(U(T ), S )  Top(T, I(S ))assert that the discrete, forgetful, and indiscrete functors define a pair of adjunctions:
|
Top
b

D
⊥
U
⊥

/ Set .

I

An adjunction between preorders is called a (monotone) Galois connection. Recall,from Exercise 1.3.ii, that functors F : A → B and G : B → A between preorders A and B
are simply order-preserving functions. An adjunction F a G asserts thatFa ≤ bif and only if

a ≤ Gbfor all a ∈ A and b ∈ B. In this context, F is the lower adjoint and G is the upper adjoint.
Example 4.1.7. The inclusion of posets Z ,→ R, with the usual ordering ≤, has both left
and right adjoints, namely, the ceiling and floor functions. For any integer n and real numberr, n ≤ r if and only if n ≤ brc, where brc denotes the greatest integer less than or equal to r.
This function is order-preserving, defining a functor b−c : R → Z that is right adjoint to theinclusion Z ,→ R. Dually, r ≤ n if and only if dre ≤ n, and this order-preserving function
defines a functor d−e : R → Z that is left adjoint to the inclusion.
 
Z]

d−e
⊥
⊥

/ R.

b−c

Example 4.1.8. Consider a function f : A → B between sets. The subsets of A and subsets
of B form posets, PA and PB, ordered by inclusion. The map f induces direct image andinverse image functors f∗ : PA → PB and f −1 : PB → PA. The inverse image is rightadjoint to the direct image: for A0 ⊂ A and B0 ⊂ B, f (A0 ) ⊂ B0 if and only if A0 ⊂ f −1 (B0 ).
The inverse image functor has a further right adjoint f! that carries a subset A0 ⊂ A to
the subset of elements of B whose fibers lie entirely in A0 . With this definition, B0 ⊂ f! (A0 )if and only if f −1 (B0 ) ⊂ A0 . These functors define a pair of adjunctions:

PB`

}

f∗
⊥
f −1
⊥

/ PA .

f!

Example 4.1.9. A propositional function is a function P : X → Ω = {⊥, >}, which weinterpret as declaring, for each x ∈ X, whether P(x) is true or false. The set ΩX is then theset of propositional functions on X. The set Ω is given the partial order ⊥ ≤ > from whichΩX inherits a pointwise-defined order: P ≤ Q if and only if P(x) ≤ Q(x) for all x ∈ X,
which is the case if and only if P implies Q.
The logical operations of universal and existential quantification define functors∀X , ∃X : ΩX ⇒ Ω4.1. ADJOINT FUNCTORS

119

in the expected way: ∀X P = > if and only if P(x) = > for all x ∈ X, and ∃X P = > if andonly if there exists x ∈ X with P(x) = >. There is also a constant “dummy variable” functor∆X : Ω → ΩX , and one can verify that these functors define a triple of adjoints:
Ω_

∃X

~

⊥
∆X
⊥

/ ΩX .

∀X

See [Awo96] for more.
Example 4.1.10. There is a large and very important family of “free a forgetful” adjunctions
A

o

F
⊥
U

/ S,

with the forgetful functor U defining the right adjoint and the free functor F defining theleft adjoint. If one can construct a universal object of type A on an object of type S, andthis construction defines a left adjoint to a forgetful functor U : A → S, then the term
“free” is used in place of “universal” to convey this particular relationship. The reason whyleft adjoints to forgetful functors are more common than right adjoints has to do with thehandedness of the universal properties that one meets in practice. “Cofree” constructions,which define right adjoints to some forgetful functor, are somehow less common.
The following forgetful functors admit left adjoints, defining “free” constructions.
(i) U : Set∗ → Set. The left adjoint carries a set X to the pointed set X+ B X t {X},with a freely-adjoined basepoint.
`(ii) U : Monoid → Set. The free monoid on a set X is the set n≥0 X ×n of finite lists ofelements of X, including the empty list.
(iii) U : Ring → Ab, forgetting the multiplicative structure. The free ring on an abelianLgroup A is n≥0 A⊗n .
Exercise 5.1.i explores the connection between Examples (ii) and (iii): as observed in
Definition 1.6.3, rings are monoids in the category of abelian groups.
L(iv) U : Ab → Set. The free abelian group on a set X is the set Z[X] B X Z of finite
formal sums of elements of X with integer coefficients. More generally:L(v) U : ModR → Set. The free R-module on a set X is R[X] B X R. A special casedefines the free k-vector space on a set, defining the left adjoint to U : Vectk → Set
considered in the introduction to this chapter.
(vi) U : Ring → Set. By composing the left adjoints to the forgetful functors Ring →Ab → Set, the free ring on a set X is the free monoid on the free abelian group on
LL
the set: n≥0 ( X Z)⊗n .
(vii) (−)× : Ring → Group, carrying a ring to its group of units. The free ring on a group
L
G is the group ring Z[G] = G Z, whose elements are finite formal sums of groupelements, using the group operation to define the bilinear multiplication law.
(viii) U : Group → Set. The free group on a set is described in more detail in Examples
4.2.4 and 4.6.6.
(ix) Ab ,→ CMonoid, the inclusion of abelian groups into the category of commutative
monoids. The left adjoint Gr carries a commutative monoid (M, +, 0) to its groupcompletion, also called the Grothendieck group: as a set Gr(M, +, 0) is the quotientof M × M by the relation (a, b) ' (a0 , b0 ) if and only if there is some c ∈ M so thata + b0 + c = a0 + b + c.

120

4. ADJUNCTIONS(x) Group ,→ Monoid. The left adjoint constructs the “group completion” of a monoid
as a quotient of the free group on its underlying set modulo relations arising from themultiplication and unit in the monoid structure.
(xi) The restriction of scalars functor φ∗ : ModS → ModR associated to a ring homomorphism φ : R → S . Its left adjoint, given by S ⊗R − : ModR → ModS , is called
extension of scalars.
(xii) U : ModR → Ab, forgetting the scalar multiplication. This is a special case of theprevious example induced from the unique (unital) ring homomorphism Z → R.
(xiii) φ∗ : SetBG → SetBH , another restriction of scalars functor induced from a grouphomomorphism φ : H → G. The left adjoint is also called induction and can be
described analogously to (xi).
Each of these free a forgetful adjunctions is an instance of a monadic adjunction, a
special class of adjoint functors that is the subject of §5.5.
Example 4.1.11 (Frobenius reciprocity). Example 4.1.10(xiii) can be generalized: Example 6.2.8 proves that for any complete and cocomplete category C and any group homomorphism φ : H → G, the restriction functor φ∗ : CBG → CBH admits both left and rightadjoints. These are most commonly considered for subgroup inclusions H ⊂ G, in whichcase the left adjoint is called induction and the right adjoint is called coinduction:
indGH

CBG\

⊥
resGH
⊥

/ CBH

coinGHTaking C = Vectk , the adjunction indGH a resGH between the category VectBk H of Hrepresentations and the category of VectBkG of G-representations is referred to in the representation theory literature as Frobenius reciprocity.
Example 4.1.12. None of the functors
U

Field −→ Ring,U

Field −
→ Ab,

(−)×

Field −−−→ Ab,

U

Field −
→ Set

that forget algebraic structure on fields admit left or right adjoints. To see that no left
adjoint is possible, note that there exist maps in the target categories from Z to fieldsof any characteristic. As there are no field homomorphisms between fields of differingcharacteristic, it is not possible to define the value of a hypothetical left adjoint on Z. A
similar argument can be used to show that no right adjoints to these functors exist.
Example 4.1.13. The forgetful functor U : Cat → DirGraph admits a left adjoint F,
defining the free category on a directed graph. A directed graph G consists of a set V ofvertices, a set E of edges, and two functions s, t : E ⇒ V defining the source and target ofeach directed edge. The free category on G has V as its set of objects. The set of morphisms
consists of identities for each vertex together with finite “paths” of edges. Composition isdefined by concatenation of paths.
The adjunction supplies a natural bĳection between functors F(G) → C and morphisms
G → U(C) of directed graphs. A functor F(G) → C, or equally a directed graph morphism
G → U(C), defines a diagram in C with no commutativity requirements, since directed
graphs do not encode composites. The data of such a diagram is uniquely determined by theimages of the vertices and edges in the directed graph G. These edges form atomic arrows
in the category F(G), admitting no non-trivial factorizations. For instance, as described in4.1. ADJOINT FUNCTORS

121

Example 1.1.4(iv), the category ω is free on the directed graph whose vertices are indexedby natural numbers and with edges n → n + 1. On account of the adjunction, an ω-indexed
diagram is defined by specifying only the images of the atomic arrows, from an ordinal toits successor.
Example 4.1.14. Let n + 1 denote the ordinal category, freely generated by the directed
graph 0 → 1 → 2 → · · · → n. For each 0 ≤ i ≤ n, there is an injective functor
di : n → n + 1 where i ∈ n + 1 is the unique object missing from the image. For each0 ≤ i < n, there is also a surjective functor si : n + 1 → n for which i ∈ n is the unique
object with two preimages. These functors define a sequence of 2n + 1 adjoints:
/

dn

o

⊥ n−1
s
⊥
n−1
d
⊥ n−2
s
⊥
.

o

/
n+1

..

n
o
o

s1
⊥
d1
⊥ 0
s
⊥
d0

/
/

proving that arbitrarily long finite sequences of adjoint functors exist.
Example 4.1.15. The inclusion Groupoid ,→ Cat admits both left and right adjointsyGroupoid

e

⊥

/ Cat

⊥

The right adjoint carries a category to its maximal subgroupoid considered in Lemma
1.1.13: since functors preserve isomorphisms, the image of a functor G → C whose
domain is a groupoid necessarily factors (uniquely) through the maximal subgroupoid ofC.
The left adjoint “formally inverts” the morphisms of a category, constructing thecorresponding “category of fractions.” The category of fractions for C can be constructed
as a quotient of the free category on the directed graph obtained by gluing the directed graphunderlying C to the directed graph underlying Cop along the common discrete subcategory
ob C. Objects in the category of fractions are objects of C and morphisms are finite zig-zagsof morphisms in C, subject to relations that, for instance, equate the zig-zags
a

f

/bo

f

a

bo

f

a

f

/b

with identities. More details on this construction can be found in [Bor94a, §5.2].
Exercises.
Exercise 4.1.i. Show that functors F : C → D and G : D → C and bĳections D(Fc, d) C(c, Gd) for each c ∈ C and d ∈ D define an adjunction if and only if these bĳections induce
a bĳection between commutative squares (4.1.4). That is, prove Lemma 4.1.3.
Exercise 4.1.ii. Define left and right adjoints to(i) ob : Cat → Set,(ii) Vert : Graph → Set, and(iii) Vert : DirGraph → Set.

122

4. ADJUNCTIONS

Exercise 4.1.iii. Show that any triple of adjoint functors
C^

~

L
⊥
U
⊥

/D

R

gives rise to a canonical adjunction LU a RU between the induced endofunctors of C.
Exercise 4.1.iv. Prove that the construction of the free R-module on a set described in
Example 4.1.10(v) is functorial.3
Exercise 4.1.v. Use Lemma 4.1.3 to show that if F : C  D : G are adjoint functors, withF a G, then the comma categories F ↓ D and C ↓ G, introduced in Exercise 1.3.vi, areisomorphic, via an isomorphism that commutes with the forgetful functors to C × D.
4.2. The unit and counit as universal arrows
Recall that an adjunction is comprised of an opposing pair of functors together withnatural isomorphisms
F
⊥
G

Co

(4.2.1)/D(Fc, d)  C(c, Gd) .

D

Fixing c ∈ C, this natural isomorphism asserts that the object Fc ∈ D represents the functor
C(c, G−) : D → Set. By the Yoneda lemma, the natural isomorphism D(Fc, −)  C(c, G−)is determined by an element of C(c, GFc), the transpose of 1Fc , denoted by ηc . The

naturality of adjoint transposition implies that the maps ηc assemble into the components
of a natural transformation η : 1C ⇒ GF:
Lemma 4.2.2. Given an adjunction F a G, there is a natural transformation η : 1C ⇒ GF,
called the unit of the adjunction, whose component ηc : c → GFc at c is defined to be thetranspose of the identity morphism 1Fc .
Proof. To prove that η is natural, we must show that the left-hand square commutesfor every f : c → c0 in C.
c
f


c0

ηc

/ GFc
GF f

ηc0


/ GFc0

Fc
!

1Fc

Ff


Fc0

/ Fc
Ff

1Fc0


/ Fc0

This follows from Lemma 4.1.3 and the obvious commutativity of the right-hand transposedsquare.

Dually, fixing d ∈ D the defining natural isomorphism (4.2.1) of an adjunction F a G
says that the object Gd ∈ C represents the functor D(F−, d) : Cop → Set. By the Yoneda
lemma, the natural isomorphism C(−, Gd)  D(F−, d) is determined by an element of
D(FGd, d), the transpose of 1Gd , denoted by d . By the dual of Lemma 4.2.2, the maps dassemble into the components of a natural transformation  : FG ⇒ 1D :3This exercise intends to ask for an explicit description of the homomorphism between two formally defined
finite formal sums associated to a function between their generating sets. But the reader who is disinclined to think
about explicit constructions could instead use the Yoneda lemma to prove functoriality. (Hint: See Proposition
4.3.4.)

4.2. THE UNIT AND COUNIT AS UNIVERSAL ARROWS

123

Lemma 4.2.3. Given an adjunction F a G, there is a natural transformation  : FG ⇒ 1D ,called the counit of the adjunction, whose component d : FGd → d at d is defined to be
the transpose of the identity morphism 1Gd .
Example 4.2.4. The left adjoint F to the forgetful functor U : Group → Set defines thefree group on a set S . Elements of the group F(S ) are finite “words” whose “letters” areelements s of S and formal “inverses” s−1 , modulo some evident relations. The empty
word serves as the identity element and multiplication is by concatenation; the relations, inparticular, identify ss−1 and s−1 s with the empty word.
The component of the unit of the adjunction at a set S is the function ηS : S → UF(S )that sends an element s ∈ S to the corresponding singleton word. The component of thecounit of the adjunction at a group G is the group homomorphism G : FU(G) → G that
sends a word, whose letters are elements of the group G and formal inverses, to the productof those symbols, interpreted using the multiplication, inverses, and identity present in thegroup G.
A similar description, with the unit defining some sort of “singleton” map and thecounit defining some form of “evaluation” homomorphism, can be given for many of theother adjunctions listed in Example 4.1.10.
In the terminology of Definition 2.3.3, the components of the unit and the counit define
universal elements characterizing the natural isomorphisms
Group(F(S ), −)  Set(S , U(−))
Set(−, U(G))  Group(F(−), G)!
!

ηS ∈ Set(S , UF(S ))G ∈ Group(FU(G), G)

By Proposition 2.4.8, the components of the unit and the counit enjoy certain dual universal
properties to be explored in Exercise 4.2.iv, which will feature prominently in the argumentsof §4.6.
Lemmas 4.2.2 and 4.2.3 prove that any adjunction has a unit and a counit. Conversely,
if F : C  D : G are opposing functors equipped with natural transformations η : 1C ⇒ GF
and  : FG ⇒ 1D satisfying a dual pair of conditions, then this data can be used to define anatural bĳection D(Fc, d)  C(c, Gd), exhibiting F and G as adjoint functors. That is, thenotion of adjunction may be redefined as follows:
Definition 4.2.5 (adjunction II). An adjunction consists of an opposing pair of functors
F : C  D : G, together with natural transformations η : 1C ⇒ GF and  : FG ⇒ 1D thatsatisfy the triangle identities:FFη

+3 FGF

1F

& 
F

F

G

ηG

+3 GFG

1G

& 
G

G

The left-hand triangle asserts that a certain diagram commutes in DC , while the righthand triangle asserts that the dual diagram commutes in CD . The natural transformationsFη, F, ηG, and G are defined by whiskering; recall Remark 1.7.6. Their components aredefined just as the notation would suggest: Gd is G(d ), while Fc is Fc .
Together the triangle identities assert that “the counit is a left inverse of the unit
modulo translation.” They cannot literally be inverses because the components of the unit
ηc : c → GFc lie in C while the components of the counit d : FGd → d lie in D, andsuch morphisms are not composable. But if we apply F to the unit, we obtain a morphismFηc : Fc → FGFc in D whose left inverse is Fc . And if we apply G to the counit, we
obtain a morphism Gd : GFGd → Gd whose right inverse is ηGd .

124

4. ADJUNCTIONS

Proposition 4.2.6. Given a pair of functors F : C  D : G, there exists a natural isomorphism D(Fc, d)  C(c, Gd) if and only if there exists a pair of natural transformations
η : 1C ⇒ GF and  : FG ⇒ 1D satisfying the triangle identities.
Proposition 4.2.6 asserts that the definitions of adjunction given in 4.1.1 and in 4.2.5
are equivalent.
Proof. The dual Lemmas 4.2.2 and 4.2.3 demonstrate that a natural isomorphism

D(Fc, d)  C(c, Gd) gives rise to a pair of natural transformations η : 1C ⇒ GF and : FG ⇒ 1D , whose components are transposes of identity morphisms. It remains todemonstrate the triangle identities. By Lemma 4.1.3, the upper-left-hand and lower-righthand squares
Fc

1Fc

Fηc


FGFc

FGd

/ Fc
1Fc

Fc

1FGd


/ Fc


FGd


/d

d

1GFc


GFc

!


/ GFc

1GFc
ηGd

Gd

d

/ GFc

ηc

!

/ FGd

1FGd

ηc

c

/ GFGd
Gd

1Gd


Gd

1Gd


/ Gd

commute because the transposed squares manifestly do. By Lemma 1.7.1, which defines
vertical composition for natural transformations, this is what we wanted to show.
Conversely, the unit and counit can be used to define a natural bĳection D(Fc, d) C(c, Gd). Given f ] : Fc → d and g[ : c → Gd, their adjuncts are defined to be thecomposites:f[ B c

ηc

/ GFc

Gf]

/ Gd

g] B Fc

Fg[

/ FGd

d

/d

The triangle identities imply that these operations are inverses. The transpose of thetranspose of f ] : Fc → d is equal to the top composite
Fc

Fηc

]

/ FGFc FG f / FGd

d

=/ d

Fc
1Fc

$,

f]

Fc

By naturality of  and one triangle identity, this returns the original f ] : Fc → d. The
dual diagram chase, left as an exercise for the reader, demonstrates that the transpose of thetranspose of g[ : c → Gd returns this map.
Remark 4.2.7. Proposition 4.2.6 reveals that the data of a fully-specified adjunction canbe presented in two equivalent forms: it consists of a pair of functors F : C  D : G
together with(i) a natural family of isomorphisms D(Fc, d)  C(c, Gd) for all c ∈ C and d ∈ D,or, equivalently,(ii) natural transformations η : 1C ⇒ GF and  : FG ⇒ 1D so that G · ηG = 1G andF · Fη = 1F .

4.2. THE UNIT AND COUNIT AS UNIVERSAL ARROWS

125

Indeed, either of the unit and the counit alone, satisfying an appropriate universal property,
suffices to determine a fully specified adjunction: (i) and (ii) are each equivalent to eitherof(iii) a natural transformation η : 1C ⇒ GF so that the function
D(Fc, d)G(ηc )∗/ C(GFc, Gd)/ C(c, Gd)

defines an isomorphism for all c ∈ C and d ∈ D,or, equivalently, and dually,(iv) a natural transformation  : FG ⇒ 1D so that the function
C(c, Gd)F/ D(Fc, FGd)(d )∗/ D(Fc, d)defines an isomorphism for all c ∈ C and d ∈ D.
In particular, a morphism of adjunctions, introduced in Exercise 4.2.v, is defined to be amorphism of fully-specified adjunctions. On account of the equivalence between (i), (ii),(iii), and (iv), this notion can be presented in several equivalent ways.
The mere presence of unit and counit transformations establishes the following fixed
point formulae for a Galois connection between posets.
Corollary 4.2.8. If A and B are posets and F : A → B and G : B → A form a Galois
connection, with F a G, then F and G satisfy the following fixed point formulae
FGF = FandGFG = G .

Proof. By the triangle identities F(a) ≤ FGF(a) ≤ F(a) for all a ∈ A, whence
F = FGF. The other formula is dual.
For instance, consider the direct-image a inverse-image adjunction
PB

o

f∗
⊥
f −1

/ PA

induced by a function f : A → B defined in Example 4.1.8. For generic subsets X ⊂ A andY ⊂ B, neither of the inclusions
X ⊂ f −1 ( f (X))

orf ( f −1 (Y)) ⊂ Y

need be equalities but nonethelessf (X) = f ( f −1 ( f (X)))

andf −1 ( f ( f −1 (Y))) = f −1 (Y) .

Exercises.
Exercise 4.2.i. Prove that any pair of adjoint functors F : C  D : G restrict to define anequivalence between the full subcategories spanned by those objects c ∈ C and d ∈ D for
which the components of the unit ηc and of the counit d , respectively, are isomorphisms.
Exercise 4.2.ii. Explain each step needed to convert the statement of Lemma 4.2.2 into
the statement of the dual Lemma 4.2.3.
Exercise 4.2.iii. Pick your favorite forgetful functor from Example 4.1.10 and prove that
it is a right adjoint by defining its left adjoint, the unit, and the counit, and demonstratingthat the triangle identities hold.
Exercise 4.2.iv. Each component of the counit of an adjunction is a terminal object insome category. What category?

126

4. ADJUNCTIONS

Exercise 4.2.v. A morphism of adjunctions from F a G to F 0 a G0 is comprised of apair of functors
CO

H

D

O

F 0 a G0

F a G



/ C0

K


/ D0

so that the square with the left adjoints and the square with the right adjoints both commute(i.e., KF = F 0 H and HG = G0 K) and satisfying one additional condition, which takes anumber of equivalent forms. Prove that the following are equivalent:(i) Hη = η0 H, where η and η0 denote the respective units of the adjunctions.
(ii) K =  0 K, where  and  0 denote the respective counits of the adjunctions.
(iii) Transposition across the adjunctions commutes with application of the functors H
and K, i.e., for every c ∈ C and d ∈ D, the diagram
D(Fc, d)K 

D0 (KFc, Kd)

/ C(c, Gd)
H

C0 (Hc, HGd)

D0 (F 0 Hc, Kd)  / C0 (Hc, G0 Kd)

commutes.
4.3. Contravariant and multivariable adjoint functors
Kan’s original exploration of adjoint functors also considered adjoint relationships
involving functors that are contravariant or have more than one variable [Kan58]. Indeed,
these examples provided the central motivation for Kan’s discovery, at least according to an
apocryphal account of a question he posed to Eilenberg in a homological algebra seminarat Columbia University [Mar09, §4.2]:
kan: You have explained how the tensor product can be definedin terms of the hom functor. Can the hom instead be definedin terms of the tensor product?
eilenberg: No, of course not. That’s absurd.
In fact, Kan’s intuition was correct. By the Yoneda lemma, any adjoint functor determines
its adjoints up to natural isomorphism. Example 4.3.11 explains how to define the hombifunctor from the tensor product.
Contravariant adjoint functors require little special consideration, arising simply as anapplication of the principle of duality. Definition 4.1.1 can be dualized in three ways, by
replacing C, D, or both C and D by their opposite categories. The latter dualization recovers
the original notion of adjunction, with G : Dop → Cop left adjoint to F : Cop → Dop if andonly if F a G. In particular, any theorem about left adjoints has a dual theorem about rightadjoints, which is a very useful duality principle for adjunctions. The other two dualizations
lead to new types of adjoint functors.
Definition 4.3.1. A pair of contravariant functors F : Cop → D and G : Dop → C are
mutually left adjoint if there exists a natural isomorphism
D(Fc, d)  C(Gd, c) ,

4.3. CONTRAVARIANT AND MULTIVARIABLE ADJOINT FUNCTORS

127

or mutually right adjoint if there exists a natural isomorphism
D(d, Fc)  C(c, Gd) .

Dualizing Definition 4.2.5, a pair of mutual left adjoints F : Cop → D and G : Dop → C
come equipped with a pair of “counit” natural transformations GF ⇒ 1C and FG ⇒ 1D ,while a pair of mutual right adjoints F : Cop → D and G : Dop → C come equipped witha pair of “unit” natural transformations 1C ⇒ GF and 1D ⇒ FG. The formulation of thetriangle identities in each case is left to Exercise 4.3.i.
Mutual right adjoints between preorders form what is sometimes called an antitone
Galois connection.
Example 4.3.2. For a fixed natural number n and algebraically closed field k, consider thesets of elements in the polynomial ring k[x1 , . . . , xn ] and in the vector space kn . There arecontravariant functors
V

P(k[x1 , . . . , xn ])op −→ P(kn ) andI

P(kn )op →− P(k[x1 , . . . , xn ])between the posets of subsets of these sets defined for S ⊂ k[x1 , . . . , xn ] and T ⊂ kn by:V(S ) B (t1 , . . . , tn ) ∈ kn f (t1 , . . . , tn ) = 0, ∀ f ∈ S
I(T ) B f ∈ k[x1 , . . . , xn ] f (t1 , . . . , tn ) = 0, ∀(t1 , . . . , tn ) ∈ T ,and these define mutual right adjoints because
T ⊂ V(S ) if and only if

S ⊂ I(T ) .

This Galois connection is a starting point of modern algebraic geometry.
The unit and counit encode relations
T ⊂ V(I(T ))andS ⊂ I(V(S )) .

A set of points T ⊂ kn defines a fixed point T = V(I(T )) if and only if T is closed in the
Zariski topology on kn = Spec(k[x1 , . . . , xn ]), introduced in Example 1.3.7(iv). A set ofpolynomials S ⊂ k[x1 , . . . , xn ] defines a fixed point S = I(V(S )) if and only if S defines aradical ideal of k[x1 , . . . , xn ]; this is Hilbert’s Nullstellensatz. Exercise 4.2.i now implies
that the poset of Zariski closed subsets of kn is isomorphic to the opposite of the poset ofradical ideals of k[x1 , . . . , xn ].
Example 4.3.3. Let Axiomσ be a set of axioms, i.e., sentences in a fixed first-orderlanguage whose signature σ, specifies a list of function, constant, and relation symbols to
be used with the standard logical symbols. Let Structσ be a set of σ-structures, i.e., setswith interpretations of the given constant, relation, and function symbols. For instance, thelanguage of the natural numbers has a constant symbol “0,” a binary function symbol “+,”and a binary relation symbol “≤.” A structure for this language is any set with a specified
constant, binary function, and binary relation. Axioms include sentences such as∀x, y, z, ((x ≤ y) ∧ (y ≤ z)) → (x ≤ z)

or∀x, x + 0 = x ,asserting transitivity of the relation “≤” and that the constant “0” serves as a unit for thebinary function “+.”Given a set of σ-structures M and a set of axioms A, we write M  A if each of theaxioms in A is satisfied by—meaning “is true in”—each of the σ-structures in M. For
instance, the first displayed sentence is satisfied by a structure if and only if its interpretationof the relation “≤” is transitive.

128

4. ADJUNCTIONSForm the poset categories P(Axiomσ ) and P(Structσ ) ordered by inclusion. There arecontravariant functors
True inP(Axiomσ )op −−−−−→ P(Structσ )Satisfying

P(Structσ )op −−−−−−−→ P(Axiomσ )andwhich send a set of axioms to the set of σ-structures, called models, that satisfy thoseaxioms and send a set of σ-structures to the set of axioms that they satisfy. These are
mutual right adjoints, forming what is called the Galois connection between syntax andsemantics [Smi].
Before turning our attention to multivariable functors, we make a useful observationthat applies equally to ordinary adjunctions:
Proposition 4.3.4. Consider a functor F : A → B so that for each b ∈ B there exists anobject Gb ∈ A together with an isomorphism(4.3.5)B(Fa, b)  A(a, Gb),

natural in a ∈ A .

Then there exists a unique way to extend the assignment G : ob B → ob A to a functor
G : B → A so that the family of isomorphisms (4.3.5) is also natural in b ∈ B.
In other words, if F : A → B is a functor admitting representations for each functor
B(F−, b) : Aop → Set

for each b ∈ B, then this data assembles into a right adjoint F a G.
Proof. Naturality of (4.3.5) in f : b → b0 demands that the functionf∗A(a, Gb)  B(Fa, b) −→ B(Fa, b0 )  A(a, Gb0 )equals post-composition by the yet-to-be-defined morphism G f : Gb → Gb0 . This composite function defines a natural transformation A(−, Gb) ⇒ A(−, Gb0 ), which by the Yoneda
lemma must equal post-composition by a unique morphism G f : Gb → Gb0 in A. Uniqueness of this definition implies that the assignment G : mor B → mor A is functorial, as we
saw in the proof of Theorem 1.5.9.4
The argument used to prove Proposition 4.3.4 can be extended to functors of many
variables.
Proposition 4.3.6. Suppose that F : A × B → C is a bifunctor so that for each objecta ∈ A, the induced functor F(a, −) : B → C admits a right adjoint Ga : C → B. Then:(i) These right adjoints assemble into a unique bifunctor G : Aop × C → B, defined sothat G(a, c) = Ga (c) and so that the isomorphisms
C(F(a, b), c)  B(b, G(a, c))

are natural in all three variables.
If furthermore for each b ∈ B, the induced functor F(−, b) : A → C admits a right adjoint
Hb : C → A, then:4More concisely, G is defined to be the unique restriction of the functor
Aop
/ Set
=

B(F−,−)

B



G

 .

y

A

along the Yoneda embedding so that the triangle commutes up to the specified natural isomorphisms (4.3.5).

4.3. CONTRAVARIANT AND MULTIVARIABLE ADJOINT FUNCTORS

129(ii) There is a unique bifunctor H : Bop × C → A defined so that H(b, c) = Hb (c) and theisomorphisms
C(F(a, b), c)  B(b, G(a, c))  A(a, H(b, c))

are natural in all three variables.
(iii) In this case, for each c ∈ C, the functors G(−, c) : Aop → B and H(−, c) : Bop → A
are mutual right adjoints.


Proof. Exercise 4.3.ii.
Definition 4.3.7. A triple of bifunctors
F

G

H

A×B−
→ C , Aop × C −→ B , Bop × C −→Aequipped with a natural isomorphism
C(F(a, b), c)  B(b, G(a, c))  A(a, H(b, c))defines a two-variable adjunction.
Particularly, when F : C × C → C defines some sort of monoidal product, its pointwisedefined right adjoints G and H are called its left and right closures, respectively. Whenthese are isomorphic, the bifunctor F is called closed.
Example 4.3.8. The product bifunctor×

Set × Set −→ Set

is closed: the operation called currying in computer science defines a family of natural
isomorphismsfff{A × B →− C}  {A →− C B }  {B →− CA} .
Thus, the product and exponential bifunctors
×

Set × Set −→ Set,(−)(−)

Setop × Set −−−−→ Set,(−)(−)

Setop × Set −−−−→ Set

define a two-variable adjunction.
Definition 4.3.9. A cartesian closed category is a category C with finite products inwhich the product bifunctor
×
C×C−
→C
is closed.
In addition to Set, the categories Fin, Cat, SetBG , and, more generally, SetC arecartesian closed; this was proven for Cat in Exercise 1.7.vii.
Example 4.3.10 (a convenient category of spaces). The category Top is not cartesianclosed. However, a famous paper of Steenrod proves that there exists a convenient categoryof topological spaces that is complete, cocomplete, cartesian closed, and sufficiently large
so as to contain the CW complexes [Ste67]. Steenrod’s convenient category is a subcategoryof the category Haus of Hausdorff spaces and continuous functions. Previous researchers
had argued that a “convenient category of spaces” satisfying Steenrod’s list of conditions
did not exist, but Steenrod notes:
The arguments are based on a blind adherence to the customary
definitions of the standard operations. These definitions are suitable
for the category of Hausdorff spaces, but they need not be for a
subcategory. The categorical viewpoint enables us to defrost thesedefinitions and bend them a bit.

130

4. ADJUNCTIONSWhat he means is that the “product” on a yet-to-be-defined subcategory cgHaus should be
the categorical product of Definition 3.1.9, which, as it turns out, is not preserved by thefull inclusion cgHaus ,→ Haus.
Objects in cgHaus are compactly generated Hausdorff spaces: Hausdorff spaces X
with the property that any subset A ⊂ X that intersects each compact subset K ⊂ X in aclosed subset A ∩ K is itself closed in X. The inclusion cgHaus ,→ Haus has a right adjointk : Haus → cgHaus called k-ification: the space k(X) refines the topology on X by adding
to the collection of closed sets those subsets A ⊂ X whose intersections with all compact
subsets K are closed.
By Exercise 4.6.ii and the dual of Proposition 4.5.15, the presence of this adjoint
implies that cgHaus is complete and cocomplete. The product on cgHaus is the k-ificationof the product on Haus; this latter product is preserved by the inclusion Haus ,→ Top. The
pointwise right adjoint is given by the following construction of function spaces Map(X, Y):
the underlying set of Map(X, Y) is the set of continuous maps X → Y, and the topology is
the k-ification of the compact-open topology. See [Ste67] for a proof.
Other categories admit a hom bifunctor that is not necessarily right adjoint to thecartesian product. The most famous of these are the “tensor a hom” adjunctions thatinspired Kan:
Example 4.3.11. Consider the hom bifunctor
Hom

Abop × Ab −−−→ Ab ,where Hom(A, B) is the group of homomorphisms A → B with addition defined pointwisein the abelian group B.5 Fixing the contravariant variable, there is an adjunction(4.3.12)

A⊗Z −
/
⊥
Ab
Hom(A,−)Ab o

Ab(A ⊗Z B, C)  Ab(B, Hom(A, C))

defining the tensor product. Once the objects A ⊗Z B ∈ Ab have been defined, Proposition
4.3.4 uses the isomorphisms (4.3.12) to extend this data into a functor A ⊗Z − : Ab → Ab.
Proposition 4.3.6(i) then extends this data into the tensor product bifunctor⊗Z

Ab × Ab −−→ Abin such a way that the tensor product and the hom define a two-variable adjunction.
As Kan suspected, this process can be reversed. Given the tensor product bifunctor⊗Z : Ab × Ab → Ab, the abelian group Hom(B, C) can be defined as a representation for
the functor
Ab(−⊗Z B,C)

(4.3.13)Abop −−−−−−−−−→ Set

Ab(A, Hom(B, C))  Ab(A ⊗Z B, C) .

Propositions 4.3.4 and 4.3.6(i) then imply that there is a unique way to extend this construction Hom : ob Ab × ob Ab → ob Ab into a bifunctor Hom : Abop × Ab → Ab in such a
way that the isomorphisms (4.3.13) are natural in B and C, as well as A.
Example 4.3.14. Temporarily, let Top denote not the ordinary category of topologicalspaces but the cartesian closed category of spaces defined in Example 4.3.10. In this
5Recall from §1.1 that this is why the collection of morphisms between a fixed pair of objects in a generalcategory is often denoted “Hom.” Here we use “Ab” for the mere set of homomorphisms, which in this instanceplays a secondary role.

4.3. CONTRAVARIANT AND MULTIVARIABLE ADJOINT FUNCTORS

131

context, the two-variable adjunction specializes to define an adjunction:
Top o

S 1 ×− /⊥Top

Map(S 1 ,−)where S 1 ∈ Top is the unit circle. The space Map(S 1 , X) is the free loop space on X; points
in Map(S 1 , X) are loops in the space X.
The slice category of Top under the singleton space ∗ defines a convenient categoryTop∗ of based topological spaces. By Exercise 4.3.iv(ii), Top∗ admits a two-variableadjunction
∧

Map∗

op

Top∗ × Top∗ −→ Top∗ ,Top∗ × Top∗ −−−−→ Top∗ ,

op

Map∗

Top∗ × Top∗ −−−−→ Top∗ ,where Map∗ ((X, x), (Y, y)) denotes the based space of basepoint-preserving continuous functions (X, x) → (Y, y). The bifunctor ∧ is called the smash product.
The spaceΩX B Map∗ (S 1 , (X, x))is the based loop space on (X, x); the basepoint x ∈ X implicit in the common notation.
The functor Ω has a left adjoint ΣX B S 1 ∧ X, which constructs the reduced suspensionof the based space (X, x). This defines the loops a suspension adjunction between based
topological spaces:Top∗ oΣ⊥Ω/Top∗ (ΣX, Y)  Top∗ (X, ΩY)

Top∗

Definition 4.3.7 can be generalized to define multivariable adjunctions. Applying
Proposition 4.3.6, an n-variable adjunction is determined by a functor F : A1 ×· · ·× An → B
admitting pointwise right adjoints when any (n − 1) of its variables are fixed. See [CGR14]
for more.
Exercises.
Exercise 4.3.i. Dualize Definition 4.2.5 to define mutual left adjoints and mutual right adjoints as a pair of contravariant functors equipped with appropriate natural transformations.
Exercise 4.3.ii. Prove Proposition 4.3.6.
Exercise 4.3.iii. Show that the contravariant power set functor P : Setop → Set is mutuallyright adjoint to itself.
Exercise 4.3.iv. Define pointwise adjoints to the following bifunctors, giving rise toexamples of two-variable adjunctions.
(i) There is a bifunctor
op

Hom∗

Set∗ × Set∗ −−−−→ Set∗ ,where Hom∗ ((X, x), (Y, y)) is defined to be the set of pointed functions (X, x) → (Y, y),with the constant function at y serving as the basepoint. Define a two-variableadjunction determined by this bifunctor, the pointwise left adjoints to Hom∗ ((X, x), −).
∧(ii) Describe the left adjoint bifunctor Set∗ × Set∗ −→ Set∗ constructed in (i) in a sufficiently categorical way so that Set can be replaced by any cartesian closed category
with pushouts and pullbacks.

132

4. ADJUNCTIONS(iii) The discussion in Example 4.3.11 extends to any category ModR of modules overa commutative ring R. If R is not commutative, for a pair of R-modules A and B,
Hom(A, B) is not necessarily an R-module, but it is still an abelian group. This
construction defines a bifunctor
Homop

ModR × ModR −−−→ Ab .

Extend this data to a two-variable adjunction.
(iv) In a similar fashion, define a two-variable adjunction determined by the hom bifunctorop

Hom

ChR × ChR −−−→ ModR ,where the ring R is commutative and the set Hom(A• , B• ) of chain homomorphisms
A• → B• inherits an R-module structure with addition and scalar multiplication
defined elementwise.
4.4. The calculus of adjunctions
Perhaps the purpose of categorical algebra is toshow that which is formal is formally formal.
J. Peter May, [May01]

This section introduces the basic calculus of adjoint functors: proving that adjoints are
unique up to unique natural isomorphism, that adjunctions can be composed and whiskered,and that any equivalence can be promoted to an adjoint equivalence in which the inverse
equivalences are also adjoints, with either choice of handedness. What further unites
these results is that they all admit “formal categorical”6 proofs, meaning that they can beproven syntactically using objects (categories), morphisms (functors), and morphisms ofmorphisms (natural transformations), which can be composed horizontally and vertically inthe sense described in §1.7. As our interest is as much in these categorical proof techniques
as in the results themselves, we give secondary proofs of these results, to illustrate alternate
Yoneda-style arguments, and briefly comment on the merits of each approach.
Proposition 4.4.1. If F and F 0 are left adjoint to G, then F  F 0 , and moreover there
is a unique natural isomorphism θ : F  F 0 commuting with the units and counits of the
adjunctions:
η

1C
η0

+3 GF



FG
θG

Gθ

% 
GF 0

+3 1D
9A
0



0

FG

Proof 1. To define a natural transformation θ : F ⇒ F 0 , it suffices, by Lemma 4.1.3,
to define a transposed natural transformation, which we take to be η0 : 1C ⇒ GF 0
Fc
(4.4.2)

θc

Ff


Fc0

/ F0c


θ c0

F0 f

/ F 0 c0

c
!

f


c0

η0c

/ GF 0 c
GF 0 fη0c0


/ GF 0 c06Here, formal category theory is meant in the rather narrow sense of 2-category theory.

4.4. THE CALCULUS OF ADJUNCTIONS

133

Remark 4.2.7(iv) provides an explicit formula for θ:
Fη0

θB F

+3 FGF 0

F 0+3 F 0 .

Exchanging the roles of F 0 and F, we also define a natural transformation θ0 : F 0 ⇒ F to
be the transpose of η : 1C ⇒ GF, given by the formula:F0η

θ0 B F 0

0 F+3 F 0GF+3 F.

The hope is that these natural transformations define inverse isomorphisms. To provethat θ0 · θ = 1F , it suffices to demonstrate this equality after transposing both θ0 · θ and 1F ,i.e., to show that η : 1 ⇒ GF equals the composite
1

η

+3 GF

GFη0

+3 GFGF 0

GF 0 +3

GF 0

GF 0 +3

GF 0

GF 0 η

+3 GF 0GF

G 0 F +3

GF .

GF 0 η

+3 GF 0GF

G 0 F +3

GF .

By naturality of η, this composite equals
1

η0

ηGF 0

+3 GF 0

+3 GFGF 0

By the triangle identity G · ηG = 1G , this reduces toη0

1+3 GF 0

GF 0 η

+3 GF 0GF

G 0 F +3

GF .

+3 GF

η0 GF

+3 GF 0GF

G 0 F +3

GF ,By naturality of η0 , this equals
η1and by the triangle identity G · η G = 1G , this reduces to
0

0

1

η

+3 GF

as desired. The other diagram chase, proving that θ · θ0 = 1F 0 , is dual.
From the formula for θ, there are easy diagram chases, left to the reader, that verify thatthe triangles of natural transformations displayed in the statement commute. The left-handtriangle asserts that the transpose of θ across the adjunction F a G is η0 , as indeed has beentaken as the definition in (4.4.2), thereby proving uniqueness.
Proof 2. By the Yoneda lemma, the composite natural isomorphism
D(F 0 c, d)  C(c, Gd)  D(Fc, d) ,(4.4.3)defines a natural isomorphism θ : F  F 0 , whose component θc is defined to be the imageof 1F 0 c under the bĳection (4.4.3). The first isomorphism carries 1F 0 c to η0c : c → GF 0 c.
The second isomorphism carries it to its transpose along F a G. Thus,Fη0c

θc B Fc/ FGF 0 c

Fc0

/ F0c ,

which is the formula used in Proof 1; here we see that it was not necessary to guess thedefinition of θ. Setting d = F 0 c in the commutative triangle of natural isomorphisms (4.4.3)
proves the compatibility of θ : F  F 0 with the units of the adjunctions; setting c = Ud
proves compatibility with the counits.
Proposition 4.4.4. Given adjunctions F a G and F 0 a G0
Co

F
⊥
G

/

Do

F0
⊥
G0

/

E

the composite F 0 F is left adjoint to the composite GG0 .

Co

F0 F
⊥
GG0

/

E

134

4. ADJUNCTIONSProof 1. There are natural isomorphisms
E(F 0 Fc, e)  D(Fc, G0 e)  C(c, GG0 e) ,the first defined using F 0 a G0 and the second defined using F a G.



Proof 2. The only reasonable definitions for the unit and counit of F 0 F a GG0 thatcan be defined from the data of the two adjunctions are:η̄ B 1C

η

+3 GF

Gη0 F

+3 GG0 F 0 F

¯ B F 0 FGG0

F 0 G0 +3

F 0G0

0

+3 1E

which indeed are the natural transformations that result from passing the identities 1F 0 F and
1GG0 across the natural isomorphisms in Proof 1. The proof of the triangle identities is anentertaining diagram chase left to the reader.
Proposition 4.4.5. Any equivalence
Co

/D

F

η : 1C  GF,

G

 : FG  1D

can be promoted to an adjoint equivalence, in which the natural isomorphisms satisfy thetriangle identities, by replacing either one of the originally specified natural isomorphisms
by a new unit or counit.
Note that the symmetry in the definition implies that an equivalence of categories
F : C → D may be regarded as either a left or right adjoint; indeed, any chosen equivalence
inverse G : D → C is both left and right adjoint to F.
Proof 1. Because η : 1C  GF and  : FG  1D are isomorphisms, so is the composite
ηG

γB G

+3 GFGG

+3 G

but it need not be an identity. Redefining either  or η so as to absorb the isomorphism
γ−1 —it will not matter which—we will show that the resulting pair of natural isomorphisms
define the unit and the counit of an adjunction F a G.
Let  0 B  · Fγ−1 . By naturality of η, the diagram
ηG

G

−1

+3 GFG GFγ 3+ GFG
6>

G+319 G

ηG

γ−1

&

γ

G

commutes, proving one triangle identity G 0 · ηG = 1G . By naturality of η and  0 , and by
this first triangle identity, the diagram
F

Fη

Fη


FGF

+3 FGF

F0

FGFη

FηGF


+3 FGFGF

Fη

0
FGF

FGF0


FGF

+3 F

F0


+3 FGF+3 F

F0commutes, proving that F0 · Fη is an idempotent. By cancellation, any idempotent isomorphism is necessarily an identity; thus F0 · Fη = 1F , as desired.


4.4. THE CALCULUS OF ADJUNCTIONS

135

Proof 2. If η : 1C  GF is one of the natural isomorphisms defining an equivalenceof categories F : C  D : G, then the function
D(Fc, d)G

/ C(GFc, Gd)(ηc )∗/ C(c, Gd)

defines a natural isomorphism for all c ∈ C and d ∈ D: the first map is an isomorphism
because G is fully faithful and the second map is an isomorphism because ηc is an isomorphism. By Remark 4.2.7, it follows that F and G define an adjunction with unit η.
Proposition 4.4.6. Given an adjunction
Co

/

F
⊥
G

D

post-composition with F and G defines a pair of adjoint functors
CJ o

/

F∗
⊥
G∗

DJ

for any small category J, and pre-composition with F and G also defines an adjunction
EC o

G∗
⊥
F∗

/

ED

for any locally small category E.7
As in the case of the other results in this section, this can be proven by appealing toeither Definition 4.1.1 or Definition 4.2.5. Rather than spell out the details, which are left
for the exercises, we instead sketch a third proof.
Definition 4.2.5 reveals that an adjunction can be defined internally to the 2-category
CAT introduced in Definition 1.7.8: an adjunction consists of a pair of objects, a pair of
1-morphisms, and a pair of 2-morphisms satisfying certain composition relations. Now
a 2-functor is a morphism between 2-categories: a map on objects, 1-morphisms, and2-morphisms preserving composition and identities at all levels. There is a 2-dimensional
analog of Lemma 1.6.5—that functors preserve commutative diagrams—which says that 2functors preserve “2-dimensional diagrams” including, in particular, adjunctions. Ignoring
size issues, Proposition 4.4.6 is an immediate corollary of the fact that(−)J : CAT → CAT

and E(−) : Catop → CAT

define 2-functors for any small category J and locally small category E.
Remark 4.4.7. The “syntactic” proofs of Propositions 4.4.1, 4.4.4, 4.4.5, and 4.4.6 (thelast of these left to Exercise 4.4.ii) apply immediately to adjunctions defined internally to
any 2-category, in place of Cat. This suggests a reason to prefer these proofs to the alternate“Yoneda-style” arguments, which apply only to Cat. However, using analogous arguments
to those made in §3.4, these “Yoneda-style” arguments can be bootstrapped from Cat to a
general 2-category. As (co)limits in a general category are defined representably in termsof limits in Set, so too—again by the Yoneda lemma—are adjunctions and equivalences
in a 2-category are defined representably in terms of adjunctions and equivalences in Cat.
This line of reasoning supports the category theorist’s joke that “all theorems are Yoneda.”
7Note that these functor categories will not be locally small unless C and D are small. If local smallness ofthe functor categories is not a concern, then the hypothesis that J is small can be dropped.

136

4. ADJUNCTIONS

Exercises.
Exercise 4.4.i. Complete the second proof of Proposition 4.4.4 by verifying the triangle
identities.
Exercise 4.4.ii. Use the unit and counit associated to an adjunction to prove Proposition
4.4.6.
Exercise 4.4.iii. Use the natural bĳection between hom-sets to prove the first half of
Proposition 4.4.6, that an adjunction F a G induces an adjunction F∗ a G∗ by postcomposition. (Hint: Exercise 3.2.iv might help.)

4.5. Adjunctions, limits, and colimits
In this section, we explore the interaction between adjoint functors and limits andcolimits. Our first result provides an adjoint characterization of completeness and cocompleteness of a category C with regard to diagrams of a fixed shape J.
Proposition 4.5.1. A category C admits all limits of diagrams indexed by a small category
J if and only if the constant diagram functor ∆ : C → CJ admits a right adjoint, and admits
all colimits of J-indexed diagrams if and only if ∆ admits a left adjoint:

C_

~

colim
⊥
∆ / CJ .
⊥lim

When these adjoints exist, they define the limit and colimit functors introduced in
Proposition 3.6.1. Recall that the axiom of choice is needed to define the action of the limit
or colimit functor on objects.
Proof. These dual statements follow immediately from the defining universal properties of a limit and colimit. For c ∈ C and F ∈ CJ , the hom-set CJ (∆c, F) is the set of naturaltransformations from the constant J-diagram at c to the diagram F. This is precisely the setof cones over F with summit c, as defined in 3.1.2. There is an object lim F ∈ C together
with an isomorphism
CJ (∆c, F)  C(c, lim F)

that is natural in c ∈ C if and only if this limit exists. If such natural isomorphisms exist for
all diagrams F ∈ CJ , then Proposition 4.3.4 applies to extend the objects lim F to a limit
functor lim : CJ → C.
Proposition 4.5.1 reveals that limits assemble into a right adjoint to the constantdiagram functor. The reason that limits appear on the right is because limits are defined as
representations for contravariant functors: the universal property of the limit of F : J → C
characterizes the functor C(−, lim F).
Similarly, the value of a right adjoint functor G : D → C on an object d ∈ D isdetermined by a characterization of the contravariant representable functor C(−, Gd). Basedon these observations, one might expect right adjoints to interact nicely with limits andindeed this is the case:
Theorem 4.5.2 (RAPL). Right adjoints preserve limits.

4.5. ADJUNCTIONS, LIMITS, AND COLIMITS

137

Proof. Consider a diagram K : J → D admitting a limit cone λ : lim K ⇒ K in D,
illustrated here in the case where the indexing category J is the poset of integers.
lim K
··· s

/ K j−2

λ−2

···

u

K f−2

{

/ K j−1

λ0

λ−1

K f−1

λ1



/ K j0

K f0

#

λ2

···

/ K j1

)/

K f1

K j2

/+ · · ·Applying a right adjoint G : D → C, we obtain a cone Gλ : G lim K ⇒ GK over the diagram
GK in C.
G lim K··· s

/ GK j−2t···
GK f−2Gλ−2y/ GK j−1

Gλ−1

GK f−1

Gλ0

Gλ1/ GK j0

GK f0

Gλ2

$
/ GK j1

···
GK f1

*/

GK j2

+/ · · ·The statement asserts that this is a limit cone for the diagram GK : J → C. To prove this,consider another cone µ : c ⇒ GK.
c
µ−2

µ−1

···

···

y t

vr

/ GK j−2···
GK f−2Gλ−2 y/ GK j−1

Gλ−1

GK f−1µ0

G lim K
Gλ0

µ1

···
Gλ1

 
/ GK j0

µ2

GK f0

Gλ2

% 
/ GK j1

···
GK f1

*/ %
GK j2

/+ ( · · ·The legs of this cone transpose across the adjunction F a G to define maps in D
Fc
τ

···

vr

···

/ K j−2

yt

···
K f−2

µ]−2
λ−2

 y

/ K j−1

µ]−1
λ−1

K f−1

]

 µ0 ]
lim K µ1λ0 
/ K j0

λ1

K f0

µ]2
λ2

% 
/ K j1

···
···
K f1

$
/* K j 2/+ ( · · ·which define a cone µ] : Fc ⇒ K over K, by naturality of the transposition relation, as
expressed for instance by Lemma 4.1.3. There is a unique factorization τ : Fc → lim K ofthe cone µ] through the limit cone λ. The map τ transposes to define a map τ[ : c → G lim K
c
τ[

µ−2

µ−1

Gλ−2

Gλ−1

···

···

vr

y t

/ GK j−2···
GK f−2 y/ GK j−1

GK f−1

 µ0
G lim K µ1
Gλ0

 / GK j0

Gλ1

GK f0

µ2
···
Gλ2

% 
/ GK j1

···
GK f1

*/ %
GK j2

+/ ( · · ·which, by Lemma 4.1.3, defines a factorization of the cone µ through the cone Gλ. Thisfactorization is clearly unique: another such factorization c → G lim K would transpose to138

4. ADJUNCTIONS

define a factorization of µ] through λ, which, by the universal property of the limit cone
λ would necessarily equal τ, and τ[ is the unique transpose of this map. This shows that
Gλ : G lim K ⇒ GK has the universal property required to define a limit cone for GK. 
Put more concisely, the argument just presented describes a series of natural isomorphisms:
CJ (∆c, GK)  DJ (F∆c, K)  DJ (∆Fc, K)  D(Fc, limJ K)  C(c, G limJ K) ,which, by the defining universal property of the limit, says that G limJ K defines a limit forthe diagram GK.
Dually, of course:
Theorem 4.5.3 (LAPC). Left adjoints preserve colimits.
Corollaries are ubiquitous. For instance:
Corollary 4.5.4. For any function f : A → B, the inverse image f −1 : PB → PA, afunction between the power sets of A and B, preserves both unions and intersections, while
the direct image f∗ : PA → PB only preserves unions.
Proof. Example 4.1.8 describes adjunctions

PB`

}

f∗
⊥
f −1
⊥

/ PA .

f!

Unions are colimits and intersections are limits in the poset category PA.

Corollary 4.5.5. For any vector spaces U, V, W,
U ⊗ (V ⊕ W)  (U ⊗ V) ⊕ (U ⊗ W) .
Proof. The two-variable adjunction of Example 4.3.11 can be defined for the categoryof modules over any commutative ring, in particular for the category of vector spaces overa field k. It follows that the functor U ⊗ − : Vectk → Vectk is left adjoint to Hom(U, −) andconsequently preserves the coproduct V ⊕ W. This argument also proves that the tensor
product distributes over arbitrary direct sums.

Similarly, on account of the adjunctions A × − a (−)A between the product and exponential on Set or its subcategory Fin, Theorems 4.5.2 and 4.5.3 supply proofs of many ofthe basic operations in arithmetic, first discussed in Example 1.4.9.
Corollary 4.5.6. For any sets A, B, C, there are natural isomorphisms
A × (B + C)  (A × B) + (A × C)

(B × C)A  BA × C A

AB+C  AB × AC .

Consequently, for any cardinals α, β, γ, cardinal arithmetic satisfies the laws:
α × (β + γ) = (α × β) + (α × γ)

(β × γ)α = βα × γα

αβ+γ = αβ × αγ .

Proof. The left adjoint A × − preserves the coproduct B + C, the right adjoint (−)A
preserves the product B × C, and the functor A− : Setop → Set, which is mutually rightadjoint to itself, carries coproducts in Set to products in Set. The laws of cardinal arithmetic
follow by applying the cardinality functor |−| : Setiso → Card, which converts these natural
isomorphisms to identities in the discrete category of cardinals.
4.5. ADJUNCTIONS, LIMITS, AND COLIMITS139

The forgetful functors of Example 4.1.10 carry any limits that exist in the categories
of groups, rings, modules, and so forth to corresponding limits of their underlying sets.
Indeed, in §5.6, we show that these forgetful functors create, and not merely preserve, alllimits. The dual result, that left adjoints preserve colimits, is less directly useful in suchcontexts, characterizing only those colimit constructions involving “free” diagrams. For
instance:
Corollary 4.5.7. The free group on the set X t Y is the free product of the free groups on
the sets X and Y.
The free product of not-necessarily free groups will be constructed in §5.6 using
language that also describes the construction of the coproducts in the categories of abeliangroups, rings, and modules, among many others.
Theorems 4.5.2 and 4.5.3 also have important applications to homological algebra.
Corollary 4.5.8. For any R-S bimodule M, the tensor product M ⊗S − is right exact.
Before proving Corollary 4.5.8, we should explain its statement. The term “right exact”
comes from homological algebra, which studies functors between abelian categories. An
abstract definition is given in Definition E.5.1, but by a powerful result, Theorem E.5.2, itsuffices to declare that a category is abelian if it is a full subcategory of category of modules
that contains the zero object and is closed under direct sums, kernels, and cokernels.8
The following general definitions of left and right exactness makes sense for any
functor, not necessarily between abelian categories.
Definition 4.5.9. A functor is right exact if it preserves finite colimits and left exact if it
preserves finite limits.
The following proposition connects Definition 4.5.9 to the notions of left and rightexactness that are used in homological algebra.
Proposition 4.5.10. A functor F : A → B between abelian categories is left exact if andonly if it preserves direct sums and kernels. For such functors, if
0 → A → A0 → A00
is an exact sequence in A, then
0 → FA → FA0 → FA00
is exact in B.
Proof. Direct sums and kernels are both finite limits, so if F is left exact, these
constructions are preserved. Conversely, Theorem 3.4.12 proves that all finite limits can bebuilt from finite products and equalizers. In an abelian category, the equalizer of a parallel
pair of maps f, g : A ⇒ A0 is the kernel of the map f − g : A → A0 . A functor that preserves
direct sums also preserves differences of maps, so if F preserves direct sums and kernels it
preserves all finite limits.
Now a sequence
/ A0
/ A00
/A
0
is exact in A just when i is the kernel of j (equivalently, when j is a monomorphism), andwhen j is the kernel of k. A left exact functor preserves these kernels and also the zero
object 0.

i

j

k

8A zero object is an object that is both initial and terminal; see Exercise 1.6.i. Direct sums are defined in
Remark 3.1.27. As in Example 3.1.14, the kernel of a map f : A → A0 is the equalizer of f with the zero
homomorphism, while the cokernel is the coequalizer of this pair.

140

4. ADJUNCTIONS

Immediately from Definition 4.5.9 and Theorems 4.5.2 and 4.5.3:
Corollary 4.5.11. For any adjoint functors between abelian categories, the left adjoint
is right exact and the right adjoint is left exact. Moreover, both functors are additive.9
Corollary 4.5.8 is a special case:
Proof of Corollary 4.5.8. For any R-S bimodule M, there is a pair of adjoint functors
/

M⊗S −

ModS o⊥ModR ,
HomR (M,−)

where the right adjoint carries a left R-module N to the left S -module of R-module homomorphisms M → N. As a left adjoint, Corollary 4.5.11 proves that M ⊗S − is rightexact.
For a certain important class of adjoint functors, Theorems 4.5.2 and 4.5.3 take on astronger form.
Definition 4.5.12. A reflective subcategory of a category C is a full subcategory D sothat the inclusion admits a left adjoint, called the reflector or localization:
o

D 

L
⊥

/ C.

Where possible, we identify the full subcategory D with its image in C, declining inparticular to introduce notation for the inclusion functor. With this convention in mind, thecomponents of the unit have the form c → Lc; the mnemonic is that “an object looks at
its reflection.” The following lemma implies that the components of the counit Ld  d are
isomorphisms. Via the counit, any object in a reflective subcategory D ,→ C is naturally
isomorphic to its reflection back into that subcategory.
Lemma 4.5.13. Consider an adjunction
Co

F
⊥
G

/

D

with counit  : FG ⇒ 1D . Then:(i) G is faithful if and only if each component of  is an epimorphism.
(ii) G is full if and only if each component of  is a split monomorphism.
(iii) G is full and faithful if and only if  is an isomorphism.
Dually, writing η : 1C ⇒ GF for the unit:(i) F is faithful if and only if each component of η is a monomorphism.
(ii) F is full if and only if each component of η is a split epimorphism.
(iii) F is full and faithful if and only if η is an isomorphism.
Proof. Exercise 4.5.vi.



Example 4.5.14. The following define reflective subcategories:(i) Compact Hausdorff spaces define a reflective subcategory cHaus ,→ Top of thecategory of all topological spaces. The reflector is the functor β : Top → cHaus
sending a space to its Stone–Čech compactification, which is constructed in Example 4.6.12. The universal property of the unit says that any continuous function
9A functor between abelian categories is additive if it preserves direct sums.

4.5. ADJUNCTIONS, LIMITS, AND COLIMITS141

X → K from a space X to a compact Hausdorff space K extends uniquely to theStone–Čech compactification:
/K
>

X
βX

(ii) Abelian groups define a reflective subcategory Ab ,→ Group of the category ofgroups. The reflector carries a group G to its abelianization, the quotient G/[G, G] by
the commutator subgroup, the normal subgroup generated by elements ghg−1 h−1 .
The quotient maps G → G/[G, G] define the components of the unit. For any
abelian group A, there is an isomorphism A  A/[A, A]; the commutator subgroup[A, A] is trivial if and only if the group A is abelian. The adjunction asserts that ahomomorphism from a group G to an abelian group A necessarily factors throughthe abelianization of G. A similar construction defines a left adjoint to the inclusion
CRing ,→ Ring of commutative rings into the category of all rings.
(iii) The inclusion Abtf ,→ Ab of torsion-free abelian groups is reflective. The reflector
sends an abelian group A to the quotient A/T A by its torsion subgroup. The quotient
maps A → A/T A define the components of the unit. Any map from A to a torsionfree group factors uniquely through this quotient homomorphism because any torsion
elements of A must be contained in its kernel.
(iv) As described in Example 4.1.10(xi), for any ring homomorphism φ : R → T , there
exist adjoint functors
ModT

o T ⊗R −⊥φ∗/ ModR ,the right adjoint being restriction of scalars and the left adjoint being extension ofscalars. The restriction of scalars functor is always faithful because both categories
of modules admit faithful forgetful functors to Ab, and is full if and only if φ : R → T
is an epimorphism. For such homomorphisms, restriction of scalars identifies ModT
as a reflective subcategory of ModR .
Epimorphisms in Ring include all surjections but are not limited to the surjections. The localizations define another important class of epimorphisms. Let S ⊂ R
be a monoid under multiplication; that is, S is a multiplicatively closed subset ofthe ring R. The localization R → R[S −1 ] is an initial object in the category whose
objects are ring homomorphisms R → T that carry all of the elements of S to units
in T . For integral domains, the ring R[S −1 ] can be constructed as a field of fractions.
A similar construction exists for general commutative rings.
(v) Recall that a presheaf on a space X is a contravariant functor O(X)op → Set fromthe poset of open subsets of X to sets. A presheaf is a sheaf if it preserves certain
limits (see Definition 3.3.4). The sheaves define a reflective subcategory ShvX of thecategory of presheaves, with the left adjoint called sheafification:o sheafify O(X)op⊥ / SetShvX  (vi) The category of small categories defines a reflective subcategory of the category
opSet∆ of simplicial sets via an adjunction that is constructed in Exercise 6.5.iv:
oCat  

h
⊥
N

∆op

/ Set

142

4. ADJUNCTIONS

Here ∆ ⊂ Cat is the full subcategory whose objects are the finite non-empty ordinals,
in this context denoted by [0], [1], [2], . . ., and whose morphisms are all functors, i.e.,
oporder-preserving functions, between them. The embedding N : Cat ,→ Set∆ carries
a small category C to its nerve: N C : ∆op → Set sends [n] = 0 → 1 → · · · → n
opto the set of functors N Cn := Cat([n], C). The left adjoint h : Set∆ → Cat sends
a simplicial set to its homotopy category. Restricting to the objects [0], [1] ∈ ∆, asimplicial set X : ∆op → Set has an underlying reflexive directed graph/
X0 o/ X1 .
The homotopy category hX is a quotient of the free category generated by thisreflexive directed graph modulo relations that arise from elements of the set X2 . In
particular, the counit defines an isomorphism hN C  C for any category C, provingthat the inclusion is full and faithful.
The following result explains our particular interest in reflective subcategories. The
presence of a left adjoint to the inclusion of a full subcategory provides complete information
about limits and colimits in that subcategory:
Proposition 4.5.15. If D ,→ C is a reflective subcategory, then:(i) The inclusion D ,→ C creates all limits that C admits.
(ii) D has all colimits that C admits, formed by applying the reflector to the colimit in C.
By Theorems 4.5.2 and 4.5.3, if D has limits or colimits, they must be constructed in
the way described in Proposition 4.5.15: limits are preserved by the inclusion D ,→ C andcolimits of diagrams in D, regarded as diagrams in C, are preserved by L : C → D. The
real content of this result is that these (co)limits necessarily exist in D. A special case ofthis completes some unfinished business from §3.5:
Corollary 4.5.16. Cat is complete and cocomplete.
op

Proof. By Proposition 3.3.9, Set∆ is complete and cocomplete, with limits and colimits defined objectwise in Set. Proposition 4.5.15 implies that the reflective subcategory
Cat inherits these limits, defined objectwise in Set, and also these colimits, defined byopapplying the homotopy category functor to the colimit in Set∆ .
Proposition 4.5.15(i) appears as Corollary 5.6.6, where this result is deduced as a
special case of the more general Theorem 5.6.5.
Proof of 4.5.15(ii). For clarity, write i : D ,→ C for the inclusion, the right adjoint
to the reflector L. Consider a diagram F : J → D and let λ : iF ⇒ c be a colimit cone
for the diagram iF in C. By Theorem 4.5.3, the left adjoint L sends this to a colimit cone
Lλ : LiF ⇒ Lc in D. Now the counit supplies a natural isomorphism Li  1D . Composingwith this natural isomorphism yields a colimit cone F  LiF ⇒ Lc for the original diagramin D.

Exercises.
Exercise 4.5.i. When does the unique functor ! : C → 1 have a left adjoint? When does it
have a right adjoint?
Exercise 4.5.ii. Suppose the diagonal functor ∆ : C → CJ admits both left and rightadjoints. Describe the units and counits of these adjunctions.
Exercise 4.5.iii. Use Proposition 4.5.1 to prove that in any complete category limits
commute with limits in the sense of the natural isomorphism of limit functors (3.8.2).

4.6. EXISTENCE OF ADJOINT FUNCTORS

143

Exercise 4.5.iv. If G : D → C has a left adjoint F and if D and C admit all limits indexed
by a category J, use Propositions 4.5.1, 4.4.4, and 4.4.6 to argue that right adjoints preserve
limits by considering the diagram of adjoint functors:
o

OC

G
>
F

∆ a limlim ` ∆



CJ o

/ DO

F∗
⊥
G∗

/ J
D

The characterization of preservation of limits provided by Exercise 3.3.i might prove useful.
Exercise 4.5.v. Show that a morphism f : x → y in C is a monomorphism if and only if
the square
x
1x

1x

/x
f


x

f

/ y

is a pullback. Conclude that right adjoints preserve monomorphisms, and that left adjoints
preserve epimorphisms.
Exercise 4.5.vi. Prove Lemma 4.5.13.
Exercise 4.5.vii. Consider a reflective subcategory inclusion D ,→ C with reflector
L : C → D.
(i) Show that ηL = Lη, and that these natural transformations are isomorphisms.
(ii) Show that an object c ∈ C is in the essential image of the inclusion D ,→ C,
meaning that it is isomorphic to an object in the subcategory D, if and only if ηc is
an isomorphism.
(iii) Show that the essential image of D consists of those objects c that are local for theclass of morphisms that are inverted by L. That is, c is in the essential image if andonly if the pre-composition functionsf∗C(b, c) −→ C(a, c)

are isomorphisms for all maps f : a → b in C for which L f is an isomorphism in D.
This explains why the reflector is also referred to as “localization.”
Exercise 4.5.viii. Find an example which demonstrates that the inclusion of a reflective
subcategory does not create all colimits.
4.6. Existence of adjoint functors
Does the inclusion Ring ,→ Rng of the category of unital rings into the category of
possibly non-unital rings have any adjoints? A first strategy to probe a question of thisform might be called the “initial and terminal objects test”: by Theorems 4.5.2 and 4.5.3, afunctor admitting a left adjoint must necessarily preserve all limits while a functor admitting
a right adjoint must necessarily preserve all colimits. The ring Z is initial in Ring but notin Rng, so there can be no right adjoint. The zero ring is terminal in both categories, so a

144

4. ADJUNCTIONS

left adjoint to the inclusion might be possible. Indeed, Ring has all limits and the inclusion
preserves them.10
The search for a left adjoint to Ring ,→ Rng might be thought of as some sortof formulaic (that is to say functorial) optimization problem, whose aim is to adjoin amultiplicative unit to a possibly non-unital ring R in the “most efficient way possible.”
The Yoneda lemma can be used to make this intuition precise. To define the value of ahypothetical left adjoint on a possibly non-unital ring R, we seek a unital ring R∗ together
with a natural isomorphism
Ring(R∗ , S )  Rng(R, S )for all unital rings S . Note that even if R has a unit, the ring R∗ will still differ from R
because homomorphisms in Ring must preserve units, while homomorphisms in Rng neednot.
To define this natural isomorphism is to define a representation R∗ for the functor
Rng(R, −) : Ring → Set. RBy Proposition 2.4.8, a representation defines an initial object in
the category of elements Rng(R, −). Objects in this category are homomorphisms R → S
whose codomain is a unital ring; morphisms are commutative triangles
R
S



/ S0

whose leg opposite R is a unital ring homomorphism but whose legs with domain R do
not necessarily preserve the multiplicative unit, if R happens to have one. This categoryof elements is isomorphic to the comma category R ↓ Ring of non-unital homomorphisms
from R to a unital ring. The optimization problem is solved if we can find a unital ring R∗
and ring homomorphism R → R∗ that is initial in this category.11
The same line of reasoning proves the following general result.
Lemma 4.6.1. A functor U : A → S admits a left adjoint if and only if for each s ∈ S thecomma category s ↓ U has an initial object.
Comma categories are defined in Exercise 1.3.vi: here, objects of s ↓ U are pairs(a ∈ A, f : s → Ua ∈ S) and a morphism from f : s → Ua to f 0 : s → Ua0 is a map
h : a → a0 ∈ A so that the evident triangle commutes in S.
Proof. The comma category s ↓ U is isomorphic to the category of elements for thefunctor S(s, U−) : A → Set. If a left adjoint F exists, then the component of the unit at s
defines an initial object η s : s → UF s in this category; see Proposition 2.4.8 or Exercise
4.2.iv.
10As an application of Theorem 5.6.5, the forgetful functors from Ring or from Rng to Set both create all
limits. On account of the commuting triangle
Ring



U

#

/ Rng
Set

{

U

it follows that the inclusion preserves all limits.
11The optimization problem intuition for the construction of adjoint functors is explained very well on
Wikipedia’s “adjoint functors” entry (retrieved on April 14, 2015). However, Wikipedia’s suggestion that “picking the right category [to express the universal property of the adjoint construction] is something of a knack” isincorrect. A left adjoint to a functor U : A → S at an object s ∈ S defines an initial object in the category ofelements of the functor S(s, U−) : A → Set; dually, a right adjoint defines a terminal object in the category ofelements of S(U−, s) : Aop → Set. See Proposition 2.4.8.

4.6. EXISTENCE OF ADJOINT FUNCTORS

145

Conversely, suppose each s ↓ U admits an initial object, which we suggestively denoteby η s : s → UF s. This defines the value of a function F : ob S → ob A, which we now
extend to a functor; the proof of Proposition 4.3.4 gives an alternate account of this same
construction. For each morphism f : s → s0 in S, define F f : F s → F s0 to be the uniquemorphism in A making the square
s
f


s0

ηs

/ UF s
UF f

η s0


/ UF s0

commute; the fact that η s is initial in s ↓ U implies the existence and uniqueness ofsuch a map. The functoriality of F follows from the uniqueness of these choices, so
this construction defines a functor F : S → A together with a natural transformation
η : 1S ⇒ UF.
This data allows us to define a natural transformation φ : A(F−, −) ⇒ S(−, U−) withcomponents
φ s,a : A(F s, a) → S(s, Ua)
defined as in Remark 4.2.7(iii): given a map g : F s → a in A, define
φ s,a (g) := sηs

/ UF sUg/ Ua .

Injectivity and surjectivity of φ s,a follows immediately from the uniqueness and existenceof morphisms from η s to any particular object s → Ua in s ↓ U. By Remark 4.2.7, thisnatural isomorphism proves that F a U with unit η.

Recall that a limit-preserving functor is called continuous; a colimit-preserving functor
is called cocontinuous. Lemma 4.6.1 reduces the problem of finding a left adjoint to acontinuous functor U : A → S to the problem of finding an initial object in the comma
category s ↓ U defined for each s ∈ S. This comma category, as the category of elements
for S(s, U−) : A → Set, comes with a canonical forgetful functor Π : s ↓ U → A that carries
an object s → Ua to the object a.
Lemma 4.6.2. For any functor U : A → S and object s ∈ S, the associated forgetful functor
Π : s ↓ U → A strictly creates the limit of any diagram whose limit exists in A and ispreserved by U. In particular, if A is complete and U is continuous, then s ↓ U is complete.
Proof. Lemma 4.6.2 is a special case of Exercise 3.4.iii, or can be proven directly via
a straightforward extension of the argument used to prove Proposition 3.3.8.
Lemma 4.6.2 can be used to produce an initial object in s ↓ U. By Lemma 3.7.1,
an initial object is equally the limit of the identity functor. Applying Lemma 4.6.2, alimit of the identity functor on s ↓ U exists if and only if the limit of the forgetful functor
Π : s ↓ U → A exists in A.
This line of reasoning seems to imply that all continuous functors whose domains arecomplete should admit left adjoints. This is not the case: the problem is that s ↓ U is notin general a small category, so even if A admits all small limits it may not admit a limit of
the large diagram Π : s ↓ U → A. The adjoint functor theorems, the two most common ofwhich are discussed here, supply conditions under which this large limit can be reduced toa small one that A possesses.

146

4. ADJUNCTIONS

Theorem 4.6.3 (General Adjoint Functor Theorem). Let U : A → S be a continuousfunctor whose domain is locally small and complete. Suppose that U satisfies the following
solution set condition:• For every s ∈ S there exists a set of morphisms Φ s = { fi : s → Uai } so that any
f : s → Ua factors through some fi ∈ Φ s along a morphism ai → a in A.
Then U admits a left adjoint.
The solution set condition says exactly that { fi : s → Uai } is a set of jointly weakly
initial objects in the category s ↓ U.
Definition 4.6.4.
(i) An object c in a category C is weakly initial if for every x ∈ C there exists a morphismc → x (but possibly more than one).
(ii) A collection of objects Φ = {ci } in a category C is jointly weakly initial if for everyx ∈ C there exists some ci ∈ Φ for which there exists a morphism ci → x.
In the presence of certain limits, an initial object can be built from a jointly weakly
initial set of objects:
Lemma 4.6.5. If C is complete, locally small, and has a jointly weakly initial set of objects
Φ, then C has an initial object.
Proof. Form the limit ` of the inclusion into C of the full subcategory spanned by theobjects of Φ; because C is locally small, this diagram is small and because C is complete,
this limit exists. Using the limit cone (κk : ` → k)k∈Φ , we may define a morphism λc : ` → c
for each c ∈ C:
• For the objects k ∈ Φ, define λk B κk .
• For the remaining objects, choose a morphism hc : k → c, with k ∈ Φ, and defineκkhc

λc B ` −→ k −→ c.
The maps λc prove that ` is weakly initial. To prove that ` is initial, we show that themorphisms λc define a cone λ : ` ⇒ 1C with summit ` over the identity functor 1C : C → C
with the property that the component λ` is the identity morphism. By the proof of Lemma
3.7.1, this implies that ` is initial.
To prove that the maps λc define a cone, consider any morphism f : c → c0 in C andform the pullback p of hc0 : k0 → c0 , the chosen morphism for c0 with k0 ∈ Φ, along thecomposite of f with hc : k → c:
`
κk00


k00

κk0

κk


p

k

 
/ k0

y

hc 0


hc

/c

f


/ c0

Because Φ is jointly weakly initial, there is some k00 → p with k00 ∈ Φ. The dashedcomposite morphisms live in the full subcategory spanned by the objects in Φ, which

4.6. EXISTENCE OF ADJOINT FUNCTORS

147

means that the top triangles, involving the cone legs κ commute. The commutativity of therest of the displayed diagram now implies that λ defines a cone.
Finally, the cone condition for λ implies that the triangle
λ`

`

`

λk =κk





κk

/k

commutes for all k ∈ Φ. This tells us that λ` defines a factorization of the limit cone κ
through itself; whence λ` = 1` .
The proof of the General Adjoint Functor Theorem proceeds directly by assembling
the lemmas in this section.
Proof of Theorem 4.6.3. By Lemma 4.6.1, U admits a left adjoint if and only if foreach s ∈ S the comma category s ↓ U has an initial object; these initial objects define thevalue of the left adjoint on objects and the components of the unit of the adjunction. The
solution set condition says that s ↓ U has a jointly weakly initial set of objects. Because A
is locally small, s ↓ U is as well. Because A is complete and U is continuous, Lemma 4.6.2
applies to prove that s ↓ U is also complete. So we may apply Lemma 4.6.5 to construct an
initial object in s ↓ U.

Example 4.6.6. For example, consider the functor U : Group → Set and a set S . The
following construction of the free group on S can be found in [Lan02, §I.12]. Let Φ0 be
the set of representatives for isomorphism classes of groups that can be generated by a setof elements of cardinality at most |S |. Let Φ be the set of functions S → UG, with G ∈ Φ0 ,whose image generates the group G. This is a jointly weakly initial set of objects in thecomma category S ↓ U and a solution set for U : Group → Set.
To construct the initial object, which by Lemma 4.6.1 defines the free group on theset S , form the product, indexed by the set Φ, of the groups appearing as codomains andconsider the induced map



 Y
η̂
G .
S→
− U 
S →UG∈ΦThe map η̂ is the product of the objects of Φ in the category S ↓ U. Let Γ be the subgroupQof the product S →UG∈Φ G generated by the image of η̂. We show that Γ is the free groupon S by proving that the codomain restriction η : S → UΓ is initial in S ↓ U.
Given a group H and a function ψ : S → UH, let G ⊂ H be the subgroup generated by
the image of S . By construction, ψ factors through its codomain restriction φ : S → UG,
and since G is generated by the image of this map, φ ∈ Φ. The map φ equals the compositeof η : S → UΓ with the projection to the component indexed by φ, so we have found a way
to factor ψ through η via a homomorphism Γ → H. The uniqueness of this factorization
follows from the fact that Γ is generated by the image of S ; as ψ : S → UH specifies theimages of these generators no alternate choices are possible.
Our next adjoint functor theorem requires a few preliminary definitions.
Definition 4.6.7. A separating set or separator for a category C is a set Φ of objects
that can distinguish between distinct parallel morphisms in the following sense: givenf, g : x ⇒ y, if f , g then there exists some h : c → x with c ∈ Φ so that f h , gh. A
coseparating set in C is a separating set in Cop .

148

4. ADJUNCTIONSDefinition 4.6.8. A subobject of an object c ∈ C is a monomorphism c0  c with
codomain c. Isomorphic subobjects, that is, subobjects c0  c  c00 with a commuting
isomorphism c0  c00 , are typically identified.
Definition 4.6.9. The intersection of a family of subobjects of c is the limit of the diagramof monomorphisms with codomain c.
The induced map from the limit to c is again a monomorphism. By Lemma 1.2.11(ii),the legs of the limit cone are also monomorphisms. Thus, the intersection is the minimal
subobject that is contained in every member of the family of subobjects.
Theorem 4.6.10 (Special Adjoint Functor Theorem). Let U : A → S be a continuousfunctor whose domain is complete and whose domain and codomain are locally small.
Furthermore, if A has a small coseparating set and every collection of subobjects of a fixed
object in A admits an intersection, then U admits a left adjoint.
In many categories, each object admits only a set’s worth of subobjects up to isomorphism, in which case completeness implies that every collection of subobjects admits anintersection. The purpose for these hypotheses is that they are used to construct the initial
objects sought for in Lemma 4.6.1.
Lemma 4.6.11. Suppose C is locally small, complete, has a small coseparating set Φ, andhas the property that every collection of subobjects has an intersection. Then C has an
initial object.
Q
Proof. Form the product p = k∈Φ k of the objects in the coseparating set and form
the intersection i ,→ p of all of the subobjects of p. We claim that i is initial.
To say that Φ is coseparating is to say that for every c ∈ C the canonical map
Yc
kC(c,k) ,k∈Φwhose component at an element f ∈ C(c, k) is that morphism f : c → k, is a monomorphism.
Q
Q
There is a map k∈Φ k → k∈Φ kC(c,k) , which is the product over k ∈ Φ of the maps
∆ : k → kC(c,k) defined to be the identity on each component of the power kC(c,k) . The
pullback
/c
pc

 y
p=

Q

k∈Φ k Qk∈Φ ∆
/ Qk∈Φ kC(c,k)

defines a subobject pc of p and thus a map i → pc → c from the intersection to c. Therecannot be more than one arrow from i to c because the equalizer of two distinct such would
define a smaller subobject of p, contradicting minimality of the intersection i. Thus, weconclude that i is an initial object.
Proof of Theorem 4.6.10. Again, for each s ∈ S, the category s ↓ U is locally small
and complete. Monomorphisms in s ↓ U are preserved and reflected by Π : s ↓ U → A, andso the comma category s ↓ U has intersections of subobjects created by this functor. If Φ isa coseparating set for A, then the setΦ0 = s → Ua a ∈ Φ
is coseparating for s ↓ U; because S is locally small, Φ0 is again a set. Applying Lemma
4.6.11, there is an initial object in each comma category s ↓ U, which provides a left adjoint
to U by Lemma 4.6.1.
4.6. EXISTENCE OF ADJOINT FUNCTORS

149

Example 4.6.12. The Special Adjoint Functor Theorem is an abstraction of the constructionof the Stone–Čech compactification β : Top → cHaus, which defines a left adjoint to theinclusion cHaus ,→ Top. The unit interval I = [0, 1] ∈ R is a coseparating object in cHaus.
To see this, note that if f , g : X ⇒ Y, there must be some x ∈ X with f (x) , g(x). Then
Urysohn’s lemma can be used to define a continuous function h : Y → I with h f (x) = 0
and hg(x) = 1, so that in particular h f , hg. Given a topological space X, Theorem 4.6.10
constructs an initial object in X ↓ cHaus in the following manner. A coseparating family inX ↓ cHaus is given by the set of all maps X → I. By Proposition 3.3.8, the product of thesemaps, considered as objects in X ↓ cHaus, is the canonical map
Y
η̂
X→
−
I.
Hom(X,I)Q
A subobject of this object is a compact Hausdorff subspace K ⊂ Hom(X,I) I containing theQ
image of η̂. Because Hom(X,I) I is compact Hausdorff, a subspace is compact Hausdorff ifQ
and only if it is closed. Thus, the intersection of all subobjects of η̂ : X → Hom(X,I) I issimply the codomain restriction η : X → β(X), where β(X) is the closure of the image of η̂.
This constructs the Stone–Čech compactification.
The adjoint functor theorems have a number of corollaries.
Corollary 4.6.13. Suppose C is locally small, complete, has a small coseparating set,and has the property that every collection of subobjects of a fixed object has an intersection.
Then C is also cocomplete.
Proof. For any small category J, if C is locally small, then CJ is again locally small.
The constant diagram functor ∆ : C → CJ preserves limits by Exercise 3.3.vi. Applying
Theorem 4.6.10, ∆ has a left adjoint, which by Proposition 4.5.1 demonstrates that C has
all J-shaped colimits.
The arguments used to prove the adjoint functor theorems also specialize to giveconditions under which a set-valued limit-preserving functor is representable.
Corollary 4.6.14. Suppose C is locally small, complete, has a small coseparating set,and has the property that every collection of subobjects of a fixed object has an intersection.
Then any continuous functor F : C → Set is representable.
Proof. By Theorem 4.6.10, F has a left adjoint L : Set → C. In particular, there is anatural isomorphism
C(L(∗), c)  Set(∗, Fc)  Fc
where ∗ ∈ Set denotes the singleton set. The object L(∗) ∈ C represents F.

Theorem 4.6.15 (Freyd’s Representability Theorem). Let F : C → Set be a continuousfunctor and suppose that C is complete and locally small. If F satisfies the solution set
condition:• There exists a set Φ of objects of C so that for any c ∈ C and any element x ∈ Fc, there
exists an s ∈ Φ, an element y ∈ F s, and a morphism f : s → c so that F f (y) = x.
then F is representable.
Proof. The solution
set defines a jointly weakly initial set of objects in the commaR
category ∗ ↓ F  F, where ∗ ∈ Set is a singleton set. By Lemma 4.6.2, this category iscomplete, and so by Lemma 4.6.5 it has an initial object. By Proposition 2.4.8, this defines
a representation for F.
150

4. ADJUNCTIONS

These corollaries aside, the Adjoint Functor Theorems 4.6.3 and 4.6.10 tend to be bothuseless and widely applicable. Their ineffectualness stems from the fact that solution setsor coseparating sets tend to be hard to find in practice. At the same time, a very simple
general characterization of adjoint functors between a large family of categories can beobtained from these theorems.
Definition 4.6.16. Let κ be a regular cardinal.12 A locally small category C is locallyκ-presentable if it is cocomplete and if it has a set of objects S so that:(i) Every object in C can be written as a colimit of a diagram valued in the subcategory
spanned by the objects in S .
(ii) For each object s ∈ S , the functor C(s, −) : C → Set preserves κ-filtered colimits.
A functor between locally κ-presentable categories is accessible if it preserves κ-filteredcolimits.
For example, a large variety of categories whose objects are sets equipped with some
sort of “algebraic” structure are locally finitely presentable (meaning locally ω-presentable);see Definition 5.5.5. A locally κ-presentable category is also locally λ-presentable for any
λ > κ. Between locally presentable categories, meaning categories that are locallyκ-presentable for some κ, the adjoint functor theorem takes a particularly appealing form.
Theorem 4.6.17. A functor F : C → D between locally presentable categories(i) admits a right adjoint if and only if it is cocontinuous.
(ii) admits a left adjoint if and only if it is continuous and accessible.
Proof sketch. Theorem 4.6.3 proves (ii): any accessible functor satisfies the solution
set condition; see [AR94, 1.66].
The dual of Theorem 4.6.10 proves (i): the colimit-generating set of objects S defines
a separator for C. A hard theorem proves that any object in a locally presentable category
admits at most a set’s worth of epimorphic quotients, up to isomorphism [AR94, 1.58].
Cocompleteness then implies that cointersections of quotient objects always exist.

The categories Ring and Rng are locally presentable and the inclusion is accessible,
and so Theorem 4.6.17 implies that a left adjoint to the inclusion Ring ,→ Rng exists. But
this general result does not tell us anything useful about how it is constructed. Instead, we
sketch a direct construction of the left adjoint that will preview some of the themes fromthe next chapter.
The inclusion Ring ,→ Rng commutes with the underlying abelian group functors:
/ Rng
=

⊥
U

a

b

F

"

U

}

a

Ring

(−)∗

 s

F

Ab

Thus, by Proposition 4.4.4, its left adjoint must commute with the corresponding freefunctors up to natural isomorphism.
As observed in Example 4.1.10(iii), the free unital ring on an abelian group A is theLLgraded ring n≥0 A⊗n . Similarly, the free non-unital ring on A is n>0 A⊗n . Commutativity
12A regular cardinal is an infinite cardinal κ with the property that every union of fewer than κ sets each of
cardinality less than κ has cardinality less than κ.

4.6. EXISTENCE OF ADJOINT FUNCTORS

151

of the left adjoints tells us that
(

L

L
⊗n ∗
⊗n
n>0 A ) n≥0 A .

That is, the free unital ring on a free non-unital ring is constructed by adjoining a copy ofA⊗0 B Z and using the componentwise addition and graded multiplication in the gradedLring n≥0 A⊗n to define the ring structure.
By Proposition 5.4.3, one of the main results to be proven in the next chapter, any nonunital ring R is the coequalizer of a pair of maps between free non-unital rings. By Theorem
4.5.3, R∗ is then the coequalizer of the images of those maps between the corresponding
free unital rings. From this description we see that R∗  Z ⊕ R and can easily guess andverify that the addition is defined componentwise, while the multiplication in R∗ is definedby the “graded multiplication” formula:(n, r) · (n0 , r0 ) B (nn0 , n0 r + nr0 + rr0 ) .
Exercises.
Exercise 4.6.i. Give a direct proof of Lemma 4.6.2 along the lines of the argument usedin Proposition 3.3.8.
Exercise 4.6.ii. Use Theorem 4.6.3 to prove that the inclusion Haus ,→ Top of the fullsubcategory of Hausdorff spaces into the category of all spaces has a left adjoint. The
left adjoint carries a space to its “largest Hausdorff quotient.” Conclude, by applying
Proposition 4.5.15, that the category of Hausdorff spaces, as a reflective subcategory of acomplete and cocomplete category, is cocomplete as well as complete.
Exercise 4.6.iii. Suppose C is a locally small category with coproducts. Show that afunctor F : C → Set is representable if and only if it admits a left adjoint.

CHAPTER 5

Monads and their Algebras
A monad is just a monoid in the category of
endofunctors, what’s the problem?
Philip Wadler, fictional attribution by James Iry[Iry09]Consider an adjunction
Co

F
⊥
U

/

D

η : 1C ⇒ UF,

 : FU ⇒ 1D

from the perspective of the category C (“home”), in ignorance of the category D (“abroad”).
From C, only part of the data of the adjunction is visible: the composite endofunctor
UF : C → C but neither U nor F individually, and the unit natural transformation but
only a whiskered version of the counit. In this chapter, we study the “shadow” cast by
an adjunction on the codomain of its right adjoint, the data of which forms a monad on
C. Monads are interesting in their own right but additionally so because, in unexpectedly
common situations, the category D can be recovered from the monad on C underlying
the adjunction. When this is the case, objects of D can be represented as algebras forthe monad on C. In general, a monad can be thought of as a syntactic representation ofalgebraic structure that is potentially borne by objects in the category on which it acts.1
The general definition of a monad is introduced in §5.1 together with a range ofexamples. As hinted here, any adjunction gives rise to a monad. The converse question,of constructing an adjunction that presents a given monad, is investigated in §5.2, where
two universal solutions to this problem are described: the Kleisli and Eilenberg–Moore
categories, which can be constructed for any monad, also arise naturally in several areas ofmathematics.
The Eilenberg–Moore category, also called the category of algebras for the monad,
is our focus for the remainder of the chapter. In §5.3, we pose the question of identifying
whether a right adjoint functor U : D → C is equivalent to the forgetful functor from thecategory of algebras of its monad, in which case the functor is called monadic. When D isequivalent to the category of algebras, there are a number of pleasing consequences. The
first consequence, proven in §5.4, is that every algebra admits a canonical presentation asa quotient of free algebras. This construction generalizes the presentation of a group via
generators and relations.
The monadicity theorem, proven in §5.5, establishes necessary and sufficient conditions
for a right adjoint to be monadic. The concluding §5.6 explains our interest in recognizing
categories of algebras: this characterization has significant implications for the construction
1Dually, the “shadow” cast by an adjunction on the codomain of its left adjoint defines a comonad, encoding
coalgebraic structure, examples of which are somewhat less common. So that the exposition aligns with the mostprominent examples, this dual theory will largely remain unacknowledged.
153

154

5. MONADS AND THEIR ALGEBRAS

of limits and colimits, which were hinted at in Chapters 3 and 4. For example, monoids,groups, rings, modules over a fixed ring, sets with an action of a fixed group, pointed sets,compact Hausdorff spaces, lattices, and so on all arise as algebras for a monad on the
category of sets. Fields do not, which explains why the category of fields shares few of theproperties that are common to the categories just described.
5.1. Monads from adjunctions
The abstract definition of a monad is relatively simple to state:
Definition 5.1.1. A monad on a category C consists of• an endofunctor T : C → C,
• a unit natural transformation η : 1C ⇒ T , and• a multiplication natural transformation µ : T 2 ⇒ T ,so that the following diagrams commute in CC :T3Tµ

+3 T 2

ηT


+3 T

µ

+3 T 2 ks T η

T

µ

µ

µT


T2

T

1T

#  {T

1T

Remark 5.1.2. The diagrams in Definition 5.1.1 are reminiscent of the diagrams in Definition 1.6.3. This is no coincidence. Monads, like (topological) monoids, unital rings, andk-algebras, are all instances of monoids in a monoidal category. A monoidal category V isa category equipped with a binary functor V × V → V and a unit object, together with some
additional coherence natural isomorphisms satisfying conditions that are described in §E.2.
A monad on C is precisely a monoid in the monoidal category CC of endofunctors on C,
where the binary functor CC × CC → CC is composition, and the unit object is the identity
endofunctor 1C ∈ CC . The category of endofunctors is a strict monoidal category, in which
the coherence natural isomorphisms can be taken to be identities. Nonetheless, the notionof monoid in a monoidal category is rather more complicated than the notion of monad ona category, which is the point of the following joke from Iry’s “A Brief, Incomplete, andMostly Wrong History of Programming Languages” [Iry09]:
1990—A committee formed by Simon Peyton-Jones, Paul Hudak,
Philip Wadler, . . . creates Haskell, a pure, non-strict, functional language. Haskell gets some resistance due to the complexity of using
monads to control side effects. Wadler tries to appease critics by explaining that “a monad is a monoid in the category of endofunctors,
what’s the problem?”One source of intuition is that a monad is the “shadow” cast by an adjunction on the
category appearing as the codomain of the right adjoint. Consider the adjunction
Set o

F
⊥
U

/

Ab

η : 1Set ⇒ UF,

 : FU ⇒ 1Ab ,and suppose we have forgotten entirely about the category of abelian groups. What structure
remains visible on the category of sets? First, there is an endofunctor, UF, which sends
a set to the set of finite formal sums of elements with integer coefficients. There is also
the natural transformation η, the “unit map” whose component ηS : S → UFS sends anelement of the set S to the corresponding singleton sum. By contrast, the “evaluation

5.1. MONADS FROM ADJUNCTIONS

155map” , whose component A : FUA → A is the group homomorphism that evaluates afinite formal sum of elements of the abelian group A to its actual sum in A, is not directly
visible. There is, however, a related natural transformation UFS : UFUFS → UFS
between endofunctors of Set, which is the evaluation map regarded as a function, not agroup homomorphism, and considered only in the special case of free groups.
For any adjunction, this triple of data defines a monad:
Lemma 5.1.3. Any adjunction
Co

F
⊥
U

/

η : 1C ⇒ UF,D

 : FU ⇒ 1D

gives rise to a monad on the category C serving as the domain of the left adjoint, with• the endofunctor T defined to be UF,• the unit η : 1C ⇒ UF serving as the unit η : 1C ⇒ T for the monad, and• the whiskered counit UF : UFUF ⇒ UF serving as the multiplication µ : T 2 ⇒ T
for the monad.
Proof. The triangles
UF

ηUF+3 UFUF ks UFη UF
UF(  v~
UF

1UF

UFUFUF

UFUF +3

UFUF

UF


UFUF

1UF

UFUF

UF


+3 UF

commute by the triangle identities for the adjunction. The square commutes by naturalityof the vertical natural transformation U : UFU ⇒ U.2

Example 5.1.4. The following monads come from familiar adjunctions:(i) The free a forgetful adjunction between pointed sets and ordinary sets of Example
4.1.10(i) induces a monad on Set whose endofunctor (−)+ : Set → Set adds a new
disjoint point. The components of the unit are given by the obvious natural inclusions
ηA : A → A+ . The components of the multiplication µA : (A+ )+ → A+ are defined tobe the identity on the subset A and to send the two new points in (A+ )+ to the new
point in A+ . By Lemma 5.1.3, or by a direct verification, the diagrams((A+ )+ )+µ A+(µA )+

ηA+A+/ (A+ )+ o (ηA )+ A+µA(A+ )+/ (A+ )+

µA

µA


/ A+

"  |
A+

1A+

1A+

commute. Particularly in computer science, this monad is called the maybe monad,for reasons that are explained in Example 5.2.10(i). There is a similar monad on Top,or any category with coproducts, which acts by adjoining a copy of a fixed object (inthis case a point).
2Note that the two composites define the horizontal composite natural transformation (see Lemma 1.7.4):
U

C

F

/D

?C
⇓

U

F



?C
⇓

D

F



D

U

/C

156

5. MONADS AND THEIR ALGEBRAS(ii) The free monoid monad is induced by the free a forgetful adjunction between
monoids and sets of Example 4.1.10(ii). The endofunctor T : Set → Set is defined
by
a
An ,
TA B
n≥0

that is, T A is the set of finite lists of elements in A; in computer science contexts,
this monad is often called the list monad. The components of the unit ηA : A → T A
are defined by the evident coproduct inclusions, sending each element of A to thecorresponding singleton list. The components of the multiplication µA : T 2 A → T A
are the concatenation functions, sending a list of lists to the composite list. A
categorical description of this map is given in Exercise 5.1.i, which demonstrates
that the free monoid monad can also be defined in any monoidal category withcoproducts that distribute over the monoidal product.
(iii) The free a forgetful adjunction between sets and the category of R-modules of
Example 4.1.10(v) induces the free R-module monad R[−] : Set → Set. Define
R[A] to be the set of finite formal R-linear combinations of elements of A. Formally,a finite R-linear combination is a finitely supported function χ : A → R, meaning afunction for which only finitely many elements of its domain take non-zero values.
P
Such a function might be written as a∈A χ(a) · a. The components ηA : A → R[A]of the unit send an element a ∈ A to the singleton formal R-linear combination
corresponding to the function χa : A → R that sends a to 1 ∈ R and every otherelement to zero. The components µA : R[R[A]] → R[A] of the multiplication are
defined by distributing the coefficients in a formal sum of formal sums. Special casesof interest include the free abelian group monad and the free vector space monad.
(iv) The free a forgetful adjunction between sets and groups induces the free groupmonad F : Set → Set that sends a set A to the set F(A) of finite words in the lettersa ∈ A together with formal inverses a−1 .
(v) The composite adjunction
cHaus

o

β
⊥

/ Top

o

D
⊥
U

/ Set

induces a monad β : Set → Set that sends a set to the underlying set of the Stone–Čech compactification of the discrete space on that set. There is a simpler description:β(A) is the set of ultrafilters on A. An ultrafilter is a set of subsets of A that is upward
closed, closed under finite intersections, and for each subset of A contains either that
subset or its complement but not both; see Exercise 5.1.ii.
(vi) The adjunction
DirGraph o

F
⊥
U

/

Cat

of Example 4.1.13 induces a monad on DirGraph that carries a directed graph G to
the graph with the same vertices but whose edges are finite (directed) paths of edges
in G. This is the underlying directed graph of the free category on G. The graphhomomorphism ηG : G → UF(G) includes G as the subgraph of “unary” edges.
(vii) By Example 4.3.8, the contravariant power set functor is its own mutual right adjoint:
Set o

P
⊥
P

/

Setop

Set(A, PB)  Set(B, PA)5.1. MONADS FROM ADJUNCTIONS

157

A function from A to the power set of B, or equally a function from B to the power
set of A, can be encoded as a function A × B → Ω, i.e., as a subset of A × B. The
induced double power set monad takes a set A to P2 A. The components of theunit are the “principal ultrafilter” functions ηA : A → P2 A that send an element a tothe set of subsets of A that contain a. The components of the multiplication takea set of sets of sets of subsets to the set of subsets of A with the property that oneof the sets of sets of subsets is the set of all sets of subsets of A that include thatparticular subset as an element.3 A similar monad can be defined with any otherset in place of the two-element set Ω; in computer science contexts, these are called
continuation monads. This construction can also be generalized to other cartesianclosed categories. For instance, there is a similar double dual monad on Vectk .
Example 5.1.5. Monads also arise in nature:(i) The covariant power set functor P : Set → Set is also a monad. The unit ηA : A → PA
sends an element to the singleton subset. The multiplication µA : P2 A → PA takes theunion of a set of subsets. Naturality of the unit with respect to a function f : A → B
makes use of the fact that f∗ : PA → PB is the direct image (rather than inverse
image) function. Naturality of the multiplication maps makes use of Corollary 4.5.4;
the direct image function, as a left adjoint, preserves unions.
A
f


B

ηA

/ PA

P2 A
( f∗ )∗

f∗

ηB


/ PB

µA

f∗



P2 B

/ PA

µB


/ PB

(ii) A modification of the free monoid monad yields the free commutative monoid
monad defined bya(A×n )/Σn .
TA B
n≥0

Elements are finite unordered lists of elements, or equally finitely supported functions
A → N.
(iii) There is a monad − × N : Set → Set. We might think of the second componentof an element (a, n) ∈ A × N as a discrete time variable. The unit A → A × N is
defined by a 7→ (a, 0) and the multiplication A × N × N → A × N is defined by(a, m, n) 7→ (a, m + n). A similar monad exists with (N, +, 0) replaced by any monoid,
and this example can be generalized further from Set to any monoidal category.
(iv) The Giry monad acts on the category Meas of measurable spaces and measurable
functions. A measurable space is a set equipped with a σ-algebra of “measurable”
subsets, and a function f : A → B between measurable spaces is measurable if thepreimage of any measurable subset of B is a measurable subset of A. The Girymonad sends a measurable space A to the measurable space Prob(A) of probability
measures on A, equipped with the smallest σ-algebra so that, for each measurable
subset X ⊂ A, the evaluation function evX : Prob(A) → I is measurable. The unit is
the measurable function ηA : A → Prob(A) that sends each element a ∈ A to the Diracmeasure, which assigns a subset the probability 1 if it contains a and 0 otherwise.
The multiplication is defined using integration; see [Gir82] or [Ave16] for details.
Briefly:
3This is one of those instances where it is easier to speak mathematics than to speak English: the multiplicationis the inverse image function for the map ηPA : PA → P3 A.

158

5. MONADS AND THEIR ALGEBRASDefinition 5.1.6. A comonad on C is a monad on Cop : explicitly, a comonad consists of an
endofunctor K : C → C together with natural transformations  : K ⇒ 1C and δ : K ⇒ K 2
so that the diagrams dual to Definition 5.1.1 commute in CC .
A comonad is a comonoid in the category of endofunctors of C. By the dual to Lemma
5.1.3, any adjunction induces a comonad on the domain of its right adjoint.
Example 5.1.7. A monad on a preorder (P, ≤) is given by an order-preserving function
T : P → P that is so that p ≤ T p and T 2 p ≤ T p. If P is a poset, so that isomorphic objects
are equal, these two conditions imply that T 2 p = T p. An order-preserving function T sothat p ≤ T p and T 2 p = T p is called a closure operator. Dually, a comonad on a poset
category (P, ≤) defines a kernel operator: an order-preserving function K so that K p ≤ p
and K p = K 2 p.
For example, the poset PX of subsets of a topological space X admits a closure operator
T A = A, where A is the closure of A ⊂ X, and a kernel operator KA = A◦ , where A◦ is theinterior of A ⊂ X.4
Exercises.
Exercise 5.1.i. Suppose V is a monoidal category (see §E.2), i.e., a category with abifunctor ⊗ : V × V → V that is associative up to coherent natural isomorphism5 and aunit object ∗ ∈ V with natural isomorphisms v ⊗ ∗  v  ∗ ⊗ v. Suppose also that V has
finite coproducts and that the bifunctor ⊗ preserves them in each variable.6 Show that`
T (X) = n≥0 X ⊗n defines a monad on V by defining natural transformations η : 1V ⇒ T
and µ : T 2 ⇒ T that satisfy the required conditions.
Exercise 5.1.ii. Show that the functor β : Set → Set that carries a set to the set of ultrafilterson that set is a monad by defining unit and multiplication natural transformations that satisfy
the unit and associativity laws. (Hint: The ultrafilter monad is a submonad of the double
power set monad P2 : Set → Set.)
Exercise 5.1.iii. The adjunction associated to a reflective subcategory of C induces an
idempotent monad on C. Prove that the following three characterizations of an idempotent
monad (T, η, µ) are equivalent:(i) The multiplication µ : T 2 ⇒ T is a natural isomorphism (hence, the appellation“idempotent”).
(ii) Each component of µ : T 2 ⇒ T is a monomorphism.
(iii) The natural transformations ηT, T η : T ⇒ T 2 are equal.
5.2. Adjunctions from monads
We have seen, in Lemma 5.1.3, that any adjunction presents a monad on the category
serving as the domain of its left adjoint. A natural question is whether all monads arise thisway. For instance, is there any adjunction that gives rise to the power set monad on Set?
Perhaps surprisingly, the answer is yes: any monad T on a category C can be recoveredfrom two (typically distinct) adjunctions that are moreover universal with this property. The
initial such adjunction, in a category to be introduced shortly, is between C and the Kleisli
4The closure of A is the smallest closed set containing A, equally the intersection of all closed sets containing
A; the interior is defined dually.
5Rather than worry about what this means, feel free to assume that there is a well-defined n-ary functor⊗nV×n −−→ V built from the bifunctor ⊗.

6In particular, (v t v0 ) ⊗ (w t w0 )  v ⊗ w t v0 ⊗ w t v ⊗ w0 t v0 ⊗ w0 .

5.2. ADJUNCTIONS FROM MONADS

159

category CT of T . The terminal such adjunction is between C and the Eilenberg–Moore
category CT of T , also called the category of T -algebras.
The sense in which a monad defines a syntactic encoding of a particular variety ofalgebraic structure on C is explained by the construction of the category of algebras, theintuition for which we develop in an example. Recall that an affine space, over a fixed fieldk, is like a “vector space that has forgotten its origin.” An n-dimensional affine space is aset A equipped with a translation automorphism for each n-dimensional vector. This action
is simply transitive in the sense that any two points in affine space can be “subtracted” toobtain a unique vector. Choosing an origin o ∈ A defines a bĳection between points a ∈ A
in the affine space and vectors ~v = a − o ∈ kn . In summary:Definition 5.2.1. Given a vector space V over a field k, an affine space is a non-empty+
set A together with a “translation” function V × A −→ A so that• ~0 + a = a for all a ∈ A;~ ) + a = ~v + (~~ ∈ V and a ∈ A; and• (~v + w
w + a) for all ~v, w• for any a ∈ A, the function − + a : V → A is a bĳection.
There is an equivalent definition that allows us to define an affine space without making
use of the auxiliary vector space V. If we temporarily fix an origin o ∈ A, then for any pairof elements a, b ∈ A of the affine space and any scalar λ ∈ k, we can exploit the bĳection− + o : V → A to see that there is a unique c ∈ A so thatc − o = λ(a − o) + (1 − λ)(b − o) .
This element c is sensibly denoted by λa + (1 − λ)b and is independent of the choice oforigin. More generally, for any n-tuple a1 , . . . , an ∈ A and scalars λ1 , . . . , λn ∈ k with
λ1 + · · · + λn = 1, there is a unique element λ1 a1 + · · · + λn an ∈ A defined analogously as
an affine linear combination of ai . This leads to a second equivalent definition of affine
space.
Definition 5.2.2. An affine space is a non-empty set A in which affine linear combinations
can be evaluated.
To make this precise, define Affk (A) to be the set of finite formal affine linear combiPP
nations ni=1 λi ai so that ni=1 λi = 1; a sum is affine precisely when the coefficients sum to
1 ∈ k. To say “these can be evaluated” means there is a function evA : Affk (A) → A. For
this map to define a reasonable evaluation function, a few axioms are required:• If ηA : A → Affk (A) is the “singleton” function and µA : Affk (Affk (A)) → Affk (A) is
the “distributivity” function, then the following diagramsAηA/ Affk (A)

1A

" 
A

evA

Affk (Affk (A))
Affk (evA )

µA

evA



Affk (A)/ Affk (A)

evA


/A

commute in Set.
The first condition says that the value of a singleton sum 1 · a is the element a. The second
condition says that an affine linear combination of affine linear combinations
λ1 · (µ11 a11 + · · · + µ1n1 a1n1 ) + · · · + λk · (µk1 ak1 + · · · + µknk aknk )P P
can be evaluated by first distributing—note that i j λi µi j = 1—and then evaluating orby first evaluating inside each of the k sets of parentheses and then evaluating the resulting
affine linear combination. In summary, the precise meaning of Definition 5.2.2 is:160

5. MONADS AND THEIR ALGEBRASDefinition 5.2.3. An affine space is an algebra for the affine linear combination monad
Affk (−) : Set → Set.
Let us now introduce the general definition.
Definition 5.2.4. Let C be a category with a monad (T, η, µ). The Eilenberg–Moore
category for T or the category of T -algebras is the category CT whose:• objects are pairs (A ∈ C, a : T A → A), so that the diagrams
A(5.2.5)

ηA

/ TA

1A


A

T 2A

a

Ta

µA

a



TA

/ TA

a


/A

commute in C, and• morphisms f : (A, a) → (B, b) are T -algebra homomorphisms: maps f : A → B in C
so that the square
TA
a


A

Tf

/ TB
b

f


/B

commutes, with composition and identities as in C.
For instance, algebras for the affine space monad Affk on Set are precisely affine spacesin the sense of Definition 5.2.2. This is a representative example: the abstract definition of
5.2.4, which is a priori a bit strange, precisely captures familiar notions of “algebra,” of thevariety encoded by the monad (T, η, µ).
Example 5.2.6. For instance:(i) Consider the free pointed set monad of Example 5.1.4(i). An algebra is a set A witha map a : A+ → A so that the diagrams (5.2.5) commute. The square imposes noadditional conditions, but the triangle asserts that the map a : A+ → A restricts to theidentity on the A component of the disjoint union A+ = A t {∗}. Thus, the data of analgebra is a set with a specified basepoint a ∈ A, the image of the extra point ∗ underthe map a : A+ → A. A morphism f : (A, a) → (B, b) is a map f : A → B so that
A+
a


A

f+

/ B+
b

f


/B

commutes. The map f+ carries the extra point in A+ to the extra point in B+ , so thiscondition demands that f (a) = b. In conclusion, the Eilenberg–Moore category isisomorphic to Set∗ , the category of pointed sets.
(ii) For any unital ring R, there is a monad R ⊗Z − on Ab derived from the adjunction
Ab o

R⊗Z −
/
⊥
ModR
U

5.2. ADJUNCTIONS FROM MONADS

161

of Example 4.1.10(xii). An algebra is an abelian group A equipped with a homomorphism · : R⊗Z A → A satisfying a pair of axioms. By the universal property of the tensor product, this homomorphism encodes a Z-bilinear map (r, a) 7→ r · a : R × A → A,
which we call “scalar multiplication.” The commutative diagramsAηA/ R ⊗Z A

1A

" 
A

R ⊗Z R ⊗Z A·

µA

/ R ⊗Z A
·

R⊗Z ·


R ⊗Z A·/A

ensure 1 · a = a and r · (r · a) = (rr ) · a, i.e., that this action is associative and unital.
In conclusion, an algebra for the monad R ⊗Z − on Ab is precisely an R-module.
(iii) An algebra for the free monoid monad of Example 5.1.4(ii) is a set A with a map`
α : n≥0 An → A, whose component functions αn : An → A specify an n-ary operation on A for each n, satisfying certain conditions. The unit triangle
ηA/ `n≥0 An
A
0

0

α

1A

# 
A

asserts that the unary operation α1 is the identity. The commutative square
µA
`
`
m n
/ `k≥0 Akn≥0 ( m≥0 A )(5.2.7)

n≥0 α

`

α

n

` 

n
n≥0 A

α


/A

imposes an associativity condition on the operations. An element of the upper
left-hand set is a list((a11 , . . . , a1m1 ), . . . , (an1 , . . . , anmn ))of n varying-length lists of elements of A. Commutativity of (5.2.7) demands thatthe result of applying the (m1 + · · · + mn )-ary operation to the concatenated list is thesame as applying the n-ary operation to the results of applying the mi -ary operations
to each sublist.
Recall that a monoid is a set A with an associative binary operation A2 → A anda unit element e ∈ A. A monoid defines an algebra for the free monoid monad: thenullary operation 1 → A picks out the unit element e, the unary operation A → A isnecessarily the identity, the binary operation A2 → A is given, and the operations ofhigher arity are determined by iterating the binary operation; associativity implies
that this is well-defined.
Conversely, any algebra for the free monoid monad defines a monoid, whose unit
and binary operations are defined to be the components α0 : 1 → A and α2 : A2 → A`of the map α : n≥0 An → A. The associativity condition (5.2.7) implies in particularthat the ternary product of three elements a1 , a2 , a3 ∈ A is equal to both of the iterated
binary products (a1 · a2 ) · a3 and a1 · (a2 · a3 ):α2 (α2 (a1 , a2 ), a3 ) = α3 (a1 , a2 , a3 ) = α2 (a1 , α2 (a2 , a3 )) .
The square (5.2.7) also implies that the composite of the binary operation with thenullary operation in one of its inputs is equal to the identity unary operation.

162

5. MONADS AND THEIR ALGEBRAS

By Exercise 5.2.i, algebra morphisms correspond precisely to monoid homomorphisms. Thus, we conclude that the category of algebras for the free monoid
monad is isomorphic to the category Monoid of associative unital monoids.
(iv) An algebra for the closure closure operator7 on the poset of subsets of a topologicalspace X is exactly a closed subset of X. Dually, a coalgebra8 for the interior kernel
operator is exactly an open subset.
Various notations are common for the Eilenberg–Moore category, many involving thestring “alg ” to emphasize the interpretation of its objects as “algebras” of some sort. The
notation used here, while less evocative, has the virtue of being concise.
Lemma 5.2.8. For any monad (T, η, µ) acting on a category C, there is an adjunction
Co

FT
⊥

/

CT

UT

between C and the Eilenberg–Moore category whose induced monad is (T, η, µ).
Proof. The functor U T : CT → C is the evident forgetful functor. The functor
F : C → CT carries an object A ∈ C to the free T -algebra
T

F TA B (T A, µA : T 2 A → T A)
and carries a morphism f : A → B to the free T -algebra morphism
Tf

F T f B (T A, µA ) −−→ (T B, µB ) .
Note that U T F T = T .
The unit of the adjunction F T a U T is given by the natural transformation η : 1C ⇒ T .
The components of the counit  : F T U T ⇒ 1CT are defined as follows:
T 2Aa(A,a) B (T A, µA ) →− (A, a)

µA

Ta

a



TA

/ TA

a


/A

That is, the component of the counit at an algebra (A, a) ∈ CT is given by the algebra
structure map a : T A → A; the commutative square demonstrates that this map defines a
T -algebra homomorphism a : (T A, µA ) → (A, a). Note, in particular, that U T F AT = µA ,so that the monad underlying the adjunction F T a U T is (T, η, µ). The straightforward
verifications that the unit and counit satisfy the triangle identities are left to Exercise
5.2.ii.

A second solution to the problem of finding an adjunction that induces a given monadis given by the Kleisli category construction.
Definition 5.2.9. Let C be a category with a monad (T, η, µ). The Kleisli category CT
is defined so that• its objects are the objects of C, and• a morphism from A to B in CT , depicted as A
B, is a morphism A → T B in C.
Identities and composition are defined using the monad structure:
7This is not a typo: Example 5.1.7 describes a closure operator on the poset of subsets of a topological spacethat sends a subset to its closure.
8Interpreting Definition 5.2.4 for a monad (T, η, µ) on Cop defines the category of coalgebras for the comonadon C; see Exercise 5.2.iii.

5.2. ADJUNCTIONS FROM MONADS

163• The unit ηA : A → T A defines the identity morphism A
A ∈ CT .
• The composite of a morphism f : A → T B from A to B with a morphism g : B → TCfrom B to C is defined to be
A

f

/ TB

Tg

µC

/ T 2C/ TC .

The verification that these operations are associative and unital is left as Exercise 5.2.iv.
Example 5.2.10. For instance:(i) Objects in the Kleisli category for the maybe monad on Set are sets. A map A
B
in the Kleisli category is a function A → B+ , which may be thought of as a partiallydefined function from A to B: the elements of A that are sent to the free basepoint
have “undefined” output. The composite of two partial functions is the maximal
partially-defined function. Thus, the Kleisli category is the category Set∂ of sets andpartially-defined functions.
(ii) Objects in the Kleisli category for the free monoid monad on Set are sets and a map`
from A to B is a function A → n≥0 Bn . For each input element a ∈ A, the output is`an element (b1 , . . . , bk ) ∈ n≥0 Bn , i.e., a list of elements of B.
(iii) For a fixed set S of “states,” the adjunction S × − a (−)S induces a monad (S × −)S on
Set. A map A
B in the Kleisli category is, by adjunction, a function f : A × S →B × S that takes an input a ∈ A together with the current state s ∈ S and returns
an output f s (a), dependent on both the input and the state, together with a new“updated” state s0 ∈ S . Monads of this form make an appearance in certain functional
programming languages such as Haskell, where the updated state is interpreted as a“side effect” of the computation f .
(iv) The Kleisli category for the Giry monad of Example 5.1.5(iv) is the category ofmeasurable spaces and Markov kernels. For instance, a finite set defines a measurable space with discrete σ-algebra. An endomorphism of this object in the Kleisli
category is a discrete time Markov chain.
Lemma 5.2.11. For any monad (T, η, µ) acting on a category C, there is an adjunction
Co

FT
⊥
UT

/

CT

between C and the Kleisli category whose induced monad is (T, η, µ).
Proof. The functor FT is the identity on objects and carries a morphism f : A → B in
B in CT defined by

C to the morphism Af

ηB

FT f B A →− B −−→ T B .
The functor UT sends an object A ∈ CT to T A ∈ C and sends a morphism A
represented by g : A → T B to
Tg

B

µB

UT g B T A −−→ T 2 B −−→ T B .
Elementary diagram chases, left as Exercise 5.2.v, demonstrate that both mappings arefunctorial. Note in particular that UT FT = T . From the definition of the hom-sets in CT ,there are isomorphisms
CT (FT A, B)  C(A, T B)  C(A, UT B) ,which are natural in both variables. This establishes the adjunction FT a UT .



164

5. MONADS AND THEIR ALGEBRAS

For any monad (T, η, µ) on C, there is a category AdjT whose objects are fully-specified
adjunctions
F /Co ⊥ Dη : 1C ⇒ UF,  : FU ⇒ 1D
U

inducing the monad (T, η, µ) on C. A morphism> / D0

K

D

_

F0

U

a

a



F

~

C

U0

from F a U to F 0 a U 0 is a functor K : D → D0 commuting with both the left and rightadjoints, i.e., so that KF = F 0 and U 0 K = U. Because the units of F a U and F 0 a U 0 are
the same, by Exercise 4.2.v the commutativity of K with the right and left adjoints implies
further that(i) the whiskered composites K =  0 K of K with the counits of F a U and F 0 a U 0
coincide, and(ii) K carries the transpose in D of a morphism c → Ud = U 0 Kd to the transpose of thismorphism in D0 .
Proposition 5.2.12. The Kleisli category CT is initial in AdjT and the Eilenberg–Moore
category CT is terminal. That is, for any adjunction F a U inducing the monad (T, η, µ) on
C, there exist unique functors
/D
O

J

CT

∃!

_

UT

F

U



C 

FT
a

a

a



FT

/ CT
?

K
∃!

UT

commuting with the left and right adjoints.
Because the functors J and K define adjunction morphisms, they must commute withthe counits of the adjunctions and preserve transposition, facts that can be used to deducethe correct definitions of these functors.
Proof. We argue that there exist unique adjunction morphisms from the Kleisli adjunction to F a U and from F a U to the Eilenberg–Moore adjunction. Write  : FU ⇒ 1D
for the counit of F a U. The hypothesis that this adjunction induces the monad (T, η, µ) on
C implies that UF = µ.
On objects c ∈ CT , we are forced to define Jc = Fc so that JFT = F. A morphism
f: cc0 in CT is the transpose, under FT a UT , of the representing morphism f : c →0
T c = UT c0 in C, so the requirement that J commutes with transposition forces us to define
J f to be the transpose
Ff

Fc0

J f B Fc −−→ FUFc0 −−−→ Fc0
of f under F a U. The proof that these definitions are functorial, left to Exercise 5.2.vi,
makes use of the fact that UF = µ.
To define the functor K on an object d ∈ D so that U T K = U, we seek a suitable T algebra structure for the object Ud ∈ C. For any algebra (c, γ : T c → c) ∈ CT , the algebra
structure map γ appears as the component of the counit of F T a U T at the object (c, γ); see
the proof of Lemma 5.2.8. Thus, γ can be recognized as the morphism γ : (T c, µc ) → (c, γ)

5.2. ADJUNCTIONS FROM MONADS

165

from the free T -algebra on c to the given T -algebra that is the transpose of the identity
on c = U T (c, γ). The fact that K preserves transposes now tells us that we should define
Kd = (Ud, Ud ); the proof that Ud : UFUd → Ud is an algebra structure map makes useof the fact that UF = µ. On morphisms, the condition U T K = U forces us to define theimage of f : d → d0 to be U f : (Ud, Ud ) → (Ud0 , Ud0 ), and indeed this is a morphism ofalgebras. Functoriality in this case is obvious.
Proposition 5.2.12 implies, in particular, that there is a unique functor from the Kleisli
category for any monad to the Eilenberg–Moore category that commutes with the free andforgetful functors from and to the underlying category. The following result characterizes
its image.
Lemma 5.2.13. Let (T, η, µ) be a monad on C. The canonical functor K : CT → CT fromthe Kleisli category to the Eilenberg–Moore category is full and faithful and its image
consists of the free T -algebras.
For ease of future reference, recall:Definition 5.2.14. A free T -algebra is an object
F T c B (T c, µc : T 2 c → T c)in the category of algebras that is in the image of the free functor F T : C → CT .
Proof. The proof of Proposition 5.2.12 supplies the definition of the functor on objects:
Kc B (T c, µc ), T c being the object UT c and µc being UT of the component of the counit of
the Kleisli adjunction at the object c ∈ CT .
Applying Exercise 4.2.v, for each pair c, c0 ∈ CT , the action of the functor K on thehom-set from c to c0
CT (c, c0 )K

/ CT (Kc, Kc0 ) = CT ((T c, µc ), (T c0 , µc0 ))commutes with the transposition natural isomorphisms identifying both of these hom-setswith C(c, T c0 ). In particular, this map must also be an isomorphism, demonstrating that thefunctor K : CT → CT is full and faithful.
The upshot of Lemma 5.2.13 is that the Kleisli category for a monad embeds as thefull subcategory of free T -algebras and all maps between such. Lemma 5.2.13 also tells us
precisely when the Kleisli and Eilenberg–Moore categories are equivalent: this is the casewhen all algebras are free. For instance, all vector spaces are free modules over any chosen
basis, so the Kleisli and Eilenberg–Moore categories for the free vector space monad areequivalent. Another equivalence of this form, for the maybe monad on Set, appears in
disguise in Example 1.5.6.
Exercises.
Exercise 5.2.i. Pick your favorite monad on Set induced from a free a forgetful adjunction
involving some variety of algebraic structure borne by sets, identify its algebras, and showthat algebra morphisms are precisely the homomorphisms, in the appropriate sense.
Exercise 5.2.ii. Fill in the remaining details in the proof of Lemma 5.2.8 to show that thefree and forgetful functors relating a category C with a monad T to the category CT of
T -algebras are adjoints, inducing the given monad T .
Exercise 5.2.iii. Dualize Definition 5.2.4 and Lemma 5.2.8 to define the category of
coalgebras for a comonad together with its associated forgetful–cofree adjunction.

166

5. MONADS AND THEIR ALGEBRAS

Exercise 5.2.iv. Verify that the Kleisli category is a category by checking that the composition operation of Definition 5.2.9 is associative and unital.
Exercise 5.2.v. Verify that the assignments FT and UT defined in the proof of Lemma
5.2.11 are functorial.
Exercise 5.2.vi. Fill in the remaining details in the proof of Proposition 5.2.12: verify thefunctoriality of J and K and show that the whiskered composites of the counits of thesethree adjunctions with K and J agree.
Exercise 5.2.vii. From the canonical functor K : CT → CT of Lemma 5.2.13 that embeds
the Kleisli category into the Eilenberg–Moore category for a monad T on C, one can define
a “restricted Yoneda” functor
CT (K−,−)CT(A, α)/ SetCTop

CT (K−, (A, a))7→that constructs a presheaf on the Kleisli category for each T -algebra (A, a). Show thatthe presheaves in the image of this functor restrict along FT : C → CT to representableop
functors in SetC . In fact, this functor defines an isomorphism between the Eilenberg–
Moore category and the full subcategory of presheaves on the Kleisli category that restrict
along FT to representable functors.
5.3. Monadic functors
The free a forgetful adjunction
Set o

/

F
⊥
U

Ab

between sets and abelian groups described in Example 4.1.10(iv) induces a monad Z[−] on
Set. Given a set S , Z[S ] is defined to be the set of finite formal Z-linear combinations ofelements of S , and the map Z[ f ] : Z[S ] → Z[T ] associated to a function f : S → T carries aformal sum n1 s1 +· · ·+nk sk , with ni ∈ Z and si ∈ S , to the formal sum n1 f (s1 )+· · ·+nk f (sk );see Exercise 4.1.iv. By Proposition 5.2.12, there is a unique functor/ SetZ[−]
;

Ab a
F

F

Z[−]

`

a

U

!

Set

{

U Z[−]

from the category of abelian groups to the category of Z[−]-algebras that commutes withthe left and right adjoints. The image of an abelian group A is the algebra consisting of
the underlying set of A together with the “evaluation” map A : Z[A] → A that interprets a
finite formal sum as a finite sum, a single element of A.
In fact, this functor defines an isomorphism of categories: to equip a set A with an“evaluation” function α : Z[A] → A, so that diagrams
A

ηA/ Z[A]

1A

! 
A

α

Z[Z[A]]

µA

α

Z[α]


Z[A]

/ Z[A]α/A

5.3. MONADIC FUNCTORS

167

commute is precisely to give A the structure of an abelian group. The commutative triangle
is a “sanity check” for the evaluation function, asserting that it sends a singleton sum
a ∈ Z[A] to the element a. The remaining group axioms are imposed by the commutative
rectangle: in particular, the values assigned to the sums of sums (a1 + a2 ) + (a3 ) and(a1 ) + (a2 + a3 ) are equal.
Moreover, an algebra homomorphism

Z[A]
α


A

/ Z[B]

Z[ f ]βf/B

is a function f : A → B that preserves the evaluation of formal sums, i.e., a group homomorphism.
The observations just made, that the canonical comparison functor defines an isomorphism from the category of abelian groups to the category of algebras for the free abeliangroup monad, are expressed by saying that the category of abelian groups is monadic overthe category of sets.
Definition 5.3.1.
• An adjunction C o

F
⊥
U

/

D is monadic if the canonical comparison functor of Propo-

sition 5.2.12 from D to the category of algebras for the induced monad on C defines
an equivalence of categories.
• A functor U : D → C is monadic if it admits a left adjoint that defines a monadic
adjunction.
Example 5.3.2. The Kleisli adjunction for the maybe monad is identified in Example
5.2.10(i) as the adjunction between the category of sets and the category of sets andpartially-defined functions:
Set o

⊥

/

Set∂ .

This adjunction is monadic. The associated category of algebras is Set+ and the canonicalcomparison functor Set∂ → Set+ is part of the equivalence of categories described in
Example 1.5.6.
Many of the monadic functors that one meets in practice, such as the example U : Ab →Set considered above, are in fact strictly monadic, meaning that the canonical comparison
functor defines an isomorphism of categories. Whether this comparison functor is anequivalence or an isomorphism will not make a substantial difference to the theory.
A particularly simple family of examples of monadic functors is given by inclusions ofreflective subcategories (see Definition 4.5.12). Consider a reflective subcategory D ,→ C
with reflector L. The induced endofunctor L : C → C defines a monad on C with unitηC : C → LC and multiplication a natural isomorphism L2C  LC. A monad whose
multiplication natural transformation is invertible is called an idempotent monad; see
Exercises 4.5.vii and 5.1.iii.

168

5. MONADS AND THEIR ALGEBRAS

Proposition 5.3.3. The inclusion D ,→ C of a reflective subcategory is monadic: that is,
the functor
KD o _> / CL
FL

a

`

L



C

UL

~

from D to the category of algebras CL for the induced idempotent monad L is an equivalenceof categories.
Proof. An L-algebra is an object C ∈ C together with a map c : LC → C that is
a retraction of the unit component ηC . In fact, ηC and c are inverse isomorphisms. By
naturality of η, ηC · c = Lc · ηLC , but ηLC = LηC , as both maps are left inverse to theisomorphism µC , and so Lc · ηLC = Lc · LηC = 1LC . So if C ∈ C admits the structure of an
L-algebra, then its unit component must be invertible. In fact, this necessary condition alsosuffices: the map ηC−1 : LC → C automatically satisfies the associativity condition required
to define an L-algebra structure on the object C. In conclusion, there is no additional
structure provided by a lift of an object C ∈ C to an L-algebra; rather being an L-algebra isa condition on the object C, namely whether the unit component ηC is invertible.
Exercise 4.5.vii reveals that an object C is in the essential image of D ,→ C if and only
if ηC is an isomorphism. This proves that K : D → CL is essentially surjective. Naturalityof the unit η implies that this functor is fully faithful, so we conclude that K defines anequivalence of categories.
In §5.5, we prove the monadicity theorem, which allows one to easily detect when thecomparison functor defines an equivalence or isomorphism. In particular, the reader need
not remain concerned for very long about the remaining details in the sketched proof of themonadicity of abelian groups.
Exercises.
Exercise 5.3.i. Reading between the lines in the proof of Proposition 5.3.3, prove that thecategory of algebras for an idempotent monad on C defines a reflective subcategory of C
and moreover is equivalent to the Kleisli category for that monad.
5.4. Canonical presentations via free algebras
The reason for our particular interest in monadic functors, such as U : Ab → Set, isthat the forgetful functor U T : CT → C associated to the category of algebras for a monad T
on C has certain special properties, which are enumerated in Propositions 5.4.3 and 5.4.9,
Lemma 5.6.1, Theorem 5.6.5, and Proposition 5.6.11. These properties are transferredalong the equivalence to any monadic functor. The first of these, discussed in this section
and used in the next to prove the monadicity theorem, demonstrate that any algebra for amonad admits a canonical presentation as a quotient of free algebras.
On account of the isomorphism of categories Ab  SetZ[−] , the adjunction
Z[−](5.4.1)

Set o

⊥
U

/

Ab

is monadic, and so we build intuition by first introducing these canonical presentations inthe special case of abelian groups. To avoid inelegant pedantry, we use the same notation
for the monad Z[−] on Set and for the free abelian group functor Z[−] : Set → Ab.

5.4. CANONICAL PRESENTATIONS VIA FREE ALGEBRAS

169

The context—sets and functions or groups and homomorphisms—will always indicate theappropriate target category.
Any abelian group A has a presentation that can be defined in terms of the free andforgetful functors (5.4.1). If G is any set of elements of an abelian group A, there is a
canonical homomorphism Z[G] → A that sends a finite Z-linear combination of theseelements to the element of A that is their sum. The set G is a set of generators for A
precisely when this map is surjective. Relations involving these generators, meaning theterms that are to be set equal to zero, are elements of the group Z[G], so again there is a
canonical “evaluation” homomorphism Z[R] → Z[G] from the free group on a set R of
relations to the free group on the generators. The sets G ⊂ A of generators and R ⊂ Z[G]of relations give a presentation of A if the quotient map Z[G]  A is a coequalizer of theevaluation map and the zero homomorphism(5.4.2)0Z[R]evaluation// Z[G]/ / A,

in which case one often writes A = G R .
Ad hoc presentations as described by (5.4.2) can be a useful way to define abeliangroups, but they are unlikely to be functorial: a homomorphism ϕ : A → A0 is unlikely
to carry the chosen presentation for A to sets of generators and relations for A0 . There is,however, a canonical—by which we mean functorial—presentation of any abelian group.
Rather than choose a proper subset of generators, we take all of the elements of A to begenerators; the canonical evaluation homomorphism α : Z[A]  A is certainly surjective.
Similarly, rather than choose any particular set of relations in Z[A], we take all of theelements of Z[A] to be “relations.” Here we do not intend to send every formal sum ofelements of A to zero; the result would be the trivial group. Instead, we generalize themeaning of presentation, making use of the fact that coequalizers are more flexible thancokernels. Proposition 5.4.3 will demonstrate that the diagram
Z[α]Z[Z[A]]µA// Z[A]α// A

is always a coequalizer. That is, any abelian group is the quotient of the free abelian groupon its underlying set modulo the relation that identifies a formal sum with the element towhich it is evaluated.
In general:αProposition 5.4.3. Let (T, η, µ) be a monad on C and let (A, T A →− A) be a T -algebra.
Then(5.4.4)Tα(T 2 A, µT A )// (T A, µA )µAα

/ / (A, α)

is a coequalizer diagram in CT .
Proposition 5.4.3 can be proven directly by a diagram chase, but we prefer to deduce
it as a special case of a more general result, the statement of which requires some new
definitions.
Definition 5.4.5. A split coequalizer diagram consists of maps
f

x]

g
t

// y
g

h
s

/z

170

5. MONADS AND THEIR ALGEBRAS

so that h f = hg, hs = 1z , gt = 1y , and f t = sh.
The condition h f = hg says that this triple of maps defines a fork, i.e., h is a cone
under the parallel pair f, g : x ⇒ y.
Lemma 5.4.6. The underlying fork of a split coequalizer diagram is a coequalizer. Moreover, it is an absolute colimit: any functor preserves this coequalizer.
Proof. Given a map k : y → w so that k f = kg, we must show that k factors throughh; uniqueness of a hypothetical factorization follows because h is a split epimorphism. The
factorization is given by the map ks : z → w, as demonstrated by the following easy diagramchase:
ksh = k f t = kgt = k .
Now clearly split coequalizers, which are just diagrams of a particular shape, arepreserved by any functor, so the equationally-witnessed universal property of the underlying
fork is also preserved by any functor.

Example 5.4.7. For any algebra (A, α : T A → A) for a monad (T, η, µ) on C, the diagram
T 2 A`TαµA// T A

α

j

/A

ηA

ηT A

defines a split coequalizer diagram in C.
Note that while the fork of Example 5.4.7 lifts along U T : CT → C to a diagram (5.4.4)
of algebras, the splittings ηA and ηT A do not. This situation is captured by the following
general definition.
Definition 5.4.8. Given a functor U : D → C:
• A U-split coequalizer is a parallel pair f, g : x ⇒ y in D together with an extension of
the pair U f, Ug : U x ⇒ Uy to a split coequalizer diagram
Uf

U x_

Ug

// Uy j

h

/z

s

t

in C.
• U creates coequalizers of U-split pairs if any U-split coequalizer admits a coequalizer
in D whose image under U is isomorphic to the fork underlying the given U-split
coequalizer diagram in C, and if any such fork in D is a coequalizer.
• U strictly creates coequalizers of U-split pairs if any U-split coequalizer admits aunique lift to a coequalizer in D for the given parallel pair.
Proposition 5.4.9. For any monad (T, η, µ) acting on a category C, the monadic forgetful
functor U T : CT → C strictly creates coequalizers of U T -split pairs.
Proof. Suppose given a parallel pair f, g : (A, α) ⇒ (B, β) in CT that admits a U T splitting:
f

A]

g

// B h

h

/ C.

s

t

We must show that C lifts to an algebra (C, γ) and h lifts to an algebra map that is acoequalizer of f and g in CT , and that moreover these lifts are unique with this property.
To define the algebra structure map γ, note that the functor T : C → C preserves the split

5.4. CANONICAL PRESENTATIONS VIA FREE ALGEBRAS

171

coequalizer diagram; in particular, by Lemma 5.4.6, T h is the coequalizer of T f and T g.
The algebra structure maps α and β define a diagram
Tf

TA
Tg
α

// T B/ / TC

Th

∃! γ

β


A

f
g

// B


/C

h

in which the square defined by f and T f and the square defined by g and T g both commute.
Thus,
h · β · T f = h · f · α = h · g · α = h · β · Tg ,which says that h · β defines a cone under the pair T f, T g : T A ⇒ T B. By the universal
property of their coequalizer T h, there is a unique map γ : TC → C so that the right-handsquare above commutes. Once we show that the pair (C, γ) is a T -algebra, this commutativesquare will demonstrate that h : (B, β) → (C, γ) is a T -algebra homomorphism.
It remains to check that the diagrams
ηC

/ TC

1C


C

C

µC

T 2C

γ

/ TC
γ

Tγ


TC

γ


/C

commute. This will follow from the corresponding conditions for the T -algebra (B, β) andthe fact that the coequalizer maps are epimorphisms. Specifically, we have diagrams

ηB

TB

}

β

h

B

ηC
Th /

!  1B
B

TC

Tβ

µB

}


TB

1C
γ

h

// C
! 
/C

Th

#

β

B

/ / T 2C

T 2h

T 2B

µC

#

TB
β

/ TC



#
/ TC

Th

γ
γ

h

Tγ

$/

C



in which all but the right-most face of each prism is known to commute. It follows thatγ · ηC · h = 1C · h and γ · µC · T 2 h = γ · T γ · T 2 h, and we conclude that the right-most faces
commute by canceling the epimorphisms h and T 2 h. Thus, (C, γ) is a T -algebra.
Finally, we show that h : (B, β) → (C, γ) is a coequalizer in CT . Given a conek : (B, β) → (D, δ) so that k f = kg, there is a unique factorization
f

A

g

// B

h

k

/C
∃!

 
D

j

using the universal property of the coequalizer in C. To check that j lifts to a map of
T -algebras, it again suffices to verify that j · γ = δ · T j after precomposing with theepimorphism T h. The result follows from an easy diagram chase, using the fact that h andk are algebra maps:
j · γ · T h = j · h · β = k · β = δ · T k = δ · T j · T h.



172

5. MONADS AND THEIR ALGEBRASProof of Proposition 5.4.3. Example 5.4.7 shows that the fork (5.4.4) is part ofa U T -split coequalizer. In particular, α : T A → A is an absolute coequalizer of the pairT α, µA : T 2 A ⇒ T A in C. The proof of Proposition 5.4.9 demonstrates that this coequalizer
lifts to a coequalizer in CT .

The results of Propositions 5.4.3 and 5.4.9 extend to general monadic functors with one
caveat: as monadic adjunctions are only required to be equivalent to and not isomorphic
to the adjunction involving the category of algebras, a monadic functor U : D → C might
only create, rather than strictly create, U-split coequalizers.
Corollary 5.4.10. If C o

F
⊥
U

/

D is a monadic adjunction, then(i) U : D → C creates coequalizers of U-split pairs.
(ii) For any D ∈ D, there is a coequalizer diagramFUD(5.4.11)

FUFUD

FUD

// FUD

D/D

involving the counit  : FU ⇒ 1D of the adjunction.
Proof. To say that U : D → C is monadic is to say that there is an equivalence of
categories
K
/ CT
D
'



U

C

~

UT

where T is the monad UF.
For (i), if f, g : A ⇒ B is a U-split pair in D, then commutativity U = U T K implies
that K f, Kg : KA ⇒ KB is a U T -split pair in CT . Proposition 5.4.9 lifts the fork of the
U-split coequalizer diagram in C to a coequalizer diagram in CT . Any inverse equivalence'
L : CT −→D to the functor K can be used to map this data to a coequalizer for the pairf, g : A ⇒ B. As KL  1CT , this coequalizer maps via U : D → C to a fork that is
isomorphic to the fork of the given U-split coequalizer.9 Note also that U preserves andreflects all coequalizer diagrams that are U-split because U T has this property and K
preserves and reflects all coequalizers (see Lemma 3.3.6). This completes the proof that U
creates U-split coequalizers.
For (ii), the fork (5.4.11) is U-split by the diagram
UFUD

UFUFUDaU

FUD// UFUD m

UD

/ UD

ηUD

ηUFUD

in C. By (i), the functor U : D → C reflects U-split coequalizers, so the fork (5.4.11)
defines a coequalizer diagram in D.

Exercises.
Exercise 5.4.i. The coequalizer of a parallel pair of morphisms f and g in the category
Ab is equally the cokernel of the map f − g. Explain how the canonical presentation ofan abelian group described in Proposition 5.4.3 defines a presentation of that group, in theusual sense.
9If KL = 1CT , as is the case if these functors define an isomorphism of categories, then this coequalizer in D
is a lift of the U-split coequalizer in C and U : D → C strictly creates U-split coequalizers.

5.5. RECOGNIZING CATEGORIES OF ALGEBRAS

173

Exercise 5.4.ii. Prove Proposition 5.4.3 directly by a diagram chase.
5.5. Recognizing categories of algebras
There are many versions of the monadicity theorem, which characterizes those adjunctions for which objects in the domain of the right adjoint can be regarded as algebras forthe monad defined on the codomain. The one presented here is due to Beck [Bec67].
Theorem 5.5.1 (monadicity theorem). A right adjoint functor U : D → C is monadic if
and only if it creates coequalizers of U-split pairs.
Recalling Definition 5.4.8, the meaning of this result is the following. Given an
adjunction F a U inducing a monad T on C, Proposition 5.2.12 constructs the canonicalcomparison functor:
K> / CT
D _
FT

F

`

a

U



C

~

UT

By Theorem 5.5.1, the following are equivalent:(i) K is an equivalence (respectively, isomorphism) of categories.
(ii) U creates (respectively, strictly creates) coequalizers of U-split pairs.
We leave the proof of the strict version of the monadicity theorem, that D is isomorphic
to the category of algebras if and only if U : D → C strictly creates coequalizers of U-split
pairs, to Exercise 5.5.i and focus our attention on the equivalence-invariant statement.
Proof. Corollary 5.4.10 proves the implication (i)⇒(ii), so we assume (ii) and use it
to construct an inverse equivalence L : CT → D to the functor K. We have U T K = U and
KF = F T , so if L is to define an inverse equivalence to K, we must have U T  UL andF  LF T , conditions that we use to guide the definition of L.
In fact, we define L so that the second of these conditions holds strictly, namely, foreach A ∈ C, define
L(T A, µA ) B FA
and define L to carry a free map T f : (T A, µA ) → (T B, µB ) between free algebras to themap F f : FA → FB. It remains to extend the definition of L to non-free algebras and maps.
An equivalence of categories preserves all limits and colimits. With this in mind, for
any algebra (A, α) ∈ CT define L(A, α) to be the coequalizer(5.5.2)

FUFA

Fα
FA

// FA

/ / L(A, α),in D; this construction is arranged so that the coequalizer (5.4.4) is preserved by L. By
Example 5.4.7, the parallel pair Fα, FA : FUFA ⇒ FA is U-split, so the hypothesis that
U creates U-split coequalizers implies that the coequalizer (5.5.2) exists in D. The actionof L : CT → D on a morphism f : (A, α) → (B, β) is defined to be the unique map between
coequalizers induced by a commutative diagram of parallel pairs:
FUFA

Fα
FA

FUF f


FUFB

Fβ
FB

// FA

/ / L(A, α)

Ff∃! L f// FB

/ / L(B, β)174

5. MONADS AND THEIR ALGEBRASUniqueness of factorizations through colimit cones implies that this definition is functorial;
see Proposition 3.6.1.
The functor K carries the parallel pair (5.5.2) to the parallel pair (5.4.4). As U T strictly
creates coequalizers of U T -split pairs, we see that KL  1CT . It remains only to prove that
LK  1D . Given D ∈ D, the object LKD is defined to be the coequalizer of
FUD

FUFUD

FUD

// FUDBy Corollary 5.4.10(ii), LKD must therefore be isomorphic to the coequalizer (5.4.11).
The isomorphism induced between two coequalizers for the same diagram is unique, andthus defines a natural isomorphism LK  1D .
Corollary 5.5.3. The following categories are monadic over Set via the free a forgetfuladjunctions described in Example 4.1.10.
(i) Monoid, Group, Ab, Ring, and other variants, such as commutative rings or monoids,
or non-unital versions of the preceding(ii) ModR , Vectk , Affk , SetBG(iii) Lattice or the categories of meet or join semilattices10(iv) Set∗
Proof. We prove that monoids are monadic over sets and leave the proofs of the othercases, which are similar, to Exercise 5.5.ii.
Recall that a monoid is a set M together with maps η : 1 → M and µ : M × M → M sothat the diagrams displayed in Definition 1.6.2 commute in Set. A monoid homomorphismf : (M, η, µ) → (M 0 , η0 , µ0 ) is a function f : M → M 0 so that the diagrams
M×M

1
η

f×f

/ M0 × M0

0

η


M

f

µ

µ0


M

/ M0

f


/ M0

commute in Set.
To apply Theorem 5.5.1, it suffices to prove that U : Monoid → Set strictly createsthe coequalizer of any pair of monoid homomorphisms f, g : M ⇒ M 0 whose underlying
functions extend to a split coequalizer diagram
f

M_

g

// M 0jh/ M 00st

in Set. That is, we must show that there is a unique way to turn the set M 00 into a monoid sothat h : M 0 → M 00 is a monoid homomorphism, and moreover this fork defines a coequalizer
in the category of monoids. The proof will closely resemble the argument used to prove
Proposition 5.4.9.
For h to define a homomorphism, the unit for M 00 must be defined to be the composite
η0hη00 B 1 −→ M 0 →− M 00 . To define the multiplication for M 00 , we use the fact that the split
10A lattice is a poset with all finite meets (limits) and joins (colimits), and a morphism of lattices is anorder-preserving function that preserves these. A meet semilattice only has meets, while a join semilattice only
has joins.

5.5. RECOGNIZING CATEGORIES OF ALGEBRAS

175

coequalizer diagram is preserved by any functor, in particular by (−)2 : Set → Set. This
yields a parallel pair of coequalizers
f×f

M×M

g×g

// M 0 × M 0µM

h×h / /

µ0∃! µ00// M 0fg

M 00 × M 00h
/ / M 00

the induced map between which we define to be the multiplication for µ00 . Note ourdefinition is arranged so that h defines a monoid homomorphism, and moreover, by theuniqueness of factorizations through the coequalizer h×h, no other choice for µ00 is possible.
The proofs that (M 00 , η00 , µ00 ) is associative and unital, and that this monoid defines thecoequalizer of the monoid homomorphisms f, g : M ⇒ M 0 , proceed via the same arguments
used in the proof of Proposition 5.4.9: these properties are deduced from the analogous
properties of the monoid (M 0 , η0 , µ0 ) after canceling a quotienting epimorphism.
There is a unifying term that encompasses each of the categories listed in Corollary
5.5.3.
Definition 5.5.4. A functor is finitary if it preserves filtered colimits.
In particular, a monad T : C → C is finitary if it preserves filtered colimits in C. If aright adjoint is finitary, then so is its monad because its left adjoint preserves all colimits.
Definition 5.5.5. A category A is a category of models for an algebraic theory if there
is a finitary monadic functor U : A → Set.
Any category of models for an algebraic theory is locally finitely presentable, in thesense of Definition 4.6.16 [AR94, 2.78]. Another familiar category is monadic over sets,though it is not a model of an algebraic theory because its monad is not finitary.
Corollary 5.5.6. The underlying set functor U : cHaus → Set from the category ofcompact Hausdorff spaces is monadic.
The following proof is due to Paré [Par71]. As in the proof of Corollary 5.5.3, a key
point is that split coequalizers are absolute coequalizers, preserved by any functor.
Proof. A topological space may be defined to be a set X equipped with a closure
operator (−) : PX → PX on its set of subsets satisfying four axioms for all A, B ∈ PX:(i) ∅ = ∅,(iii) A = A,(ii) A ⊂ A,(iv) A ∪ B = A ∪ B .

A function f : X → Y between topological spaces is continuous if and only if f (A) ⊂ f (A)for all A ⊂ X and closed11 if and only if f (A) = f (A). All continuous functions between
compact Hausdorff spaces are closed; that is, a function f : X → Y between compact
Hausdorff spaces lifts to a map in cHaus if and only if it preserves the closure operator.
Consider a pair of continuous functions f, g : X ⇒ Y in cHaus so that there exists anabsolute coequalizer diagram
f

X

g

// Y

h

// Z

11A continuous function is closed if it preserves closed subsets.

176

5. MONADS AND THEIR ALGEBRAS

in Set. The covariant power set functor P : Set → Set preserves this, giving rise to acoequalizer diagram of direct image functions:
f∗

PX

g∗

(−)


PX

// PY

h∗

∃! (−)

(−)

f∗
g∗


// PY

/ / PZh∗/ / PZ

Because f and g are maps of compact Hausdorff spaces, the direct image functions commute
with the closure operators, and so the commutative diagram of parallel pairs induces afunction that we optimistically denote by (−) : PZ → PZ.
It is straightforward to verify that this function defines a closure operator on Z so that
Z is a topological space. Because the function h : Y → Z is surjective, continuous, andclosed and Y is compact Hausdorff, so is Z.12 By our characterization of maps in cHaus, it
is clear that we have constructed the unique lift of the absolute coequalizer diagram along
U : cHaus → Set.
To see that h : Y → Z has the universal property of a coequalizer in cHaus, consider
a continuous function k : Y → W so that k f = kg. On underlying sets, there is a uniquefactorization ` : Z → W of k through h, so it remains only to show that ` is continuous andclosed. That is, we must prove that the right-hand square commutes
k∗

PY

h∗

(−)


PY

/ / PZ

`∗

(−)

h∗


/ / PZ

/* PW
(−)

`∗


/4 PW

k∗

which follows from the fact that the left-hand square and exterior rectangle commute andthe fact that h∗ is an epimorphism.
Example 5.5.7. Other examples of monadic functors include:(i) Example 5.2.6(ii) reveals that the forgetful functor U : ModR → Ab is monadic. The
induced monad on Ab is given by R ⊗Z − : Ab → Ab.
(ii) The proof of Corollary 5.5.3 can be adapted to show that the forgetful functor
U : Ring → Ab is monadic. The induced monad on Ab is the free monoid monadL
T A B n≥0 A⊗n .
(iii) If C has coproducts (in fact, copowers suffice) and J is small, then Exercise 5.5.v
demonstrates that the restriction functor CJ → Cob J , which carries a J-indexed diagram in C to the ob J-indexed family of objects in its image, is monadic. Special cases
include, for instance, the functor VectkBG → Vectk that carries a G-representation toits underlying vector space or analogous functors for G-objects in any category.
(iv) U : Cat → DirGraph is also monadic. The monad forms (the underlying graphof) the free category on a directed graph, where identities and composites of all
12Compactness of Y and surjectivity of the continuous function h imply that Z is compact. Because Y is
Hausdorff, its points are closed, and since h is closed, points are also closed in Z. To show that Z is Hausdorff,note that distinct points z , z0 ∈ Z have disjoint closed preimages. Because the compact Hausdorff space Y isnormal, the preimages h−1 (z) and h−1 (z0 ) have disjoint open neighborhoods. The images of their complements
are closed subsets of Z, one containing z and the other containing z0 , whose complements are the desired disjoint
open neighborhoods of z and z0 .

5.5. RECOGNIZING CATEGORIES OF ALGEBRAS

177

finite directed paths are formally added. This functor factors through a second
monadic functor U : Cat → rDirGraph, whose target is the category whose objects
are reflexive directed graphs, with specified “identity” endoarrows of each vertex.
The construction of the inverse equivalence to the canonical functor K : D → CTgiven in the proof of Theorem 5.5.1 can be adapted to various other hypotheses on thegiven right adjoint U : D → C. In the categorical literature, these variant theorems
are traditionally assigned three-letter acronyms. Theorem 5.5.1 is sometimes called PTT,
“precise tripleability13 theorem,” “precise” because the characterization involves a condition
that is necessary, as well as sufficient. Statements of the “crude tripleability theorem” (CTT)and “vulgar tripleability theorem” (VTT) can be found in the exercises to [ML98a, §VI.7].
Another variant is the “reflexive tripleability theorem”:
Proposition 5.5.8 (RTT). If U : D → C has a left adjoint and if(i) D has coequalizers of reflexive14 pairs,(ii) U preserves coequalizers of reflexive pairs, and(iii) U reflects isomorphisms,
then U is monadic.
Proof. Exercise 5.5.iii.
Proposition 5.5.8 has a neat application:

Theorem 5.5.9 (Paré). The contravariant power set functor P : Setop → Set is monadic.
The argument presented here, due to Paré [Par74], also holds when Set is replacedby any elementary topos (a further generalization of the Grothendieck toposes introducedin §E.4). It makes use of the following lemma, whose conclusion is sometimes called theBeck–Chevalley condition, that asserts that for certain commutative squares, the induced
direct and inverse image functions on power sets commute:
Lemma 5.5.10. For any pullback diagram of monomorphisms, as displayed on the left,
A0
 y
a


A

g

/ B0


PB0


/B


PB

/ PA0
a∗

b∗

b

f

g−1

f −1


/ PA

the right hand square commutes.
Proof. For any X ∈ PB0 , consider a commutative rectangle
/X


Y


A0
 y
a


A

g


/ B0

b

f


/B

13“Triple” is an antiquated synonym for “monad.”
14A parallel pair f, g : A ⇒ B is reflexive if both maps admit a common section s : B → A.

178

5. MONADS AND THEIR ALGEBRAS

To say that a∗ g−1 (X) = Y is to say that the top square is a pullback. To say that f −1 b∗ (X) = Y
is to say that the composite rectangle is the pullback. By Exercise 3.1.viii, these conditions
are equivalent.

By Exercise 4.5.v, any monomorphism f : A ,→ B defines a pullback square
y

A


A /


/B

A

f

f

so a special case of Lemma 5.5.10 tells us that for any monomorphism, the composite
PA

f∗

f −1

/ PB

/ PA

is the identity.
Proof of Theorem 5.5.9. We apply Proposition 5.5.8. On account of the cartesianclosed structure of Set:
Set(A, PB)  Set(A × B, Ω)  Set(B, PA) ,op

the functor P : Set → Set has a left adjoint (namely, itself). By Theorem 3.2.6, Set has
finite limits, in particular, equalizers of coreflexive pairs proving (i). Given an equalizer ofa pair of maps with a common retraction:
1
e
E /

f

/A

g

// B

f

A

g

// B

s

"
/A
<

1

the square

E /
 y
e

e

/A



A / g


/B

f

is a pullback of monomorphisms: the retraction s implies that f and g are split monic,
which implies that the legs of any cone over this pullback diagram must be equal, provingthat the displayed square is a pullback. Applying Lemma 5.5.10, it follows that
f −1

PB]

g−1

// PA
j

e−1

/ PE

e∗

g∗

is a split coequalizer diagram, proving (ii).
To show (iii), note that P is faithful: given a parallel pair f, g : A ⇒ B, the composites
B

η

/ PB

f −1
g−1

// PA

with the singleton map η : B → PB transpose to define functions A × B ⇒ Ω that classify
the subsets (1, f ) : A  A× B and (1, g) : A  A× B. It follows from the universal propertyof the subobject classifier Ω, described in Example 2.1.6(i), that if f −1 = g−1 , then f = g.
By Exercise 1.6.iii, any faithful functor reflects both monomorphisms and epimorphisms.

5.5. RECOGNIZING CATEGORIES OF ALGEBRAS

179

In Set, or any elementary topos, the isomorphisms are precisely those maps that are bothmonic and epic, so it follows that P reflects isomorphisms. Proposition 5.5.8 now implies
that the contravariant power set functor is monadic.

Exercises.
Exercise 5.5.i. Prove the strict version of the monadicity theorem: that the domain D of aright adjoint U : D → C is isomorphic to the category of algebras if and only if U strictly
creates coequalizers of U-split pairs.
Exercise 5.5.ii. Use the monadicity theorem to prove that another of the categories listedin Corollary 5.5.3 is monadic over sets.
Exercise 5.5.iii. Prove Proposition 5.5.8.
Exercise 5.5.iv. For any group G, the forgetful functor SetBG → Set admits a left adjointthat sends a set X to the G-set G × X, with G acting on the left. Prove that this adjunction
is monadic by appealing to the monadicity theorem.
Exercise 5.5.v. Generalizing Exercise 5.5.iv, for any small category J and any cocomplete
category C the forgetful functor CJ → Cob J admits a left adjoint Lan : Cob J → CJ that
sends a functor F ∈ Cob J to the functor LanF ∈ CJ defined byaa
LanF( j) =Fx .
x∈J J(x, j)(i) Define LanF on morphisms in J.
(ii) Define Lan on morphisms in Cob J .
(iii) Use the Yoneda lemma to show that Lan is left adjoint to the forgetful (restriction)functor CJ → Cob J .
(iv) Prove that this adjunction is monadic by appealing to the monadicity theorem.
Exercise 5.5.vi. Describe a more general class of functors K : I → J between small
categories so that for any cocomplete C the restriction functor resK : CJ → CI strictly
creates colimits of resK -split parallel pairs. All such functors admit left adjoints and are
therefore monadic. Challenge: describe the left adjoint (or see Theorem 6.2.1).
Exercise 5.5.vii. Consider the Kleisli category SetT for a monad T acting on Set and'choose a skeleton N −→Fin ,→ Set for the full subcategory of finite sets. Let L be theopposite of the full subcategory of the Kleisli category spanned by the objects 0, 1, 2, . . . inN, so that there is an identity-on-objects functor:
Nop

/L


I

'




Finop /

/ SetopFT/ Setop
T(i) Show that the categories Nop and L have strictly associative finite products that arepreserved by the functor I : Nop → L.
When the monad T is finitary, the functor I : Nop → L defines its associated Lawvere
theory. Because the objects in the category L are all iterated finite products of the object
1, the essential data in the Lawvere theory is the set L(n, 1) of “n-ary operations.” It is
possible to recover the category of T -algebras, that is, the category of models for thisalgebraic theory, from this data [Lin66]. A model of the Lawvere theory in Set is a
finite product preserving functor L → Set. A morphism between models is a naturaltransformation.

180

5. MONADS AND THEIR ALGEBRAS(ii) Define a functor from the category of T -algebras to the category of models for the
Lawvere theory I : Nop → L. (Hint: See Exercise 5.2.vii.)
5.6. Limits and colimits in categories of algebras
Recall that a category A is monadic over C if there is an adjunction
Co

F
⊥
U

/

A

that is equivalent to the adjunction between C and the category of algebras for the monad
UF. By Theorem 5.5.1, this is the case if and only if the functor U : A → C creates
coequalizers of U-split pairs, a somewhat strange technical condition that can be relatively
practical to check. Our aim in this section is to present a few results from categoricaluniversal algebra, which describe some of the common features of categories that areequivalent to categories of algebras. In particular, these properties hold for Set∗ , Monoid,
Group, Ab, Ring, CRing, ModR , Affk , SetBG , Lattice, and cHaus, all of which are monadic
over Set,15 and sometimes fail for Poset, Top, and Field, which are not.16
A basic property common to all monadic functors is:
Lemma 5.6.1. If U : A → C is monadic, then U reflects isomorphisms: for any morphismf : a → a0 in A, if U f is an isomorphism in C, then f is an isomorphism in A.
A functor is conservative if it reflects isomorphisms.
Proof. A monadic functor U : A → C is equivalent to the forgetful functor from thecategory of algebras for the induced monad on C, and so it suffices to demonstrate thatU T : CT → C has this property. Recall that a morphism f : (A, α) → (A0 , α0 ) in CT is a
map f : A → A0 in C so that the left-hand diagram
TA
α


A

Tf

/ T A0
α0

f


/ A0

T A0

T f −1

α

α0


A0

/ TA

f −1


/A

commutes in C, whence the right-hand diagram also commutes whenever the inverse
f −1 : A0 → A exists in C.
Lemma 5.6.1 answers the question motivated by Example 1.1.10.
Corollary 5.6.2. Any bĳective continuous function between compact Hausdorff spaces
is a homeomorphism.17
Remark 5.6.3. Unlike the other corollaries listed in the introduction, this is not the mostefficient proof that bĳective continuous functions between compact Hausdorff spaces are
15For ease of exposition, these applications are focused on categories that are monadic over sets, but thecategorical results apply to monads acting on any category, and more sophisticated applications frequently takeadvantage of this.
16The functor U : Field → Set cannot be monadic: Example 4.1.12 observes that it fails to admit any adjoints.
Proofs that the forgetful functors U : Poset → Set and U : Top → Set are not monadic will appear shortly.
17More generally, any continuous bĳection from a compact space to a Hausdorff one is a homeomorphism, but
this statement is no true generalization: compactness of the domain and surjectivity of the map imply that thecodomain is also compact, while Hausdorffness of the codomain and injectivity of the map imply that the domain
is also Hausdorff.

5.6. LIMITS AND COLIMITS IN CATEGORIES OF ALGEBRAS

181

homeomorphisms,18 but we submit that this roundabout proof of Corollary 5.6.2 is nonetheless interesting. By monadicity of the functor U : cHaus → Set and Proposition 5.4.3,
any compact Hausdorff space is canonically a quotient of the Stone–Čech compactificationof its underlying set. A continuous bĳection induces a natural isomorphism between the
Stone–Čech compactifications, and thus a homeomorphism between the canonical quotients.
Corollary 5.6.4. Any bĳective homomorphism in a category of models for an algebraic
theory is an isomorphism.
For instance, the inverse of a bĳective homomorphism between groups is also a homomorphism. This, of course, is not difficult to prove. Corollary 5.6.4 eliminates theredundancy of proving the same result over and over again in many similar contexts.
The contrapositive to Lemma 5.6.1 demonstrates that the categories Top or Poset are notmonadic over sets: there exist non-invertible maps that act as the identity on underlyingsets.
We now turn our focus to limits and colimits in a category of algebras.
Theorem 5.6.5. A monadic functor U : A → C creates(i) any limits that C has, and(ii) any colimits that C has and the monad and its square preserve.

Proof. By Lemma 3.3.6, an equivalence A ' CT creates all limits and colimits present
in either category, and so it suffices to prove this result for the forgetful functor U T : CT → C
from the category of algebras for the induced monad.
To prove (i), consider a diagram D : J → CT , spanning objects (D j, γ j ) ∈ CT , so thatthe underlying diagram U T D : J → C admits a limit cone (µ j : L → D j) j∈J in C. We wish to
lift the summit L and the legs of the limit cone to a limit cone for the diagram D in CT . The
fact that D is a diagram of algebras implies that the algebra structure maps assemble into thecomponents of a natural transformation γ : T D ⇒ D ∈ CJ from the underlying C-valued
diagram T D to the underlying C-valued diagram D. The composite natural transformation
Tµ

TL+3 T Dγ+3 D

defines a cone with summit T L over the diagram D : J → C, which factors through thelimit cone to define a unique map λ : T L → L in C so that the diagrams

TL
λ

Tµj

/ TDj
γj


L

µj


/ Dj

18To wit, a closed subset of a compact space is compact and so is its continuous image. If the target space is
Hausdorff, then this compact subspace is closed. A continuous closed bĳection is a homeomorphism.

182

5. MONADS AND THEIR ALGEBRAS

commute for each j ∈ J. To verify that (L, λ) is a T -algebra, we must show that the left-mostfaces in the prisms

L

µj
ηL



1L

 
L

TL

/ Dj
Tµj
1D j

λ
µj

 |
/ Dj

T 2µ j

T 2L
ηD j

µL

"
/ T Dj


TL

γj

λ

Tµj
λ

"

Tλ

"

L

µD j

/ T 2D j

TL

Tµj


/ T DjTγj#/ T Dj

γj
γj

µj


/ Dj
$

commute in C, knowing already that the remaining faces do. Appealing to the universal
property of the limit cone (µ j : L → D j ) j∈J , it suffices to prove that these faces commute
after composing with µ j for each j ∈ J, and this is what the commutativity of the remaining
faces of each prism demonstrates.19 The verification that this is a limit cone proceeds
similarly and is left to the reader.
A similar argument works for (ii) with the additional hypothesis that T and T 2 preserve
the colimit cone in C under consideration. If L ∈ C is the nadir of a colimit cone undera diagram U T D : J → C, then its algebra structure map λ : T L → L is induced by theuniversal property of the colimit cone under T U T D with nadir T L.20 The details are left
as Exercise 5.6.ii.
An immediate corollary completes the unfinished business of Proposition 4.5.15.
Corollary 5.6.6. The inclusion of a reflective subcategory creates all limits. In particular,a reflective subcategory of a complete category is complete.
Proof. Proposition 5.3.3 proves that the inclusion of a reflective subcategory ismonadic. By Theorem 5.6.5(i), it follows that this inclusion creates all limits that exist in the codomain.
Another corollary is the completeness result promised in §3.5.
Corollary 5.6.7. Any category that is monadic over Set is complete, with limits createdby the monadic forgetful functor.
Proof. Theorem 3.2.6 proves that Set is complete. Theorem 5.6.5 demonstrates thatany category that is monadic over Set is also complete.

Example 5.6.8. For instance, the p-adic integers are defined to be the limit of an ωop indexed diagram of rings
Z p B limn Z/pn

/ ···/ Z/p3/ Z/p2/ Z/p

By Example 3.2.10, because U : Ring → Set preserves limits, as a setnoZ p = (a1 ∈ Z/p, a2 ∈ Z/p2 , a3 ∈ Z/p3 , . . .) an ≡ am mod pmin(n,m) .
That is, a p-adic integer is a sequence of elements an ∈ Z/pn that are compatible modulo
congruence.
The underlying set functor also creates the limit cone, which tells us that the ring
structure on this set of elements must be defined in such a way so that the projection functions
19This is the dual of the argument given in the proof of Proposition 5.4.9.
20The fact that the monad T preserves split coequalizers was used in this way in the proof of Proposition 5.4.9.

5.6. LIMITS AND COLIMITS IN CATEGORIES OF ALGEBRAS

183

Z p → Z/pn are ring homomorphisms. This tells us that addition and multiplication ofelements is “componentwise.”Corollary 5.6.9. Set is cocomplete.
Proof. The contravariant power set functor P : Setop → Set is monadic, so the colimitof a diagram is created from the limit of the corresponding diagram on power sets.

Corollary 5.6.10. ModR → Ab creates all colimits that Ab admits.
Proof. By Example 4.3.11, for any pair of abelian groups A and A0 , there is a natural
isomorphism
Ab(R ⊗Z A, A0 )  Ab(A, HomZ (R, A0 ))involving the tensor product of abelian groups and the abelian group HomZ (R, A0 ) of grouphomomorphisms R → A0 . In particular, the monad R ⊗Z − : Ab → Ab has a right adjoint
HomZ (R, −). By Theorem 4.5.3, R ⊗Z − preserves all colimits and so Theorem 5.6.5(ii)applies to all diagrams in Ab.

In fact, Ab and any category of models for an algebraic theory is cocomplete. However,the construction of those colimits that are not preserved by the monad is more complicated.
To explore this problem, consider the monadic forgetful functor U : Group → Set. BothSet and Group admit coproducts; in Set these are simply disjoint unions, while in Groupthey are given by the free product. The free product of groups G and H is the groupG ∗ H of words whose letters are drawn from the elements of G and H together with formal
inverses, modulo relations defined using the group operations in each group. Note, inparticular, that U : Group → Set does not preserve (and so, in particular, does not create)
coproducts. However, there is a more precise description of the free product G ∗ H that canbe stated entirely in terms of the free a forgetful adjunction
Set o

F
⊥
U

/

Group ,

which generalizes to define coproducts relative to any monadic adjunction.
A first approximation to the free product G ∗ H is given by F(UG t UH), the freegroup on the disjoint union of the underlying sets of G and H. Elements of this group arewords whose letters are elements of G or H, but it remains to impose the relations generated
by words in G and words in H. These words in G and words in H are elements of theset UFUG t UFUH, defined to be the disjoint union of the underlying sets of the freegroups on the underlying sets of G and H. The free group on this set F(UFUG t UFUH)
is the group of words of words in the letters G and H so that each subword is exclusively
comprised of letters drawn from a single one of these groups. The desired relations thatdefine G ∗ H as a quotient of F(UG t UH) identify a word of words of this type with theword obtained by evaluating each subword using the group structure of G or of H.
To encode the desired relations, we define a natural pair of group homomorphisms.
F(UG tUH )F(UFUG t UFUH)Fκ,FUF(UG t UH)/
2 F(UG t UH)F(UGtUH)

The fact that G and H are groups is encoded by a pair of homomorphisms G : FUG → G
and H : FUH → H, the components of the counit, that evaluate a word to the group elementthat it represents. The top map sends an element in the group F(UFUG t UFUH), a word

184

5. MONADS AND THEIR ALGEBRAS

of words in which these subwords are exclusively in the group G or in the group H, to aword in UG t UH by evaluating each subword to the corresponding group element.
The bottom composite homomorphism first makes use of the natural map κ of Exercise
3.3.i, which compares the coproduct of the image of two objects under a functor, in thiscase UF, with the image of the coproduct under that functor. Here this has the effect ofregarding a word of words, with subwords exclusively in either G or H, as a word of words
whose letters might belong to either G or H, but happen in this case to belong to only one
or the other. The map F(UGtUH) , again a counit component, then concatenates to produce asingle word in letters of G and H. From this point, we see that the coequalizer of these twogroup homomorphisms imposes exactly the relations desired to define the free product:/ F(UG t UH)
2

F(UG tUH )F(UFUG t UFUH)Fκ,FUF(UG t UH)// G∗HF(UGtUH)

This construction of the coproduct generalizes to any monadic functor U : A → C overa cocomplete category whenever the coequalizer just described exists in A.
Proposition 5.6.11. Suppose C is cocomplete and U : A → C is monadic. Then thefollowing are equivalent:(i) A is cocomplete.
(ii) A has coequalizers.
Proof. The implication (i) ⇒ (ii) is trivial. For (ii) ⇒ (i), we may replace A by theequivalent category of algebras CT . It suffices by Theorem 3.4.12 to prove that a categoryof algebras that has coequalizers also has coproducts. The coproduct (A, α) of a family of
T -algebras (Ai , αi ) ∈ CT is defined to be the coequalizer`
T ( i αi )`(T ( i T Ai ), µ`i T Ai )Tκ,`(T 2 ( i Ai ), µT (`i Ai ) )µ`i Ai/ (T (`i Ai ), µ` A )i i
3/ / (A, α) .

Note that the objects in the coequalizer diagram are all free algebras. Using the adjunction
F T a U T , it is straightforward to show that (A, α) has the desired universal property of the
coproduct of the objects (Ai , αi ). The proof is left as Exercise 5.6.iii.
It remains to enumerate conditions that suffice to prove that the category of algebras
has coequalizers and is therefore cocomplete.
Theorem 5.6.12. If T : C → C is a finitary monad on a complete and cocomplete, locallysmall category C, then the category CT of T -algebras is also complete and cocomplete.
The proof presented here can be found in [Bor94b].
Proof. By Theorem 5.6.5, completeness of C implies that CT is complete. By Proposition 5.6.11, to show that CT is cocomplete, it suffices to show that CT has coequalizers. By
Proposition 4.5.1, to show that CT has coequalizers, we must show that the constant diagram
functor from the category of T -algebras to the category of parallel pairs of T -algebrasCTo colim⊥∆T •⇒•/ (C )

has a left adjoint. By Proposition 3.3.9, which says that limits in a functor category
are computed objectwise, the constant diagram functor preserves limits. To prove that

5.6. LIMITS AND COLIMITS IN CATEGORIES OF ALGEBRAS

185

the desired left adjoint exists, we apply the General Adjoint Functor Theorem 4.6.3; theconclusion follows as soon as we show that the functor ∆ satisfies the solution set condition.
To apply Theorem 4.6.3, we must produce a solution set for each object in the category(CT )•⇒• , i.e., for each parallel pair of T -algebra morphisms f, g : (A, α) ⇒ (B, β). We willproduce a solution set consisting of a single element by defining a fork// (B, β)f(A, α)gq/ (Q, u)so that any other fork under the pair f, g in CT factors through q. If we knew how toform coequalizers in CT , this would be easy; instead, the T -algebra (Q, u) and T -algebramorphism q is a reasonable approximation21 to the coequalizer defined using an inductive
construction.
To start, we define the coequalizer q0 : B  Q0 in C of the pair f, g : A ⇒ B. If
T q0 were the coequalizer of T f and T g in C, then its universal property would enable us
to equip Q0 with the structure of a T -algebra, but this is unlikely as we do not assume
that the monad preserves coequalizers. Instead, we form the coequalizer p0 : T B  P0 of
T f, T g : T A ⇒ T B and the induced maps
T q0
Tf

TA
Tg
α

// T B

p0

β


A

g

∃! v0

/ / P0
∃! u0

// B

f

T
< Q
O 0

q0


/ / Q0Continuing inductively, for each n ≥ 0, we will produce a diagram in C

T qn
Tf

TA
Tg
α


A

// T B

pn

β

f
g

// B

T
< Q
O n

T qn,n+1/ T Qn+1
O

vn

/ Pn

vn+1
pn,n+1/ Pn+1

un

qn


/ Qn

qn,n+1un+1/ Qn+1so that• the pn and qn define cones under the parallel pairs (T f, T g) and ( f, g), respectively,• pn+1 = pn,n+1 · pn and qn+1 = qn,n+1 · qn , and• the ω-indexed diagram (Pn )n∈ω is a shifted version of the ω-indexed diagram (T Qn )n∈ω .
For the inductive step, define Pn+1 = T Qn , define Qn+1 to be the coequalizerµQn ·T vn

T Pn

T un

// T Qun+1n/ / Qn+1 ,definepn,n+1 B vn ,qn,n+1 B un+1 · ηQn ,vn+1 B T qn,n+1 ,and define pn+1 and qn+1 to be the necessary composites.
21The solution set condition does not demand that factorizations through q are unique.

186

5. MONADS AND THEIR ALGEBRASThis construction yields a pair of ω-indexed diagrams (Pn )n∈ω and (Qn )n∈ω in C witha natural transformation u : P ⇒ Q between them. We extend this data by forming thecolimits:
7 T Q0

p0,1

/ P1

u0


Q0

T q0,1
v1
p1,2

u1
q0,1

/ T Q1 T q1,2 / · · ·/ colimn T Qn  T colimn Qn  T Qω

6

/ P2

C

P0

v0

/ ···

p2,3

/ colimn Pn
∃! uω

u2


/ Q1

q1,2


/ Q2


/ colimn Qn =: Qω

/ ···

q2,3

As the category ω is filtered and the monad T preserves filtered colimits, the functor Tqn,ω
carries the colimit cone (Qn −−−→ Qω )n∈ω under the bottom sequence to a colimit cone
T qn,ω

(T Qn −−−−→ T Qω )n∈ω under the top one. Adding the initial leg, this data extends to define acolimit cone over the ω-diagram of Pn ’s. In particular, the natural transformation u : P ⇒ Q
induces a map uω : T Qω → Qω between the colimits so that uω · T qn,ω = qn+1,ω · un+1 .
We claim that (Qω , uω ) is a T -algebra. For the unit condition it suffices, by the universal
property of Qω to show that uω · ηQω · qn,ω = qn,ω for all sufficiently large n. Indeed,uω · ηQω · qn,ω = uω · T qn,ω · ηQn = qn+1,ω · un+1 · ηQn = qn+1,ω · qn,n+1 = qn,ωby naturality of η, the definition of uω , the definition of qn,n+1 , and the fact that the qn,ωform a cone.
For the associativity condition, observe that there is an ω-indexed sequence of coequalizer diagrams defining the maps un
..
.
2
T
8 Qn

..
.
T vn


T Pn

..
.

µ Qn

..
.


/ T Qn&T un

 T qn,n+1
2
T
7 Qn+1un+1/ / Qn+1

2

T pn,n+1


T Pn+1
..
.

T vn+1

T un+1µQn+1

qn+1,n+2

T qn,n+1' / T Qn+1


..
.

un+2


..
.


/ / Qn+2
..
.

Passing to the sequential colimits, which the finitary monad T preserves, observe that vω isthe identity. So we conclude that uω : T Qω → Qω is the coequalizer of T uω and µQω , andso in particular uω · T uω = uω · µQω . Thus, (Qω , uω ) is a T -algebra.
By construction, the map qω := q0,ω · q0 : (B, β) → (Qω , uω ) is a T -algebra morphism.
It remains to show that any other T -algebra morphism h : (B, β) → (C, γ) that defines afork under the pair f, g : (A, α) ⇒ (B, β) factors through qω . By the universal property ofthe sequential colimit Qω = colimn Qn , to define the factorization k : Qω → C it suffices todefine its components
kn B Qn

qn,ω

/ Qω

k

/C.

5.6. LIMITS AND COLIMITS IN CATEGORIES OF ALGEBRAS

187

For each n ∈ ω, we will show that the diagram
Pn(5.6.13)vn/ T Qn

un

!

/ TC

T kn

γ

Qn


/C

kn

commutes in C. Passing to sequential colimits, vω is an identity, so it will follow thatk : (Qω , uω ) → (C, γ) defines an algebra map.
Define k0 to be the unique factorization of h through the coequalizer Q0 of f, g : A ⇒ B.
To prove (5.6.13) in the case n = 0, it suffices to prove commutativity after restricting along
the epimorphism p0 : T B  P0 , and indeed
γ · T k0 · v0 · p0 = γ · T k0 · T q0 = γ · T h = h · β = k0 · q0 · β = k0 · u0 · p0 .
Inductively, define kn+1 to be the unique factorization of γ · T kn : T Qn → C through thecoequalizer un+1 : Pn+1  Qn+1 ; a straightforward diagram chase shows that γ · T kn defines
a fork under the pair coequalized by un+1 . Another diagram chase shows that the condition(5.6.13) is satisfied by kn+1 . A final diagram chase verifies that the maps kn : Qn → C
assemble into a cone under the ω-indexed diagram whose colimit is Qω . Thus, they induce
the desired map k : Qω → C. Because T preserves these filtered colimits, the pentagon
conditions (5.6.13) imply that
T Qω

Tk

γ

uω


Qω

/ TC

k


/C

so k defines a T -algebra homomorphism. With the solution set condition so-verified,
Theorem 4.6.3 implies that the constant diagram functor ∆ : CT → (CT )•⇒• has a left
adjoint and thus that CT has coequalizers.
In particular, by Theorems 3.2.6 and 5.6.9, Set is complete and cocomplete. By
Definition 5.5.5, categories of models for an algebraic theory are algebras for a finitarymonad. Applying Theorem 5.6.12, we conclude:
Corollary 5.6.14. Any category of models for an algebraic theory is cocomplete.
While the monad β for compact Hausdorff spaces is not finitary, the category cHaus is
nonetheless cocomplete. Example 4.5.14(i) observes that compact Hausdorff spaces forma reflective subcategory of Top. The claim follows from Propositions 3.5.2 and 4.5.15.
Exercises.
Exercise 5.6.i. Exercise 3.3.iv suggests an alternate form of the monadicity theorem: afunctor U : D → C is monadic if U has a left adjoint, if U reflects isomorphisms, and if D
has and U preserves U-split coequalizers. Use this, together with a direct proof that any
continuous bĳection between compact Hausdorff spaces is a homeomorphism, to re-prove
Corollary 5.5.6 by showing that cHaus has U-split coequalizers, constructed as in Top.
(Hint: It is easy to see that the coequalizer of maps f, g : X ⇒ Y of compact spaces iscompact. To prove that a U-split coequalizer is Hausdorff, use the split coequalizer diagramin Set to prove that the kernel pair E ,→ Y × Y of the quotient map Y  Z is closed.)
Exercise 5.6.ii. Prove Theorem 5.6.5(ii).

188

5. MONADS AND THEIR ALGEBRAS

Exercise 5.6.iii. Prove that the T -algebra constructed as a coequalizer in the proof of
Proposition 5.6.11 defines the coproduct in the category of T -algebras.

CHAPTER 6

All Concepts are Kan Extensions
The notion of Kan extensions subsumes all theother fundamental concepts of category theory.
Saunders Mac Lane, Categories for the Working
Mathematician [ML98a]

Extension problems are pervasive in mathematics. For instance, iterated products ofa fixed positive real number, such as 2, define its natural number powers. The resulting
function 2− : N → R>0 can be extended to a homomorphism from the additive group of
integers to the multiplicative group of non-negative reals by declaring 2−n B ( 21 )n for n ∈ N.
The resulting function 2− : Z → R>0 can be extended
further to an additive homomorphism√n1ndefined on the rationals by declaring 2 B 2 for n ∈ N. Finally, 2− : Q → R>0 canbe extended to an order-preserving function on the reals, not via any explicitly describedarithmetic formula, but by using the fact that R>0 has limits of all bounded increasing
sequences.
The construction of the function 2− : R → R>0 as an order-preserving function extending 2− : Q → R>0 is a special case of a solution to a categorically-defined problem
of extending a functor F : C → E along another functor K : C → D, possibly but notnecessarily a subcategory inclusion. In general, strict extensions may or may not exist.
Obstructions can take several forms: two arrows in C with distinct images in E might beidentified in D, or two objects in C might have a morphism between their images in D
but not in E. For this reason, it is more reasonable to ask for a “best approximation” toan extension taking the form of a universal natural transformation pointing either from or
to F. The resulting categorical notion, quite simple to define, is surprisingly ubiquitousthroughout mathematics, as we shall soon discover.
Solutions to this categorical extension problem, the first examples of which were studied
in [Kan58], are called Kan extensions and come in dual left-handed and right-handed forms.
The general definition and some basic examples of Kan extensions are introduced in §6.1,
which explains how Kan extensions define partial adjoints to the functor K ∗ : ED → EC
defined by restriction along K : C → D. A formula, defining left Kan extensions as certain
colimits and right Kan extensions as certain limits, is presented in §6.2, which in particularprovides criteria guaranteeing that certain Kan extensions exist. In §6.3, we discoverthat the (co)limit formulae imply that the Kan extensions so-defined satisfy a stronger
universal property than was originally supposed: namely, Kan extensions defined by the(co)limit formulae are pointwise Kan extensions. In §6.4, we study total derived functors,
which are defined as certain Kan extensions, and introduce an abstract general frameworkthat constructs “point-set level” lifts of these derived functors. Unexpectedly—since total
derived functors take values in homotopy categories, which have few limits and colimits—
the total derived functors constructed in this manner have the stronger universal property of
being pointwise and in fact absolute Kan extensions, a fact that has significant consequence
189

190

6. ALL CONCEPTS ARE KAN EXTENSIONS

for derived adjunctions. We conclude in §6.5 by showing how simple special cases of Kan
extensions can be used to redefine adjunctions, limits, colimits, and monads and generalize
the Yoneda lemma.
6.1. Kan extensions
The data of a Kan extension consists of a functor and a natural transformation satisfying
a universal property relative to other similar pairs. For the reasons discussed in §4.4, thisdefinition can also be stated for 2-categories other than the 2-category of categories, functors, and natural transformations, in which case the eponym “Kan” is frequently dropped.
Since we are only interested in this one example of a 2-category at present, we retain the
traditional name.
Definition 6.1.1. Given functors F : C → E, K : C → D, a left Kan extension of F along
K is a functor LanK F : D → E together with a natural transformation η : F ⇒ LanK F · K
such that for any other such pair (G : D → E, γ : F ⇒ GK), γ factors uniquely through η as
illustrated.1
F

C

/E
?

⇓η



K

/E
?

F

C

=

⇓γ



K

LanK F

D

G

F

C

LanK F
u
∃!

⇓η
K

D



/3 E
K
G

D

Dually, a right Kan extension of F along K is a functor RanK F : D → E together with anatural transformation  : RanK F · K ⇒ F such that for any (G : D → E, δ : GK ⇒ F), δ
factors uniquely through  as illustrated.
F

C

/E
?

⇑
K



/E
?

F

C

⇑δ



K

RanK F

D

G

D

F

C

=

⇑
K



D

RanK F
v
∃!

/3 E
K
G

Remark 6.1.2. When K : C → D is fully faithful, then assuming certain (co)limits exist,
the left and right Kan extensions do in fact extend the functor F along K, at least up to
natural isomorphism; see Corollary 6.3.9. But even a genuine on-the-nose extension of F
along K might not necessarily define a Kan extension; see Exercises 6.1.i and 6.1.ii.
Example 6.1.3. For any object c ∈ C and any F : C → Set, there is a bĳection between
elements x ∈ Fc and natural transformations with boundary as displayed
/ Set
>

∗

1

⇓x
c



F

C

where ∗ denotes the functor that picks out the singleton set. By the Yoneda lemma, therepresentable functor C(c, −) and the identity 1c ∈ C(c, c) define the left Kan extension of
1Writing α for the natural transformation LanK F ⇒ G, the right-hand pasting diagrams express the equality
γ = αK · η, i.e., that γ factors as F

η

+3 LanK F · K

αK+3 GK .

6.1. KAN EXTENSIONS

191∗ : 1 → Set along c : 1 → C. The required unique factorization
∗

1

/ Set
>

⇓1c
c



/ Set
>

∗

1

⇓x
c

C(c,−)



C

F

∗

1

C(c,−)

=

⇓1c
c

C



/4 Set
J

u
Ψ(x)

C

F

is the natural transformation Ψ(x) : C(c, −) ⇒ F defined in the proof of Theorem 2.2.4.
The pasting identity asserts that Ψ(x)c (1c ) = x.
Replacing C by Cop , this argument shows that the contravariant representable functor
C(−, c) defines the left Kan extension of ∗ : 1 → Set along c : 1 → Cop .
Example 6.1.4. For any group G, there is a unique functor 1 → BG picking out the single
object in this category. For any category C, object c ∈ C, and G-object X in C, naturaltransformations as displayed below-left and below-right
1

/C
>

c

/C
>

c

1

⇓

⇑
X

X

BG

BG

correspond to maps in C between c and the underlying object of the G-object X, with thegroup action forgotten.
If C has coproducts, then morphisms c → X correspond to G-equivariant maps`c
→ X, where G acts on the left of the indexing set of the coproduct. This bĳecGtion is implemented by restricting the G-equivariant map to the component indexed by theidentity element e ∈ G. Thus, we see that
1

c

/C
>

⇓ιe
`
Lan= G c

BG

is a left Kan extension diagram.
Dually, if C has products, then morphisms X → c correspond to G-equivariant maps
Q
X → G c, where G acts on the right2 of the indexing set of the product. This bĳection is
implemented by projecting the G-equivariant map to the component indexed by the identity
element. Thus, we see that
c
/C
1
>
⇑πe
Q
Ran= G c

BG

is a right Kan extension diagram.
The universal property that defines Kan extensions posits a bĳection between certainclasses of natural transformations, implemented by composing with a given natural transformation. In practice, the functor categories EC and ED referred to in Definition 6.1.1
might not be locally small, in which case the bĳection may be between a pair of properclasses.
2By the universal property of the product, a function f : A → B between indexing sets induces a map
QQ
f ∗ : B c → A c defined at the component indexed by a ∈ A by projecting from the domain to the component
indexed by f (a) ∈ B.

192

6. ALL CONCEPTS ARE KAN EXTENSIONS

Passing to a higher set-theoretical universe, we can nonetheless think of a left Kan
extension of F : C → E along K : C → D as a representation for the functor
EC (F, − · K) : ED → SET

that sends a functor D → E to the collection of natural transformations from F to its
restriction along K. By the Yoneda lemma, any pair (G, γ) as in Definition 6.1.1 defines anatural transformationγ+3 EC (F, − · K).
ED (G, −)The universal property satisfied by the left Kan extension (LanK F, η) is equivalent to theassertion that the corresponding map
η

ED (LanK F, −)+3 EC (F, − · K)

is a natural isomorphism, i.e., that (LanK F, η) represents the functor EC (F, − · K).
Fixing K : C → D but letting F ∈ EC vary:
Proposition 6.1.5. If, for fixed K : C → D and E, the left and right Kan extensions ofany functor F : C → E along K exist, then these define left and right adjoints to thepre-composition functor K ∗ : ED → EC .
(6.1.6)LanK

ED (LanK F, G)  EC (F, GK)

EC o

⊥
K∗
⊥



EC (GK, F)  ED (G, RanK F)

ED D

RanK

Proof. If the functors LanK F, RanK F : D ⇒ E exist for any F : C → E, then Proposition 4.3.4 applied to the natural isomorphisms of Definition 6.1.1 tells us that these values
assemble to define adjoint functors LanK a K ∗ a RanK . The 2-cell η : F ⇒ LanK F · K
is the component of the unit for LanK a K ∗ indexed by the object F ∈ EC ; dually, : RanK F · K ⇒ F is a component of the counit for K ∗ a RanK . The universal propertyof the left Kan extension declares that η : F ⇒ LanK F · K is initial in the comma category
F ↓ K ∗ ; see Lemma 4.6.1.
Conversely, by uniqueness of adjoints (Proposition 4.4.1), the objects in the image ofany left or right adjoint to a pre-composition functor are Kan extensions. This observation
leads to several immediate examples.
Example 6.1.7. The objects of the functor category VectBkG are G-representations overthe field k, and arrows are G-equivariant linear maps. If H is a subgroup of G, restriction resGH : VectBkG → VectBk H of a G-representation to an H-representation is simply
pre-composition by the inclusion functor BH ,→ BG. This functor has a left adjoint, called
induction, and a right adjoint, called coinduction.
indGH

}

VectBkG

a

⊥
resGH
⊥

/ VectBHkcoindGH

By Proposition 6.1.5, the induction functor is given by left Kan extension along the inclusion
BH ,→ BG, while coinduction is given by right Kan extension. The reader unfamiliar withthe construction of induced representations need not remain in suspense for very long;6.2. A FORMULA FOR KAN EXTENSIONS

193

see Theorem 6.2.1 and Example 6.2.8. Similar remarks apply for G-sets, G-spaces, based
G-spaces, or indeed G-objects in any category, although if the ambient category has few
limits and colimits, these adjoints need not exist.
Example 6.1.8. The category ∆ of finite non-empty ordinals and order-preserving maps isa full subcategory containing all but the initial object of the category ∆+ , of finite ordinals
and order-preserving maps. Presheaves on ∆ are simplicial sets, while presheaves on ∆+ are
called augmented simplicial sets. Left Kan extension defines a left adjoint to restriction
π0

|

⊥
res
⊥

op

Set∆^ +

/ Set∆optriv

that augments a simplicial set X with its set π0 X of path components. Right Kan extension
assigns a simplicial set the trivial augmentation built from the one-point set, as can easily
be verified by readers familiar with the combinatorics of simplicial sets.
Exercises.
Exercise 6.1.i. Construct a toy example to illustrate that if F factors through K along somefunctor H, it is not necessarily the case that (H, 1F ) is either the left or right Kan extensionof F along K.
Exercise 6.1.ii. Consider the functor d1 : 2 ,→ 3 whose image omits the middle object.
Show that for any category C and any functor 2 → C, there exist both left and right Kan
extensions along d1 : 2 ,→ 3 by giving an explicit construction. Observe that both the left
and right Kan extensions extend the original diagram? / ?C

2
Lan
d1



Ran

3

but, nonetheless, these functors are not typically isomorphic.
Exercise 6.1.iii. Define the category in which the right Kan extension of F : C → E along
K : C → D is terminal and explain the connection between this category and the functor
EC (− · K, F) : (ED )op → Set, in the case where these functor categories are locally small.
6.2. A formula for Kan extensions
In this section, we will see that right and left Kan extensions along any functor
K : C → D whose domain is small and whose codomain is locally small exist whenever
the target category E has certain limits or colimits, respectively. Moreover, these (co)limits
provide an explicit formula for the left and right Kan extension functors.
In the case of left Kan extensions, we are seeking to define a functor LanK F : D → E
that is the “closest approximation to an extension of F along K from the left,” i.e., up to anatural transformation η
F

C

/E
?

⇓η
K



⇓γ
LanK F

D

/E
?

F

C
K



G

D

F

C

=

⇓η
K



D

LanK F
u
∃!

/3 E
K
G

194

6. ALL CONCEPTS ARE KAN EXTENSIONS

that is universal among natural transformations γ : F ⇒ GK.
So, to define the value of LanK F(d) at an object d ∈ D, we should consider all possible
approximations to d “from the left” that come from the category C. This leads us toconsider the comma category K ↓ d whose objects are morphisms Kc → d from the imageof a specified c ∈ C and whose morphisms are morphisms c → c0 in C that give rise tocommutative triangles in D. The objects of the category K ↓ d parametrize approximations
to the object d coming from C from the left.
Recall that K ↓ d is the category of elements of the functor D(K−, d) : Cop → Set,
and as such comes with a canonical projection functor Πd : K ↓ d → C. The composite
ΠdFK ↓ d −−→ C −→ E of the projection functor with F sends each approximation Kc → d to thecorresponding object Fc in E that approximates the yet-to-be-defined LanK F(d). Basedon this intuition, it seems reasonable to define LanK F(d) to be the colimit of this diagram,and indeed if these colimits exist, they define the left Kan extension LanK F : D → E.
Theorem 6.2.1. Given functors F : C → E and K : C → D, if for every d ∈ D the colimit(6.2.2)Πd

F

LanK F(d) B colim(K ↓ d −−→ C −→ E)

exists, then they define the left Kan extension LanK F : D → E, in which case the unittransformation η : F ⇒ LanK F · K can be extracted from the colimit cone. Dually, if forevery d ∈ D the limit(6.2.3)

Πd

F

→ E)
RanK F(d) B lim(d ↓ K −−→ C −exists, then they define the right Kan extension RanK F : D → E, in which case the counit
transformation  : RanK F · K ⇒ F can be extracted from the limit cone.
Proof. The first task is to extend the data (6.2.2) defining the objects LanK F(d), eachwith a specified colimit cone λd : FΠd ⇒ LanK F(d), to a functor LanK F : D → E. A
morphism g : d → d0 in D induces a canonical functor, which commutes with the projections
to C,

Πd

/ K ↓ d0 ,

g∗

K ↓d
"

C

{

Πd

0

and is defined on objects by composing with g. Any cone under a diagram indexed by
the category K ↓ d0 also defines a cone under the diagram defined by restricting alongg∗ : K ↓ d → K ↓ d0 ; the legs of the new cone are defined by reindexing the legs of the00
old cone. In particular, the colimit cone λd : FΠd ⇒ LanK F(d0 ) that defines LanK F(d0 )0can be reindexed to define a cone under the diagram FΠd = FΠd g∗ . By the universal
property of the colimit cone λd : FΠd ⇒ LanK F(d), there is a unique morphism between
their nadirs in E commuting with the legs of the respective colimit cones:
λdf : Kc→dx
LanK F(d)Fc
LanK F(g)0λdg f : Kc→d0'/ LanK F(d0 ),which we take to be LanK F(g). Uniqueness implies that LanK F is functorial. The leg ofthe unit transformation ηc : Fc → LanK F(Kc), indexed by an object c ∈ C, is defined to be
the legηc B λ1KcKc : Kc→Kc : Fc → LanK F(Kc)6.2. A FORMULA FOR KAN EXTENSIONS

195

of the colimit cone defining LanK F(Kc). For any h : c → c0 in C, there are commuting
triangles
/ Fc0

Fh

Fc

0

λKc
Kh

λ1Kc

Kc


LanK F(Kc)

0

λ1Kc 0(/ LanK F(Kc0 )Kc

LanK F(Kh)proving that η : F ⇒ LanK F · K is natural.
It remains to show that the pair (LanK F, η) has the universal property that defines theleft Kan extension of F along K. That is, given any pair (G, γ) as below-left, we wish toconstruct the unique factorization α : LanK F ⇒ G displayed below-right:
/E
?

F

C

(6.2.4)

=

⇓γ



K

G

/3 E
K

F

C
⇓η



K

D

LanK FuαDGEach leg αd : LanK F(d) → Gd is defined to be the unique factorization of some cone underFΠd with nadir Gd through the colimit cone λd : FΠd ⇒ LanK F(d):Fcλdf : Kc→d/ LanK F(d)γc(6.2.5)

αd


GKc

Gf


/ Gd

The leg of this cone, indexed by the object f : Kc → d ∈ K ↓ d, is the left-bottom composite
displayed above. For each morphism h : c → c0 in K ↓ d, the commutative diagram displayed
below-right
c

Kc

h

Kh


c0


Kc0

Fc

f

&

9d

∈ K ↓dFΠd (h)=Fh


Fc0

f0

γc

/ GKc

Gf

'

7 Gd

GKh


/ GKc0γc0

Gf0

implies that these legs indeed define a cone FΠd ⇒ Gd.
To show that α : LanK F ⇒ G is natural in morphisms g : d → d0 in D, it suffices, by
the universal property of LanK F(d), to demonstrate that the outer pentagon
Fc}
LanK F(d)LanK F(g)!
/ LanK F(d0 )
αd 0

αd


Gd

0

λdg f : Kc→d0

λdf : Kc→d

Gg


/ Gd0

commutes for each f : Kc → d. Using the commutative squares (6.2.5) that define thecomponents of α, this follows easily from functoriality of G and naturality of γ.

196

6. ALL CONCEPTS ARE KAN EXTENSIONS

Finally, to verify the pasting equality (6.2.4), we must show that αKc · ηc = γc for eachc ∈ C. From our definition of η : F ⇒ LanK F, this is immediate, as a special caseFc

λ1Kc : Kc→Kc
Kc

Cηc/ LanK F(Kc)

γc


GKc

αKc

G1Kc


GKc

of the commutative squares that define the components of α. To show that α : LanK F ⇒ G
is the unique natural transformation defining a factorization αK · η = γ, observe thatnaturality of α in f : Kc → dFc
λdf : Kc→dηc Bλ1Kc!
|
LanK F( f )/ LanK F(d)LanK F(Kc)
αKc

 {
GKc

γc

Gf

αd


/ Gd

forces us to define αd as in (6.2.5).

Corollary 6.2.6. If K : C → D is a functor so that C is small and D is locally small then:(i) If E is cocomplete, then left Kan extensions ED

o LanK

/E

⊥
K∗

C

exist and are given by thecolimit formula (6.2.2).
(ii) If E is complete, then right Kan extensions ED o

K∗
⊥
RanK

/

EC exist and are given by thelimit formula (6.2.3).
Proof. If C is small and D is locally small, then for any d ∈ D the comma categoriesd ↓ K and K ↓ d are small, so the respective completeness and cocompleteness hypotheses
ensure that the limits and colimits of Theorem 6.2.1 exist in E.

Example 6.2.7. Consider the posets (Q, ≤) of rationals and (R>0 , ≤) of non-negative reals.
We have the functor 2− : Q → R>0 defined at the introduction to this chapter. The poset R>0
has all limits (infima) and colimits (suprema) of bounded subsets. In particular, we may
use Theorem 6.2.1 to define the left Kan extension along Q ,→ R. The value of Lan 2 x is
the supremum (the colimit) of all 2q with q ∈ Q and q ≤ x. This is the usual definition of
2 x , so we conclude that the exponential function 2− : R → R>0 is the left Kan extension of
2− : Q → R>0 along the inclusion Q ,→ R.
In this case, the exponential function 2− : R → R>0 is also the right Kan extension of−
2 : Q → R>0 along the inclusion Q ,→ R; this is because 2 x is also the infimum of 2q for
all q ∈ Q with x ≤ q. Exercise 6.2.ii invites the reader to explore why the left and right Kan
extensions coincide in this case.
Example 6.2.8. Returning to Example 6.1.7, we can use Theorem 6.2.1 to give explicit
formulae for the induction and coinduction functors associated to a subgroup inclusion

6.2. A FORMULA FOR KAN EXTENSIONS

197

H ,→ G. When C is complete and cocomplete, Corollary 6.2.6 supplies adjunctions:
indGH
⊥
resGH
⊥

CBG\

(6.2.9)/ CBH

coinGH

Given X ∈ CBH , Theorem 6.2.1 defines indGH X as a colimit of a diagramΠ

BH ↓ ∗G(6.2.10)

/ BH

X

/C

indexed by the comma category BH ↓ ∗G , where ∗G denotes the unique object in BG.
Objects in the indexing category are morphisms in BG, i.e., group elements g ∈ G. A
morphism h : g → g0 is an element h ∈ H so that g0 h = g. The diagram (6.2.10) sends eachobject to X ∈ C with morphisms given by the left action of H. Applying Theorem 3.4.12,
this colimit is defined by the coequalizer diagramXιgh

ι(g,h)`"// ` X

X

O

G×H

G

/ / indG X

q

H

O

ιg

ι(g,h)

X/X

h∗where the top map includes the component indexed by (g, h) to the component indexed by
gh, while the bottom map acts by h and then includes into the component indexed by g.
`
By inspection, the object indGH X is isomorphic in C to G/H X via an isomorphismthat allows us to explicitly describe the epimorphism q. Fix a complete set of left coset
representatives G/H = {gi H | gi ∈ G}. Then each g ∈ G factors uniquely as g j h0 with g jamong the chosen representatives and h0 ∈ H. The g-component of the quotient map q is
defined as(h0 )∗

X

/X
ιg j

ιg

`

X

G

q


/ / ` X  indG X
HG/H`
The action of an element g0 ∈ G on the object indGH X  G/H X ∈ C is the unique quotient,`with respect to this map q, of the action of g0 on G X by left multiplication on the indexingset:`// ` X q / / ` X  indG X
X
G×H

G

(g,h)7→(g0 g,h)`
G×H

g7→g0 gX// ` XGH

G/H(g0 )∗q/ / ` X  indG X
G/H

H

198

6. ALL CONCEPTS ARE KAN EXTENSIONS

Dually, Theorem 6.2.1 defines coinGH X as an equalizer

πhgcoinGH X 

Q

G
πg

π(h,g)// Q X

m / Q
X /
X

H\G

< XO

H×G


X

h∗


/X

π(h,g)Q
There is an isomorphism coinGH X  H\G X of underlying objects of C, under which theQQmonomorphism m : H\G X  G X admits a dual description, given by choosing rightQ
coset representatives. The action of G on coinGH X 
X is defined by restricting the right
H\GQ
action of G on the indexing set G of G X along this map m.
In the category Vectk , finite products and finite coproducts coincide: these are justdirect sums of vector spaces. If V ∈ VectBk H is an H-representation and H is a finite index
subgroup of G, then the colimit and limit formulae of (6.2.2) and (6.2.3) both yield a direct
sum of copies of V indexed by a set whose size is the index of H in G. Moreover, theG-actions are compatible with this isomorphism indGH V  coinGH V. Thus, for finite index
subgroups, the left and right adjoints of (6.2.9) are the same; i.e., induction from a finite
index subgroup is both left and right adjoint to restriction.
Example 6.2.11. Recall from Example 1.3.15 that any group G has an orbit category
OG whose objects are subgroups H ⊂ G, which we identify with the left G-sets G/H ofleft cosets of H. Morphisms G/H → G/K are G-equivariant maps, each of which isrepresented by a mapping gH 7→ gγK, where γ ∈ G is an element so that γ−1 Hγ ⊂ K.
From this description, we see that the monoid of endomorphisms of the object G/e ∈ OG
is the group Gop . So there is an embedding BGop ,→ OG sending the single object to G/e.
For any left G-set X ∈ SetBG , Theorem 6.2.1 can be used to compute the right Kan
extension
X

BG p

"

⇑
op

OG

/ Set
<
Ran X

The comma category that indexes the diagram whose limit defines the value of Ran X
at a subgroup H ⊂ G turns out to be isomorphic to the translation groupoid TG (G/H)
for the left G-set G/H of left cosets; see Example 1.5.18. As a category, the translationgroupoid is equivalent to the group of automorphisms of any of its objects; in this case,we choose the coset H whose automorphism group is H. The equivalence of categories
BH ' TG (G/H) allows us to reduce the limit that defines Ran X(G/H) to the limit of the
diagram resGH X : BH → Set restricting the G-set X to the subgroup H. By Example 3.2.12,
this limit is X H , the subset of H-fixed points.
op
In summary, the right Kan extension Ran X : OG → Set is the fixed point functor,H
mapping G/H to X and carrying a morphism γ : G/H → G/K to the direct-image
function γ∗ : X K → X H ; if x ∈ X K , then γx ∈ X H because γ−1 hγ ∈ K for all h ∈ H. In thecase where G is the Galois group for a finite Galois extension E/F, the right Kan extension

6.3. POINTWISE KAN EXTENSIONS

199
opof E : BG → FieldEF defines the isomorphism of categories OG  FieldEF that categorifies
the fundamental theorem of Galois theory.3
Example 6.2.12. Write ∆≤n for the full subcategory spanned by the first n + 1 ordinals [0], . . . , [n] in ∆. Restriction along the inclusion functor in : ∆≤n ,→ ∆ is called ntruncation. As Set is complete and cocomplete, Corollary 6.2.6 implies that n-truncation
admits both left and right adjoints:
Lanin

Set∆^

}

op

⊥
i∗n
⊥

/ Set∆≤n
op

Ranin
op

The composite comonad on Set∆ is skn , the functor that maps a simplicial set to its
opn-skeleton. The composite monad on Set∆ is coskn , the functor that maps a simplicial set
to its n-coskeleton. Furthermore, skn is left adjoint to coskn , as is the case for any comonadand monad arising in this way (see Exercise 4.1.iii).
Again, Theorem 6.2.1 provides an explicit description of the skeleton and coskeleton
functors. We leave the details to those readers familiar with simplicial sets.
Exercises.
Exercise 6.2.i. Directed graphs are functors from the category with two objects E, V anda parallel pair of maps s, t : E ⇒ V to Set. A natural transformation between two suchfunctors is a graph homomorphism. The forgetful functor DirGraph → Set that maps agraph to its set of vertices is given by restricting along the functor from the terminal category
1 that picks out the object V. Use Theorem 6.2.1 to compute left and right adjoints to thisforgetful functor.
Exercise 6.2.ii. Give conditions on a monotone function f : Q → R, meaning a functorf : (Q, ≤) → (R, ≤), so that the left and right Kan extensions along Q ,→ R exist andcoincide.
6.3. Pointwise Kan extensions
Unusually for a mathematical object defined by a universal property, generic Kan
extensions are rather poorly behaved, particularly in the case of derived functors, which
are introduced in the next section. On account of this, in his much cited book on enrichedcategory theory, Kelly reserves the name “Kan extension” for pairs satisfying the condition
to be introduced shortly, calling those of our Definition 6.1.1 “weak” and writing:
Our present choice of nomenclature is based on our failure to find asingle instance where a weak Kan extension plays any mathematicalrole whatsoever. [Kel82, §4]
By the categorical community’s consensus, the important Kan extensions are pointwise
Kan extensions. To explain this, a preliminary notion is needed:
3The category FieldEF has very few limits but it does have the limits of Theorem 6.2.1 in this instance; see
Theorem 6.3.7.

200

6. ALL CONCEPTS ARE KAN EXTENSIONSDefinition 6.3.1. A functor L : E → F preserves a left Kan extension (LanK F, η) if thewhiskered composite (L LanK F, Lη) is the left Kan extension of LF along K.
/E
?

F

C

L

/F

⇓η



K

⇓



LanK F

/F
?

LF

C



K

D

LanK LF

D

Theorem 6.2.1 constructs certain left Kan extensions using colimits. As left adjoints
preserve colimits (Theorem 4.5.3), it follows that left adjoints preserve such left Kan
extensions. Indeed, left adjoints preserve all left Kan extensions, regardless of how they
are constructed:
Lemma 6.3.2. Left adjoints preserve left Kan extensions.
Proof 1. Suppose given a left Kan extension (LanK F, η) with codomain E and suppose
further that L : E → F has a right adjoint R with unit ι : 1E ⇒ RL and counit ν : LR ⇒ 1F .
Then given H : D → F there are natural isomorphisms
FD (L LanK F, H)  ED (LanK F, RH)  EC (F, RHK)  FC (LF, HK) .

Taking H = L LanK F, these isomorphisms act on the identity natural transformation as
follows:
1L LanK F 7→ ιLanK F 7→ ιLanK F·K · η 7→ νL LanK F·K · LιLanK F·K · Lη = Lη .
Hence, by the Yoneda lemma, (L LanK F, Lη) is a left Kan extension of LF along K.

Unpacking the natural isomorphisms in Proof 1 yields an explicit construction of theunique factorization of a natural transformation α : LF ⇒ HK through Lη, which is moreeasily visualized in a second proof that displays the composite natural transformations as
pasting diagrams.
Proof 2. Given a natural transformation α : LF ⇒ HK, form the pasted composite
with the unit ι : 1E ⇒ RL displayed below-left.
/E

F

C



K

⇓α

?E

D

L

H

=

⇓ι



R

/E
?

F

C

⇓η



K

/F

∃!⇓ζ

LanK F

D

H

?E
R

/F

By the universal property of the left Kan extension (LanK F, η), there is a unique factorization
through η as displayed above-right. Composing each of these natural transformations withthe counit ν : LR ⇒ 1F and applying the triangle identity νL · Lι = 1L ,
/E

F

C
K



⇓α

D

=

L


H

/F

/E

F

C
K



⇓α

D

@E
L

H



⇓ι

/F

R

L
⇓ν



C

F

=

⇓η

F

K



D

/E
@
LanK F ⇓ζ
H

/F

@E
R

L
⇓ν



F

reveals that α factors through Lη via the whiskered composite of νH · Lζ : L LanK F ⇒ H
with K.

6.3. POINTWISE KAN EXTENSIONS

201

To demonstrate uniqueness, note that given any such factorization
/E

F

C

⇓α



K

L



D

H

=

/E
?

F

C

⇓η



K

/F

LanK F

L
⇓χ

D



/F

H

composing each of these natural transformations with the unit ι : 1E ⇒ RL
/E

F

C



K

?E

⇓α

=

⇓ι

D

L

R



⇓η



K

/F

H

/E
?

F

C

LanK F

?E
L

⇓χ

D

⇓ι



H

R

/F

yields a factorization of Rα·ιF : F ⇒ RHK through η, whence ζ = Rχ·ι LanK F. Composingwith ν and applying a triangle identity
BE
LanK F

D

⇓ζ

H

BE
R

BE
L

⇓ν



/F

=

LanK F

F

⇓χ

D

H

BE
L

⇓ι

L

R



BE

⇓ν

/F



=
F

LanK F

L
⇓χ

D

H


/F

we see that the factorization χ : L LanK F ⇒ H must agree with the factorization constructedabove.

Example 6.3.3. The forgetful functor U : Top → Set has both left and right adjoints. It
follows from Lemma 6.3.2 that U preserves the left and right Kan extensions of Example
6.1.7, as constructed in Example 6.2.8.
Example 6.3.4. The forgetful functor U : Vectk → Set preserves limits, because it is aright adjoint, but not colimits because the underlying set of a direct sum is not simply
the coproduct of the underlying sets of vectors. Indeed, it is clear from the construction
given in Example 6.2.8 that the underlying set of a G-representation induced from an Hrepresentation is not equal to the G-set induced from the underlying H-set. Lemma 6.3.2
implies, however, that the underlying set of the coinduced representation does agree withthe coinduced G-set from the underlying H-set.
The notion of preservation of Kan extensions is used to define what it means for a Kan
extension to be “pointwise.”
Definition 6.3.5. When E is locally small, a right Kan extension, as displayed below-right,
is a pointwise right Kan extension if it is preserved by all representable functors E(e, −).
F

C

/E
?

F

C

⇑
K



⇑
RanK FD/ E E(e,−) / Set?

K

RanK F

D

Left Kan extensions are dual to right Kan extensions, not under reversal of the direction of functors (the morphisms in CAT), but under reversal of the direction of naturaltransformations (the 2-morphisms in the 2-category CAT). A left Kan extension may be
converted to a right Kan extension by “replacing every category by its opposite”: a functor
K : C → D is equally a functor K : Cop → Dop but the process of replacing each category
by its opposite reverses the direction of natural transformations, because their components

202

6. ALL CONCEPTS ARE KAN EXTENSIONS

are morphisms in the opposites of the target categories.4 A left Kan extension is pointwise,
if the corresponding right Kan extension is pointwise:
Definition 6.3.6. When E is locally small, a left Kan extension, as displayed below-left,
is a pointwise left Kan extension if for all e ∈ E the data displayed below-right is a rightKan extension:
F

C

/E
?

F

Cop

⇓η
K



/ Eop E(−,e) / Set<⇑η
LanK FDK"

LanK F

Dop

Because covariant representables preserve all limits (Proposition 3.4.5), it is clear that
if a right Kan extension is given by the formula of Theorem 6.2.1, then that Kan extension
is pointwise; dually, the left Kan extensions defined by Theorem 6.2.1 are pointwise. The
surprise is that the converse also holds. This characterization justifies the terminology: apointwise Kan extension can be computed pointwise as a limit in E.
Theorem 6.3.7. If D and E are locally small, a right Kan extension of F : C → E along
K : C → D is pointwise if and only if it can be computed byΠdF
RanK F(d)  lim d ↓ K −−→ C −→E ,in which case, in particular, this limit exists. Dually, a left Kan extension of F : C → E
along K : C → D is pointwise if and only if it can be computed by!
ΠdF
LanK F(d)  colim K ↓ d −−→ C −→E ,in which case, in particular, this colimit exists.
Proof. If RanK F is pointwise, then for any e ∈ E, E(e, RanK F) is the right Kan
extension of E(e, F−) : C → Set along K. By the Yoneda lemma5 and the defining
universal property of this right Kan extension
E(e, RanK F(d))  SetD (D(d, −), E(e, RanK F))  SetC (D(d, K−), E(e, F−)) Cone(e, FΠd ) ,where the final natural isomorphism is the content of Lemma 6.3.8 below. This isomorphism
exhibits RanK F(d) as the limit of FΠd : d ↓ K → E.
Lemma 6.3.8. Given functors F : C → E and K : C → D with D and E locally small andan object d ∈ D, from which we define Πd : d ↓ K → C, there is a natural isomorphism
Cone(e, FΠd )  SetC (D(d, K−), E(e, F−)) .
Note that if C is not small, then d ↓ K is not small, so even if D and E are locally small
there may be more than a set’s worth of cones with summit e over FΠd and more than aset’s worth of natural transformations D(d, K−) ⇒ E(e, F−). However, the isomorphisms
exhibited in the proof of Theorem 6.3.7 imply that the collections being considered are infact sets.
4Succinctly, “op” is a 2-functor (−)op : CATco → CAT, where “co” is used to denote the dual of a 2-category
obtained by reversing the direction of the 2-morphisms but not the 1-morphisms.
5The hypothesis that D is locally small is included so that the Yoneda lemma can be used here, but in a sufficientrobust set-theoretical framework in which the proof of Theorem 2.2.4 can be extended to classes, this hypothesis
can be dropped.

6.3. POINTWISE KAN EXTENSIONS

203

Proof. To define the legs of a cone with summit e over FΠd is to choose, for everyf ∈ D(d, Kc), a morphism λcf ∈ E(e, Fc) so that for every h : c → c0 in C, commutativityof the left-hand triangle in D implies commutativity of the right-hand triangle in E:
8 Kc

f

d

e

Kh
f0

&

8 Fc

λcf


Kc0

Fh

&

0

λcf 0


Fc0

This defines precisely the data of a natural family of functions λc : D(d, Kc) → E(e, Fc) sothat for all h : c → c0 in C, the square
D(d, Kc)
(Kh)∗

λc



D(d, Kc0 )/ E(e, Fc)


λc

0

(Fh)∗

/ E(e, Fc0 )commutes.

Importantly, all pointwise Kan extensions along fully faithful functors, by which wemean Kan extensions defined by the (co)limit formulae of Theorem 6.2.1, define genuine
extensions, up to natural isomorphism:
Corollary 6.3.9. If K is fully faithful, then the counit of any pointwise right Kan extension
defines a natural isomorphism RanK F · K  F, and dually the unit of any pointwise left
Kan extension defines a natural isomorphism F  LanK F · K.
Proof. If K is fully faithful, then for any c ∈ C the canonical functor C ↓ c → K ↓ Kc
defines an isomorphism of categories, and this isomorphism commutes with the projection
functors to C. By Theorem 6.3.7, we then haveΠF

LanK F(Kc) B colim(C ↓ c −→C−→ E) ,
but in this case the indexing category C ↓ c has a terminal object 1c . Therefore, by Exercise
3.1.ix, the leg of the colimit cone indexed by the terminal object is an isomorphism; inparticular, the colimit can be computed by evaluating at the terminal object. From thedefinition of the unit given in Theorem 6.2.1, this tells us that the component of the unit
defines an isomorphism ηc : Fc  LanK F(Kc).
The cone under a small category J is the category J. defined as the pushout of the
inclusion i1 : J ,→ J × 2 of J as the “right end” of the cylinder J × 2 along the unique functor
J → 1.
!
/1
J
i1



J×2

p/  .
J

The effect of this pushout is to collapse the right end of the cylinder to a single object.
The category J. has one new object, a freely-adjoined terminal object t that serves as thenadir of a new cone under the inclusion J ,→ J. . There are no additional new objects ormorphisms. A dual pushout along the other inclusion i0 : J ,→ J × 2 defines a category J/
with a freely-adjoined initial object. See Exercise 3.5.iv.

204

6. ALL CONCEPTS ARE KAN EXTENSIONSAs mentioned in Remark 3.1.8, the category J. indexes cones under diagrams of shape
J. The informal intuition that a colimit of a diagram should be the “closest” extension of
the diagram to a cone under it is formalized by the following result.
Proposition 6.3.10. A category C admits all colimits of diagrams indexed by a small.
category J if and only if the restriction functor CJ → CJ admits a left adjoint, defined bypointwise left Kan extension:
colim
. o
⊥ / CJ
CJ
res/

Dually, C admits all J-indexed limits if and only if the restriction functor CJ → CJ admits
a right adjoint, defined by pointwise right Kan extension:
/

CJ o

res
⊥
lim

/

CJ

Proof. By construction, there is a fully faithful inclusion J ,→ J. . Consider a diagram
F : J → C. For j ∈ J ⊂ J. , Corollary 6.3.9 implies that the colimit (6.2.2) defining the left
Kan extension always exists: it is simply F j. For the cone point t ∈ J. , the comma categoryFJ ↓ t is isomorphic to J, by construction. So if colim(J −→ C) exists, then Theorem 6.2.1 tells.
us that these values define the left Kan extension functor, which we call colim : CJ → CJ
in this context because it maps a J-indexed diagram to its J. -indexed colimit diagram. 

Most commonly, pointwise Kan extensions are found whenever the codomain category
is cocomplete (for left Kan extensions) or complete (for right), but this is not the only caseas we shall now discover.
Exercises.
Exercise 6.3.i. Prove that if E is locally small and has all small copowers, then all rightKan extensions with codomain E are pointwise right Kan extensions. Dually, if E is locallysmall and has all small powers, then all left Kan extensions with codomain E are pointwise.
Exercise 6.3.ii. Proposition 6.3.10 expresses a stronger universal property of the colimit
cone than is usually stated. What is it?
6.4. Derived functors as Kan extensions
In this section, we explain how derived functors in homological algebra or algebraic
topology are defined using Kan extensions. A derived functor is derived from a functor
whose domain is a category equipped with a specified class of morphisms called “weakequivalences” and whose codomain is either another category with weak equivalences or
an ordinary category, in which case the weak equivalences are taken to be the class of
isomorphisms.
We introduce two varieties of derived functors. The first of these are the total derived
functors, which are defined to be Kan extensions along “localization” functors that formallyinvert the weak equivalences. In practice, total derived functors are frequently accompaniedby point-set derived functors, which are lifts of total derived functors to weak-equivalencepreserving functors between the original categories.
After stating these definitions, we give an abstract account of a commonly applied
construction that can be used to construct point-set derived functors. Unexpectedly, thetotal derived functors that are constructed in this way are pointwise Kan extensions, despite
the fact that homotopy categories have notoriously few limits and colimits. In fact, the6.4. DERIVED FUNCTORS AS KAN EXTENSIONS

205

universal property enjoyed by the derived functors that are defined via this construction iseven stronger and has important implications to the theory of derived adjunctions, which
we discuss.
Throughout, we reserve the term “weak equivalences” for a specified class of morphisms in a category that contains all the identities and satisfies one further condition.
The idea, that the weak equivalences are like isomorphisms but weaker, is encoded by acondition that the class of weak equivalences must satisfy:
Definition 6.4.1. A class of weak equivalences W is a class of morphisms in a categorythat contains the identities and satisfies the 2-of-6 property: if for any composable tripleof arrows
·

f

/·

W3g f

g

hg∈W

$ 
·

⇒

f, g, h, hg f ∈ W

hg f
h

$)/

·

if hg and g f are in W so are f , g, h, and hg f .
Lemma 6.4.2. Any class of morphisms in a category that contains the identities and satisfies
the 2-of-6 property necessarily also contains the isomorphisms.
Proof. Exercise 6.4.i.

The modern viewpoint, that one can “do homotopy theory” in any category with aclass of weak equivalences, explains the following terminology.
Definition 6.4.3. A homotopical category is a category equipped with a class of weakequivalences. A functor between such categories is homotopical if it preserves the weakequivalences.
Remark 6.4.4 (the universal property of the homotopy category). Any homotopical category C has an associated homotopy category HoC characterized by the following universal property: the homotopy category HoC is initial among categories E equipped witha homotopical functor C → E, that is, a functor that sends weak equivalences in C toisomorphisms.6 In analogy with ring theory, the universal functor C → HoC is referred toas a localization functor.
The universal property of the localization functor—pre-composition with C → HoC
induces a bĳective correspondence between functors HoC → E and homotopical functors
C → E—also has a 2-dimensional aspect. Pre-composition with the localization functor
also induces a bĳection between natural transformations between functors HoC → E andnatural transformations between the corresponding homotopical functors C → E; see
Exercise 6.4.ii.
The following explicit construction of the homotopy category is due to Gabriel andZisman [GZ67, 1.1]. The category HoC has the same objects as C. Its morphisms areequivalence classes of finite zig-zags of morphisms in C, with only weak equivalences
permitted to go backward, modulo the following relations:• Adjacent arrows pointing in the same direction may be composed.
6Set-theoretic concerns make it somewhat tricky to specify what functor HoC represents. In the examplesof greatest interest, C is not small, in which case HoC is a priori not even locally small, though in practice it
frequently is.

206

6. ALL CONCEPTS ARE KAN EXTENSIONS
w w

w w

• Adjacent pairs ←−−→ or −→←− labelled by a weak equivalence w may be removed.
• Identities pointing either forward or backward may be removed.
There is a canonical identity-on-objects localization functor
/ HoC

C

in which the morphisms in C are sent to unary zig-zags pointing forward.
Example 6.4.5. When C has no specified subcategory of weak equivalences, as a default weassign it the minimal homotopical structure, where the weak equivalences are precisely
the isomorphisms and the localization functor is the identity.
Example 6.4.6. There are two homotopical structures of interest on the category ChR ofchain complexes of R-modules. In the first, the class of weak equivalences is taken to
be the chain homotopy equivalences. In the second, the class of weak equivalences isenlarged to contain all quasi-isomorphisms, that is, chain maps f : A• → B• that induce
an isomorphism on graded abelian groups after applying the homology functor introducedin Example 1.3.2(viii).
Example 6.4.7. The category of topological spaces also has two homotopical structuresof interest. In the first, the class of weak equivalences is taken to be the class of homotopy
equivalences. In the second, this class is enlarged to contain all weak homotopy equivalences, which are maps inducing isomorphisms on all homotopy groups. The homotopy
category of spaces is the homotopy category associated to the second homotopical structure. It is equivalent to the category HtpyCW of CW complexes and homotopy classes ofmaps that appears in Examples 2.1.6(vi) and (vii).
It is in this context that we may define total derived functors.
Definition 6.4.8 (total derived functors). Consider a functor F : C → D between homotopical categories.
• When the right Kan extension
C


HoC

F
⇑

LF

/D

/ HoD

exists, it defines the total left derived functor LF of F.
• When the left Kan extension
C


HoC

F
⇓

RF

/D

/ HoD

exists, it defines the total right derived functor RF of F.
Note the conflict in handedness in Definition 6.4.8. A justification for calling theright Kan extension a total left derived functor is that left derived functors approximate theoriginal functor from the left.
By the universal property of the localization functor C → HoC, LF and RF areequivalently expressible as homotopical functors LF, RF : C ⇒ HoD; these functors areoften called the left or right derived functors of F. Sometimes, though by no means
always, there exists a lift of a left or right derived functor along D → HoD. For emphasis,

6.4. DERIVED FUNCTORS AS KAN EXTENSIONS

207

we will call these lifts point-set derived functors, though other authors choose to call themsimply “derived functors” [Rie14].
Definition 6.4.9 (point-set derived functors). Consider a pair of homotopical categories
and localization functors γ : C → HoC and δ : D → HoD.
• A point-set left derived functor of F : C → D is a homotopical functor LF : C → D
equipped with a natural transformation λ : LF ⇒ F so that δLF (which by Remark
6.4.4 is equivalently encoded by a functor δLF : HoC → HoD) and δλ : δLF ⇒ δF
define a total left derived functor of F.
F

C

⇑λ

&

8D

δ

/ HoD

!

LF

C

F

γ

⇑δλ


HoC

δLF

/D
δ


/ HoD

• A point-set right derived functor of F : C → D is a homotopical functor RF : C → D
equipped with a natural transformation ρ : F ⇒ RF so that δRF (which by Remark
6.4.4 is equivalently encoded by a functor δRF : HoC → HoD) and δρ : δF ⇒ δRF
define a total right derived functor of F.
F

C

⇓ρ

'
7D

δ

/ HoD

!

RF

C

F

γ

⇓δρ


HoC

δRF

/D
δ


/ HoD

There is a common setting in which derived functors exist and admit a simple construction. Such categories have a collection of “fat” objects on which the functor of interest
becomes homotopical and a functorial reflection into this full subcategory. The details are
encoded in the following axiomatization due to [DHKS04].
Definition 6.4.10.
• A left deformation on a homotopical category C consists of an endofunctor Q together∼
with a natural weak equivalence q : Q =⇒ 1C .
• A left deformation for a functor F : C → D between homotopical categories consistsof a left deformation (Q, q) for C such that F preserves all weak equivalences between
objects in a full subcategory containing the image of Q.
∼

There is an obvious dual notion of right deformation r : 1C =⇒ R.
Interestingly, left deformations can be used to construct point-set left derived functors,
a homotopical universal property that is independent of the deformation used.
∼

Proposition 6.4.11. If F : C → D has a left deformation q : Q =⇒ 1C , then
(LF = FQ, Fq : LF ⇒ F)
is a point-set left derived functor of F.
Proof. Write δ : D → HoD for the localization functor. To show that (FQ, Fq) is apoint-set left derived functor, we must show that the functor δFQ and natural transformation
δFq : δFQ ⇒ δF define a right Kan extension. The verification makes use of Remark 6.4.4,
which identifies the functor category HoDHoC with the full subcategory of HoDC spanned by
the homotopical functors. Suppose G : C → HoD is homotopical and consider α : G ⇒ δF.

208

6. ALL CONCEPTS ARE KAN EXTENSIONS

Because G is homotopical and q : Q ⇒ 1C is a natural weak equivalence, Gq : GQ ⇒ G isa natural isomorphism. Using naturality of α, it follows that α factors through δFq as
α: G(Gq)−1αQ+3 GQ+3 δFQ

δFq

δFq

+3 δF

+3 δF

To prove uniqueness, suppose α factors asβα: G+3 δFQ

Naturality of β provides a commutative square of natural transformations:
GQ

βQ

δFQq

Gq


G

+3 δFQ2β+3 δFQ

Because q is a natural weak equivalence and the functors G and δFQ are homotopical,
the vertical arrows are natural isomorphisms, so β is determined by βQ . This restrictednatural transformation is uniquely determined: qQ is a natural weak equivalence between
objects in the image of Q. Since F is homotopical on this subcategory, this means that
FqQ is a natural weak equivalence and thus δFqQ is an isomorphism, so βQ must equal thecomposite of the inverse of this natural isomorphism with αQ .
Unusually for constructions characterized by a universal property, generic total derived
functors are poorly behaved: for instance, the composite of the total left derived functors ofa pair of composable functors is not necessarily a total left derived functor for the composite.
The defining universal property is also insufficient to prove that the total left derived functor
of a left adjoint is left adjoint to the total right derived functor of its right adjoint. The
problem with the standard definition is that total derived functors are not typically required
to be pointwise Kan extensions. In light of Theorem 6.3.7, this seems reasonable because
homotopy categories have few limits and colimits.7
Somewhat surprisingly, the particular construction of total derived functors via deformations given in Proposition 6.4.11 implies that the total derived functors produced inthis manner are pointwise and indeed absolute Kan extensions: not only are these Kan
extensions preserved by representables but they are preserved by all functors.
Proposition 6.4.12. The total left derived functor of a left deformable functor is an absoluteright Kan extension, and in particular pointwise.
Proof. If F : C → D is left deformable, it has a total left derived functor (δFQ, δFq)
constructed using a left deformation (Q, q) for F and the localization functor δ : D → HoD.
Because Kan extensions are characterized by a universal property, any total derived functorof F is isomorphic to this one. To show that this right Kan extension is absolute, our task
is to show that for any functor H : HoD → E, the pair (HδFQ, HδFq) again defines a rightKan extension.
Note that (Q, q) also defines a left deformation for HδF, simply because the functor
H : HoD → E preserves isomorphisms. The proof of Proposition 6.4.11 now demonstrates
that (HδFQ, HδFq) is a right Kan extension, as claimed.

7With the exception of products and coproducts, the so-called “homotopy limits” and “homotopy colimits” do
not define limits and colimits in the homotopy category.

6.5. ALL CONCEPTS

209

In homological algebra or homotopy theory, deformable functors often come in adjoint
pairs, with the left adjoint admitting a left deformation and the right adjoint admitting a rightdeformation. In such cases, the total derived functors are again adjoints, but interestingly
the proof requires Proposition 6.4.12: the mere universal property of being a left or rightKan extension is not enough. This is the main theorem from a paper of Maltsiniotis[Mal07]. As with many non-trivial discoveries in category theory, once the statement is
known, the proof is elementary enough to leave to the reader:
Proposition 6.4.13. Suppose F a G is an adjunction between homotopical categories andsuppose also that F has a total left derived functor LF, G has a total right derived functor
RG, and both derived functors are absolute Kan extensions. Then the total derived functors
form an adjunction LF a RG between the homotopy categories.


Proof. Exercise 6.4.iv.

Remark 6.4.14. If F a G is an adjunction satisfying the hypotheses of Proposition 6.4.13,
then the adjunction
F
/
⊥
D
Co
γ


HoC o

G

δ

LF
⊥
RG

/


HoD

between the total derived functors is the unique adjunction compatible with the localization
functors γ : C → HoC and δ : D → HoD in the sense that the diagram
D(Fc, d)  C(c, Gd)γ
HoD(Fc, d) HoC(c, Gd)
Fq∗ 
 Gr∗
HoD(LFc, d)  HoC(c, RGd)
δ 

commutes for each pair c ∈ C, d ∈ D.
Exercises.
Exercise 6.4.i. Prove Lemma 6.4.2.
Exercise 6.4.ii. Prove the 2-dimensional aspect of the universal property of the localization functor described in Remark 6.4.4. That is, consider a homotopical category C
and a homotopical functor γ : C → HoC satisfying the universal property that restriction
along γ induces a bĳection between functors HoC → E and homotopical functors C → E.
Prove that restriction along γ also induces a bĳection between natural transformations between functors HoC → E and natural transformations between the associated homotopicalfunctors C → E.
Exercise 6.4.iii. Prove that any endofunctor Q : C → C on a homotopical category that is∼
equipped with a natural weak equivalence q : Q =⇒ 1C is a homotopical functor.
Exercise 6.4.iv. Prove Proposition 6.4.13.
6.5. All concepts
To wrap up this chapter, and with it this book, we see that simple special casesof Kan extensions can be used to define the other basic categorical concepts introduced

210

6. ALL CONCEPTS ARE KAN EXTENSIONS

in Chapters 2–5. This justifies Mac Lane’s famous assertion that “The notion of Kan
extensions subsumes all the other fundamental concepts of category theory” [ML98a,§X.7]. Because much of basic category theory can be encoded in this way, generalizationsof ordinary category theory to more exotic contexts often commence with the developmentof a suitable calculus of Kan extensions.
Proposition 6.5.1 ((co)limits as Kan extensions). The left Kan extension of F : C → D
along the unique functor ! : C → 1 defines the colimit of F in D, each existing if and only
if the other does.
/D
?

F

C

⇓η
!



/D
?

F

C

=

⇓γ
!

Lan! F

1



F

C

d

Lan! F
u
∃!

⇓η
!

1



1

/3 D
K
d

Dually Ran! F defines the limit of F.
Proof. The composite of a functor d : 1 → D with the functor ! : C → 1 yields theconstant functor at the object d ∈ D. Hence, the universal property of the left Kan extensionof F along ! : C → 1 specifies that cones γ : F ⇒ d under F with nadir d correspond tomorphisms Lan! F → d in D, and that this correspondence is implemented by composingwith the displayed cone η : F ⇒ Lan! F. Thus, we recognize Lan! F as the colimit of F
with colimit cone η.
Proposition 6.5.2 (adjunctions as Kan extensions).
(i) If F a G is an adjunction with unit η : 1 ⇒ GF and counit  : FG ⇒ 1, then (G, η) isa left Kan extension of the identity functor along F and (F, ) is a right Kan extensionof the identity functor along G.
C

?C

D

?D

⇓η
F



⇑
G

GLanF 1C

DFRanG 1D

C

Moreover, both Kan extensions are absolute, and in particular pointwise.
(ii) Conversely, if (G, η : 1C ⇒ GF) is a left Kan extension of the identity along F and if
F preserves this Kan extension, then F a G with unit η.
Proof. For (i), Proposition 4.4.6 tells us that an adjunction F a G gives rise to an
adjunction
CD

o

G∗
⊥
F∗

C

/C

By uniqueness of adjoints (Proposition 4.4.1), left Kan extension along F is given by precomposition with G, with the components of the unit of G∗ a F ∗ defining the unit for theleft Kan extension. This tells us that (G, η) defines a left Kan extension of 1C along F.
This Kan extension is absolute because pre-composition commutes with post-composition
with any functor L : C → E. Put differently, there is a similar adjunction G∗ a F ∗ between
functor categories ED and EC for any E.
For (ii), recall from Definition 4.2.5 that the data of an adjunction is comprised of apair of functors and a pair of natural transformations so that the “triangle identity” pasting6.5. ALL CONCEPTS

211

identities hold:
C



F

?C

⇓η

G

C
F

⇓

"

D

=
D

F



1F
=

D



F

<C

?C

G

F

⇓

D



⇓η
G

=

CC[
1G
=

G

D

G

D

Suppose that (G, η) is a left Kan extension of 1C preserved by F, meaning that (FG, Fη) is aleft Kan extension of F along F. Using the universal property of this latter Kan extension,
there is a unique factorization
/D

F

C



F

C

=

⇓1F

F

D



⇓η

?C

F

/D

G ∃!⇓

D

of 1F through Fη defining a natural transformation  : FG ⇒ 1D . Immediately from thisdefinition, we see that F · Fη = 1F , proving one of the two triangle identities. For theother, note that by the first triangle identity, the left-hand pasted composite
?C

C
F



⇓η

D

G

⇓

F



⇓η
G

?C
=

C
1F F
w
F

D

?C

⇓η


+D

G

C

=

⇓η
F



D

G 1
G
u

3 CL
G

reduces to the right-hand ones. From this, we see that 1G : G ⇒ G and G · ηG : G ⇒ Gboth define factorizations of η through itself. By the uniqueness in the universal propertyof the left Kan extension (G, η), we conclude that G · ηG = 1G , proving that F a G withunit η.
The next aim is to use a simple pointwise right Kan extension to reprove and thengeneralize the Yoneda lemma. From the defining universal property, the right Kan extensionof a functor F : C → D along the identity 1C : C → C is (isomorphic to) F. It follows
immediately from Definition 6.3.5 that this Kan extension is also pointwise, so we canapply the limit formula of Theorem 6.3.7 to conclude that
(6.5.3)

Π

F

Fc  lim(c/C −→C−→ D) .

In the case of a functor F : C → Set, this limit formula proves the Yoneda lemma. By thedefining universal property of the limit (6.5.3),
Fc  Set(∗, Fc)  Cone(∗, FΠ)  SetC (C(c, −), Set(∗, F−))  SetC (C(c, −), F) ,where the third isomorphism is a special case of Lemma 6.3.8. This is the Yoneda lemma.
In the case of a small diagram F : C → D valued in a category that has products andequalizers, we can apply Theorem 3.4.12 to re-express this limit formula as an equalizer
diagram. On account of Exercise 3.2.v, which expresses the set of natural transformations between small diagrams as a similar equalizer, this limit formula is regarded as a
generalization of the Yoneda lemma.
Proposition 6.5.4 (Yoneda lemma). For any small diagram F : C → D valued in a
category with products and equalizers, there is an equalizer diagram
Q/ Q Fx
//
Fy
Fc /
c→x

c→x→y

212

6. ALL CONCEPTS ARE KAN EXTENSIONS

in which one of the parallel maps projects to the component indexed by the composite
c → x → y in C while the other projects to the component indexed by the map c → x andthen acts by F on the second map x → y.
Proof. Apply Theorem 3.4.12 to (6.5.3).

Like any abstract nonsense, this argument can be dualized. Dually, any F : C → D isa pointwise left Kan extension of itself along 1C . The colimit formula of Theorem 6.3.7
applies, so(6.5.5)

Π

F

Fc  colim(C/c −
→C−
→ D),

which is appropriately called the coYoneda lemma:
Proposition 6.5.6 (coYoneda lemma). For any small diagram F : C → D valued in a
category with coproducts and coequalizers, there is a coequalizer diagram`// ` F x/ / Fc.
Fy
y→x→c

x→c

in which one of the parallel maps includes the component indexed by y → x → c as thecomponent indexed by the composite morphism y → c, while the other acts by F on themap y → x and then includes as the component indexed by the map x → c.
Proof. Apply Theorem 3.4.12 to (6.5.5).

The coYoneda lemma has an important consequence in the special case of a set-valued
functor called the density theorem.
Theorem 6.5.7 (density I). For any locally small category C, any functor F : C → Set isthe colimit of the diagramRΠ / op   y /F op
C
SetC ,
R
which is indexed by the opposite of the category F of elements of F and takes values inthe subcategory of representable functors. Dually, any F : Cop → Set is the colimit ofRopΠ / y /F
C
SetC .
``
Proof. For sets S and T , there are isomorphisms S T  S × T  T S , so thecoequalizer given by the coYoneda lemma is isomorphic to the coequalizer8``/ / Fc,// C(x, c)C(x, c)Fy×C(y,x)

Fx

where the indexing sets have been swapped. Letting c vary, we conclude that F is canonicallya colimit of a diagram of representable functors. Reversing Theorem 3.4.12, the indexing
category for this diagram of representables is one whose objects are elements of the set F x
for some x ∈ C and whose morphisms are maps y → x in C that send the chosen elementof Fy to the corresponding element of F x. Thus, F is the colimit of the canonical diagramRΠ / op   y /F op
C
SetC .

8If C is not small, then the displayed coproducts, which are tacitly indexed by the objects of C, are not small.
Nonetheless, Theorem 6.3.7, which expresses
R Fc as a colimit of a large diagram (6.5.5), tells us that F is a colimitof a diagram indexed by the large category F.

6.5. ALL CONCEPTS

213

The name “density theorem” refers to a second formulation of this result, which asserts
opthat the representable functors form a dense subcategory of the presheaf category SetC .
Informally speaking, a subcategory is dense if any object in the larger category is a colimitof a diagram valued in the subcategory. The precise meaning is given by the statement of
the following theorem:
Theorem 6.5.8 (density II). For any small category C, the identity functor defines the left
op
Kan extension of the Yoneda embedding C ,→ SetC along itself.
/ SetCop
:

y

Cp

⇓1y

"
op
SetC

y

1Lany yop

Proof. By Theorem 6.2.1, for any F ∈ SetC ,


y
Π
op
Lany y(F)  colim y ↓ F −→ C→− SetC .
R
By Lemma 2.4.7, the comma category y ↓ F is isomorphic to the category of elements F,
so Theorem 6.5.7 proves that Lany y(F)  F.
Remark 6.5.9. By Corollary 6.2.6, any functor F : C → E whose domain is small andwhose codomain is locally small and cocomplete admits a left Kan extension along theop
Yoneda embedding y : C → SetC
F

Cq

/
<E

⇓1F
y

"
op
SetC

Lany F

Essentially by Corollary 6.3.9, because the Yoneda embedding is full and faithful, theopfunctor Lany F : SetC → E can be defined to be a genuine extension of F along y, so thatthe unit of the pointwise left Kan extension is an identity natural transformation. Moreover,op
op
this left Kan extension admits a right adjoint R : E → SetC . To define Re ∈ SetC , notethat the desired adjunction and the Yoneda lemma demand natural isomorphisms
opRe(c)  SetC (C(−, c), Re)  E(Lany F(C(−, c)), e)  E(Fc, e) ,which necessitates the definition
Re B E(F−, e) : Cop → Set .
For each e → e0 ∈ E, post-composition defines a map Re → Re0 , so our construction ofop
R : E → SetC is functorial.

214

6. ALL CONCEPTS ARE KAN EXTENSIONSTo prove that the functors Lany F a R are indeed adjoints, we use the density theorem.
op
For any X ∈ SetC and e ∈ E,
op

op

SetC (X, Re)  SetC ( colim
R C(−, c), Re)(c,x)∈by Theorem 6.5.7

X

op



SetC (C(−, c), Re)limRop(c,x)∈ X

by Theorem 3.4.7

lim
E(Fc, e)R
X op

by the definition of R(c,x)∈

 E( colim
R Fc, e)(c,x)∈ X

by Theorem 3.4.7

 E(Lany F(X), e)R
by Theorem 6.2.1 and y ↓ F  F .

Examples of adjunctions constructed in this manner are described in Exercises 6.5.iii-6.5.v.
Finally, we turn to monads. We have seen that a right adjoint functor G : D → C
induces a monad on C whose endofunctor is the composite of G with its left adjoint. Moregenerally, assuming C has sufficient limits, any functor G : D → C induces a monad on C.
If G has a left adjoint F, this monad is the monad GF but this construction applies moregenerally.
Definition 6.5.10 (monads as Kan extensions). The codensity9 monad of G : D → C isgiven by the right Kan extension of G along itself, whenever this exists.10
/C
?

G

D

⇑



G

RanG GCTC

The unit and multiplication natural transformations are defined using the universal propertyof  : TG ⇒ G as follows:
/C
?

G

D

=

⇑1G



G

1C

/C
?

G
G



⇑

⇑

C

T

"
/C

T

T

⇑
G

C

G

D

∃!vη



C

1C

3/ ? C

G

D

=

/7 C
P

G

D

T
⇑
G



∃!vµ

C

T

T

/C

Example 6.5.11. Consider the inclusion Field ,→ CRing. It has no left adjoint becausesuch a functor would carry Z to an initial field, which does not exist. But it does have a
codensity monad T defined byY
TA B
Frac(A/p)p∈Spec(A)9A functor G : D → C is codense if its right Kan extension along itself is the identity 1C ; compare with thestatement of Theorem 6.5.8.
10In particular, “sufficient limits” in C means those necessary to define RanG G as a pointwise right Kan
extension.

6.5. ALL CONCEPTS

215

where Spec(A) is the set of prime ideals p ⊂ A and Frac(A/p) is the field of fractions of thequotient ring A/p, which is an integral domain because the ideal p is prime.
Example 6.5.12. Familiar monads also arise as codensity monads. For instance, thecodensity monad of the inclusion Fin ,→ Set is the ultrafilter monad β of Example 5.1.4(v).
Theorem 6.2.1 characterizes the set of ultrafilters on a set X as a limitΠ
β(X)  lim X ↓ Fin −→ Fin ,→ Set .
By Theorem 3.2.6, this limit is isomorphic to the set of cones over the diagram X ↓ Fin → Set
with summit the singleton set. By Lemma 6.3.8, this is isomorphic to the set of naturaltransformations from Fin(X, −) to the inclusion Fin ,→ Set. The components of such atransformation take a function f : X → {1, . . . , n} and return an element of the set {1, . . . , n}.
An ultrafilter on X gives rise to such a natural transformation: the element is the uniquei ∈ {1, . . . , n} whose fiber f −1 (i) is in the ultrafilter. See [KG71].
Proofs of the claims made in Examples 6.5.11 and 6.5.12 and considerably more canbe found in [Lei13].
Example 6.5.13. The Giry monad of Example 5.1.5(iv) is also a codensity monad for the
inclusion of a subcategory of C ,→ Meas. The objects of C are the finite powers of theunit interval I = [0, 1] ⊂ R, equipped with the Borel σ-algebra, plus the measurable spaceof sequences in I that converge to 0. The objects can be regarded as convex subspaces of
Euclidean space, from which perspective the morphisms are defined to be all affine maps
between these spaces. Omitting the space of sequences, the subcategory of finite powersof the interval and affine maps generates a variant of the Giry monad for finitely additive
probability measures as its codensity monad. See [Ave16].
Exercises.
Exercise 6.5.i. Give a second proof of Proposition 6.5.2(i) using pasting diagrams: show
directly that the counit of an adjunction defines an absolute right Kan extension.
Exercise 6.5.ii. Use the Yoneda lemma to define the colimit cone for the canonical colimit
described in Theorem 6.5.7.
Exercise 6.5.iii. For a fixed topological space X, there is a natural inclusion O(X) → Top/X
that sends an open subset U ⊂ X to the continuous function U ,→ X. Apply the constructionof Remark 6.5.9 to this functor to define an adjunction
o

Top/X

⊥

O(X)op/ Set

between the category of presheaves on O(X) and the category of spaces over X. By
Exercise 4.2.i, this adjunction, like all adjunctions, restricts to define an adjoint equivalenceof categories, in this case between the category of sheaves on X and the category of étale
spaces over X.
Exercise 6.5.iv. Apply the construction of Remark 6.5.9 to the embedding ∆ ,→ Cat of
the category of finite non-empty ordinals and order-preserving maps to define an adjunction
Cat

o

h
⊥
N

∆op

/ Set

.

The right adjoint defines the nerve of a category, while the left adjoint constructs thehomotopy category of a simplicial set. The counit of this adjunction is an isomorphism,

216

6. ALL CONCEPTS ARE KAN EXTENSIONS

so Lemma 4.5.13 implies that the nerve is fully faithful. Hence, Cat defines a reflective subcategory of the category of simplicial sets, proving the claim made in Example
4.5.14(vi).
Exercise 6.5.v. Apply the construction of Remark 6.5.9 to the functor ∆ : ∆ → Top that
sends the ordinal [n] = 0 → 1 → · · · → n to the topological n-simplex




X




n+1
n
x
=
1,
x
≥
0
(x
,
.
.
.
,
x
)
∈
R
∆ B

i
i
0
n
.



i

The left adjoint, defined by left Kan extension, forms the geometric realization of asimplicial set. The right adjoint is the total singular complex functor, which is used todefine singular homology:
Top

o

|−|
⊥
Sing

∆op

/ Set

.

Exercise 6.5.vi. Show that the triple (T, η : 1C ⇒ T, µ : T 2 ⇒ T ) constructed in Definition
6.5.10 defines a monad on C.
Exercise 6.5.vii. If F a G, show that the codensity monad RanG G of G exists and is equal
to the monad induced by the adjunction F a G.

Epilogue: Theorems in Category Theory
We did not then regard [category theory] as a field
for further research efforts, but just as a language
and an orientation—a limitation which wefollowed for a dozen years or so, till the advent ofadjoint functors.
Saunders Mac Lane, “Concepts and categories inperspective” [ML88]

The author is told with distressing regularity that “there are no theorems in category
theory,” which typically means that the speaker does not know any theorems in category
theory. This attitude is certainly forgivable, as it was held for the first dozen years of
the subject’s existence by its two founders and might reasonably persist among those with
only a casual acquaintance with this area of mathematics. But since Kan’s discovery ofadjoint functors [Kan58] and Grothendieck’s contemporaneous work on abelian categories[Gro57]—innovations that led to a burst of research activity in the 1960s—without question
category theory has not lacked for significant theorems.
What follows is a modest collection of counterexamples to the claim of “no theorems”:§E.1 extracts some of the main theorems from the preceding text, §E.2 discusses coherence for symmetric monoidal categories, §E.3 introduces a new universal property of theunit interval, §E.4 characterizes Grothendieck toposes, and §E.5 explores the embedding
theorem for abelian categories. Of course, this list is by no means exhaustive: indeed only
one of the results appearing here was proven after the early 1970s. As with any maturemathematical discipline, this introductory textbook is rather far from the current frontiers of
category theory (see, e.g., [AR94], [Joh02a, Joh02b], [Lac10], [Lei04], [Lur09, Lur16],[Sim11b]).
E.1. Theorems in basic category theory
Significant theorems in basic category theory include, first and foremost, the Yoneda
lemma, which characterizes natural transformations whose domain is a represented functor.
Theorem (2.2.4, 2.2.8). For any functor F : Cop → Set, whose domain C is locally small,there is a bĳection
Hom(C(−, c), F)  Fc
that identifies a natural transformation α : C(−, c) ⇒ F with the element αc (1c ) ∈ Fc thatis natural in both c and F. In particular, any natural transformation C(−, c) ⇒ C(−, c0 )between represented functors is given by composing with a unique morphism c → c0 in C.
A set-valued functor is representable just when it is naturally isomorphic to a represented functor, covariant or contravariant as the case dictates. The idea that representable
217

218

EPILOGUE: THEOREMS IN CATEGORY THEORY

functors encode universal properties of their representing objects is made precise by thefollowing result:
Theorem (2.4.8). A representation for a functor F : C → Set is an initial object in its
category of elements. Dually, a representation for a functor F : Cop → Set is a terminal
object in its category of elements.
To calculate particular limits and colimits, it is very useful to know that they can bebuilt out of simpler limit and colimit constructions, when these exist.
Theorem (3.4.12). A category with small coproducts and coequalizers has small colimitsof any shape, and dually a category with small products and equalizers has all small limits.
In most cases, the cardinality of a category (meaning the cardinality of its set ofmorphisms) places a sharp bound on the sizes of limits and colimits it may possess.
Theorem (3.7.3). Any κ-small category that admits limits or colimits of all diagrams
indexed by κ-small categories is equivalent to a poset. In particular, the only small
categories that are complete or cocomplete are equivalent to posets.
The following theorem, which is quite easy to prove, has consequences throughout
mathematics.
Theorem (4.5.2, 4.5.3). Right adjoints preserve limits, and left adjoints preserve colimits.
This theorem implies that continuity or cocontinuity are necessary conditions for afunctor to admit a left or right adjoint. The adjoint functor theorems supply additional
conditions that are sufficient for an adjoint to exist.
Theorem (4.6.1, 4.6.3, 4.6.10). Let U : A → S be a continuous functor whose domain iscomplete and whose domain and codomain are locally small. If either(i) for each s ∈ S the comma category s ↓ U has an initial object,(ii) for every s ∈ S there exists a set of morphisms Φ s = { fi : s → Uai } so that any
f : s → Ua factors through some fi ∈ Φ s along a morphism ai → a in A, or(iii) A has a small coseparating set and every collection of subobjects of a fixed object inA admits an intersection,then U admits a left adjoint.
As a corollary:
Theorem (4.6.17). A functor F : C → D between locally presentable categories(i) admits a right adjoint if and only if it is cocontinuous.
(ii) admits a left adjoint if and only if it is continuous and accessible.
The proofs of the adjoint functor theorems also lead to proofs of related representability
criteria.
Theorem (4.6.14, 4.6.15). If F : C → Set preserves limits and C is locally small andcomplete, then if either(i) C has a small coseparating set and has the property that every collection of subobjectsof a fixed object has an intersection, or if(ii) there exists a set Φ of objects of C so that for any c ∈ C and any element x ∈ Fc there
exists an s ∈ Φ, an element y ∈ F s, and a morphism f : s → c so that F f (y) = x,then F is representable.
The following theorem collects together several properties of monadic functors.
Theorem (5.5.1, 5.6.1, 5.6.5, 5.6.12). If U : A → S admits a left adjoint and creates
coequalizers of U-split pairs, then:E.2. COHERENCE FOR SYMMETRIC MONOIDAL CATEGORIES

219(i) A morphism f in A is an isomorphism if and only if U f is an isomorphism in S.
(ii) U creates all limits that exist in S.
(iii) U creates all colimits that exist in S and are preserved by the induced monad on S
and its square.
(iv) If S is locally small, complete, and cocomplete, and if U preserves filtered colimits,then A admits all colimits.
The following theorem establishes the existence of and provides a formula for adjoints
to restriction functors, given by pointwise left and right Kan extensions.
Theorem (6.2.1, 6.2.6). Consider a functor K : C → D where C is small and D is locallysmall. Whenever E is cocomplete, the restriction functor K ∗ : ED → EC admits a left adjoint,
defined by a particular colimit formula, and whenever E is complete, the restriction functor
along K admits a right adjoint, defined by a dual limit formula.
LanK

ED\

⊥
K∗
⊥

/ EC

RanK

Other candidates for inclusion on this list include theorems, such as 4.2.6 or 6.3.7,
that prove that two a priori distinct categorical definitions are equivalent, or the results
describing the universal properties of the derived functors constructed via deformations
established by 6.4.11 and 6.4.12.
E.2. Coherence for symmetric monoidal categories
There are a number of coherence theorems in the categorical literature, the first andmost famous being the following theorem of Mac Lane [ML63].
The data of a symmetric monoidal category (V, ⊗, ∗) consists of a category V, abifunctor − ⊗ − : V × V → V called the monoidal product, and a unit object ∗ ∈ V together
with specified natural isomorphisms(E.2.1)v⊗ww⊗vγu ⊗ (v ⊗ w)  (u ⊗ v) ⊗ w
α∗⊗vvv⊗∗λρwitnessing symmetry, associativity, and unit conditions on the monoidal product. The
natural transformations (E.2.1) must satisfy certain “coherence conditions” that will be
discussed momentarily. A monoidal category is defined similarly, except that the firstsymmetry natural isomorphism is omitted.
Examples of symmetric monoidal categories include all categories with finite products,
such as (Set, ×, ∗), (Top, ×, ∗), or (Cat, ×, 1), where the monoidal unit in each case is theterminal object. Dually, any category with finite coproducts defines a symmetric monoidal`
category, with the initial object as the monoid unit, e.g., (Man, , ∅). The category ofmodules over a ring admits multiple monoidal structures: (ModR , ⊕, 0) and (ModR , ⊗R , R),
the last only in the case where R is commutative. The category of unbounded chain
complexes of R-modules, with R commutative, is also a monoidal category (ChR , ⊗R , R),
where “⊗R ” denotes the usual graded tensor product of chain complexes and R is thechain complex with the R-module R in degree zero and zeros elsewhere. In this case, thesymmetry natural isomorphism can be defined in two distinct ways depending on whether
or not a sign is introduced in odd degrees.
A commutative monoid, regarded as the objects of a discrete category, defines a trivial
special case of a symmetric monoidal category in which the natural isomorphisms (E.2.1)

220

EPILOGUE: THEOREMS IN CATEGORY THEORY

are identities. The natural isomorphisms (E.2.1) defining a symmetric monoidal category
are categorifications of the commutativity, associativity, and unit laws. As Mac Lane writes
in the introduction to [ML63]:
The usual associative law a(bc) = (ab)c is known to imply the “general associative law,” which states that any two iterated products ofthe same factors in the same order are equal, irrespective of the arrangement of parentheses.
Similarly, in a symmetric monoidal category, composites of the coherence natural isomorphisms define higher associativity and higher symmetry isomorphisms. A final axiom
requires these natural isomorphisms to satisfy the following coherence diagrams, expressedin terms of components at objects u, v, w, x ∈ V, which demand for instance that there is aunique natural isomorphism built from this data that witnesses the symmetry of a three-foldproduct:
αu,v,w⊗x(u
3 ⊗ v) ⊗ (w ⊗ x)αu⊗v,w,xu ⊗ (v ⊗ (w ⊗ x))+((u ⊗ v)> ⊗ w) ⊗ xαu,v,w ⊗1 x

1u ⊗αv,w,xu ⊗ ((v ⊗ w) ⊗ x) αu,v⊗w,x / (u ⊗ (v ⊗ w)) ⊗ xu ⊗ (∗ ⊗ v)
1u ⊗λvαu,∗,v/ (u ⊗ ∗) ⊗ v" | ρu ⊗1v
u⊗v

u ⊗ (v ⊗ w)
αu,v,w

γv,∗

v⊗∗
ρv

1u ⊗γv,w

"

v

/ ∗⊗v
|

/ u ⊗ (w ⊗ v)(u ⊗ v) ⊗ w γu⊗v,w / w ⊗ (u ⊗ v)

λv

αu,w,v

1v⊗w

v⊗w
γv,w

/ v⊗w
<

"
γw,v
w⊗v

/ (u ⊗ w) ⊗ v
γu,w ⊗1v

/ (w ⊗ u) ⊗ v
αw,u,v

The coherence theorem for monoidal categories, first proven by Mac Lane and thenimproved by Kelly, tells us that once these conditions are satisfied, then all diagrams whoseedges are comprised of composites of these natural isomorphisms, parenthesized in any
order, commute; see [ML98a, §XI.1] for a precise statement and a proof and [ML76] for ageneral discussion.
Theorem E.2.2 (Mac Lane, Kelly). Any diagram in a symmetric monoidal category thatis comprised of associators α, unitors γ or ρ, or symmetors γ that is “formal” commutes.
The upshot of Theorem E.2.2 is that any two “n-ary tensor product” functors V×n → V
that are built iteratively from the bifunctor ⊗ and the unit object ∗, each permuting andparenthesizing the inputs in some manner, are connected by a unique natural isomorphismthat is built out of the given natural isomorphisms α, λ, ρ, and γ. In practice, this naturality
means that we need not concern ourselves with particular parenthesizations or orderings
when defining the n-ary tensor product. Hence, the structural isomorphisms (E.2.1) are
seldom emphasized.

E.3. THE UNIVERSAL PROPERTY OF THE UNIT INTERVAL

221

E.3. The universal property of the unit interval
The unit interval is the topological space I = [0, 1] ⊂ R regarded as a subspace of thereal line, with the standard Euclidean metric topology. It is used to define the fundamentalgroupoid Π1 (X) of paths in a topological space X. A path in X is simply a continuousfunction p : I → X. The path has two endpoints p(0), p(1) ∈ X defined by evaluating at theendpoints 0, 1 ∈ I. If q : I → X is a second path with the property that p(1) = q(0), thenthere exists a composite path p ∗ q : I → X defined by the composite continuous function
δ


I

/ I∨I

p∨q

/ X.

Here I ∨ I is the space formed by gluing two copies of I together by identifying the point 1
in the left-hand copy with the point 0 in the right-hand copy:
0

∗
1

/I

/ pI ∨ I

I

The space I ∨ I is homeomorphic to the space [0, 2] ⊂ R. The map δ : I → I ∨ I is thehomeomorphism t 7→ 2t. Note that this map sends the endpoints of the domain I to theendpoints of the fattened interval I ∨ I. Thus, (p ∗ q)(0) = p(0) and (p ∗ q)(1) = q(1); thatis, p ∗ q is a path in X from the starting point of the path p to the ending point of the path q.
The fundamental groupoid Π1 (X) is the category whose objects are points of X
and whose morphisms are endpoint-preserving homotopy classes of paths in X, withcomposition defined via the gluing of the unit interval just described. The reason thatmorphisms are endpoint-preserving homotopy classes, rather than simply paths, is thatpath composition is not strictly associative or unital. Endpoint-preserving homotopies
allow in particular for re-parameterizations of the unit interval.
By a theorem of Freyd [Fre99], refined by Leinster [Lei11], the structure on the unit
interval that is used to define composition of paths also describes the universal propertyof this topological space. A bipointed space is a topological space I with two distinct
closed points 0 , 1 ∈ I, one designated as the “left” and the other as the “right.” A map of
bipointed spaces is a continuous function that sends the left point to the left point and theright point to the right point. For any bipointed space (X, x0 , x1 ), we can define X ∨ X to be
the pushout
x0

∗

/X

x1


p 
/X
X
∨X
Theorem E.3.1 (Freyd–Leinster). The unit interval is the terminal bipointed space equippedwith a map of bipointed spaces X → X ∨X. That is, for any other bipointed space (X, x0 , x1 ),there is a unique bipointed continuous map r : X → I so that
/ X∨X

X
r

commutes.


I

r∨r

δ



/ I∨I

222

EPILOGUE: THEOREMS IN CATEGORY THEORY

Put more concisely, Theorem E.3.1 says that (I, 0, 1) is the terminal coalgebra for theendofunctor X 7→ X ∨ X on the category of bipointed spaces. See [Lei11] for a discussion
and proof. The fact that the map I → I ∨ I is a homeomorphism is no coincidence: atheorem of Lambek [Lam68], whose elementary proof is left as Exercise 1.6.vi, provesthat the structure map of any terminal coalgebra is always an isomorphism.
E.4. A characterization of Grothendieck toposes
Given a small category C, a presheaf is another name for a contravariant set-valued
functor on C. A Grothendieck topos is a reflective full subcategory E of a presheaf
categoryo LopE   ⊥ / SetC
with the property that the left adjoint preserves finite limits.11 Objects in E can be characterized as sheaves on a small site, which specifies a “covering family” of morphisms( fi : Ui → U)i for each object U ∈ C satisfying a weak pullback condition. A typicalexample might take C = O(X) to be the poset of open sets for a topological space X. A
presheaf P : O(X)op → Set assigns a set P(U) to each open set U ⊂ X so that this assignment is functorial with respect to restrictions along inclusion V ⊂ U ⊂ X of open subsets.
A presheaf P is a sheaf if and only if the diagram of restriction maps// Q P(U ∩ U )/ Q P(U )
P(U)
α

α

α,β

α

β

is an equalizer for every open cover U = ∪α Uα of a U ∈ O(X); see Definition 3.3.4.
Giraud’s theorem, which first appeared in print in [GV72, IV.1.2], states that Grothendieck toposes can be completely characterized by a combination of “exactness” and size
conditions.
Theorem E.4.1 (Giraud). A category E is a Grothendieck topos if and only if it satisfies
the following conditions:(i) E is locally small.
(ii) E has finite limits.
(iii) E has all small coproducts and they are disjoint and universal.
(iv) Equivalence relations in E have universal coequalizers.
(v) Every equivalence relation in E is effective, and every epimorphism in E is a coequalizer.
(vi) E has a separating set.
Some explanation is in order:``• A coproduct α Aα is disjoint if each inclusion Aα → α Aα is a monomorphism andif the pullback of any two distinct inclusions is an initial object.
• A colimit cone (Aα → A)α is universal if the pullbacks of these maps along any B → A
define a colimit cone (B ×A Aα → B)α .
• An effective equivalence relation is one that arises, as in Example 3.5.10, as a kernel
pair of some morphism.
• Finally, recall from Definition 4.6.7 that a separating set of for E is a set of objects
G ⊂ ob E that is jointly separating: for any f, g : B ⇒ A with f , g there is someh : G → B, with G ∈ G, so that f h , gh.
11Propositions 4.5.15 and 3.3.9 imply that E is cocomplete, so this adjunction can be defined as in Remark
6.5.9 from the functor Ly : C → E that is the restriction of the left adjoint along the Yoneda embedding.

E.5. EMBEDDINGS OF ABELIAN CATEGORIES

223

A proof of Theorem E.4.1 can be found in [Joh14, §0.4], which also presents a veryclear exposition of the ideas involved.
E.5. Embeddings of abelian categories
Properties of the categories Ab of abelian groups, or ModR of modules over a unitalring, or ChR of chain complexes of R-modules are abstracted in the notion of an abeliancategory. Abelian categories were introduced by Buchsbaum [Buc55] and developed by
Grothendieck [Gro57] with the aim of unifying the “coefficients” of the various cohomology
theories then under development.
Definition E.5.1. A category is abelian if• it has a zero object 0, that is both initial and terminal,• it has all binary products and binary coproducts,• it has all kernels and cokernels, defined respectively to be the equalizer and coequalizer
of a map f : A → B with the zero map A → 0 → B, and• all monomorphisms and epimorphisms arise as kernels or cokernels, respectively.
In an abelian category A, finite products and finite coproducts coincide (in the sensethat the canonical map from the latter to the former described in Remark 3.1.27 is an
isomorphism) and are called biproducts or direct sums. These axioms imply that thehom-sets in A canonically inherit the structure of an abelian group, with the zero map ineach hom-set serving as the additive identity, in such a way that composition is bilinear.
This gives A the structure of a category enriched over12 the monoidal category (Ab, ⊗Z , Z).
The image of a morphism is defined to be the kernel of its cokernel, or equivalently,
the cokernel of its kernel; these objects are always isomorphic in an abelian category.
This permits the definition of an exact sequence in an abelian category, a sequence ofcomposable morphisms
···

fn+2

/ An+1

fn+1

/ An

fn

/ An−1

fn−1

/ ···

so that ker fn = im fn+1 . Classical results such as the five lemma, which is used to detectisomorphisms between a pair of exact sequences, can be proven by general abstract nonsense
in any abelian category using the universal properties of kernels and cokernels.
A functor is exact if it preserves exact sequences or, equivalently, if it is both left andright exact in the sense introduced in Definition 4.5.9.
Theorem E.5.2 (Freyd–Mitchell). If A is a small abelian category, then there is a ring13
R and an exact, fully faithful functor A ,→ ModR , which embeds A as a full subcategory.
A proof appears as the very last result of the book [Fre03]. Unfortunately, most of thematerial presented in the preceding 150 pages appears to be necessary.
To conclude, we invite the reader to reflect upon a lovely quote from the introduction
to Johnstone’s Topos Theory [Joh14, pp. xii–xiii] which describes the significance of theFreyd–Mitchell embedding theorem and, by analogy, the broader project of category theory:
Incidentally, the Freyd–Mitchell embedding theorem is frequently
regarded as a culmination rather than a starting point; this is becauseof what seems to me a misinterpretation (or at least an inversion) of
its true significance. It is commonly thought of as saying “If you
12Ordinary categorical notions can be extended to categories in which each Hom(X, Y) is an object of a generalmonoidal category (V, ⊗, ∗), in place of (Set, ×, ∗). See [Rie14, Chapter 3] for an introduction to enriched category
theory.
13As per our conventions elsewhere, R is unital and associative but not necessarily commutative.

224

EPILOGUE: THEOREMS IN CATEGORY THEORY

want to prove something about an abelian category, you might as wellassume it is a category of modules”; whereas I believe its true importis “If you want to prove something about categories of modules, you
might as well work in a general abelian category”—for the embedding
theorem ensures that your result will be true in this generality, andby forgetting the explicit structure of module categories you will beforced to concentrate on the essential aspects of this problem. As an
example, compare the module-theoretic proof of the Snake Lemmain [HS97] with the abelian-category proof in [ML98a].

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Saunders Mac Lane. Concepts and categories in perspective. In A century of mathematics in America,
Part I, volume 1 of Hist. Math., pages 323–365. Amer. Math. Soc., Providence, RI, 1988.
Saunders Mac Lane. Categories for the working mathematician, volume 5 of Graduate Texts in
Mathematics. Springer-Verlag, New York, second edition, 1998.
Saunders Mac Lane. The Yoneda lemma. Mathematica Japonica, 47:156, 1998.
Saunders Mac Lane. Saunders Mac Lane—a mathematical autobiography. A K Peters, Ltd., Wellesley,MA, 2005.
Robert Paré. On absolute colimits. Journal of Algebra, 19:80–95, 1971.
Robert Paré. Colimits in topoi. Bulletin of the American Mathematical Society, 80(3):556–561, 1974.
Henri Poincaré. L’avenir des mathématiques. Rendiconti del Circolo Matematico di Palermo (18841940), 26(1):152–168, December 1908.
Bjorn Poonen. Why all rings should have a 1. arXiv:1404.0135 [math.RA], 2014.
Miles Reid. Undergraduate algebraic geometry, volume 12 of London Mathematical Society Student
Texts. Cambridge University Press, Cambridge, 1988.
Emily Riehl. Categorical homotopy theory, volume 24 of New Mathematical Monographs. Cambridge
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Graeme Segal. Categories and cohomology theories. Topology, 13:293–312, 1974.
Michael Shulman. Set theory for category theory. arXiv:0810.1279, 2008.
Harold Simmons. An introduction to category theory. Cambridge University Press, Cambridge, 2011.
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Catalog of Categories
RAb, of abelian groups and homomorphisms, 4Abfg , of finitely generated abelian groups andF, the category of elements for a set-valued
functor F, 66
End, of sets equipped with an endomorphism andcommuting functions, 69
Euclid∗ , of pointed Euclidean spaces and pointed
differentiable functions, 14

homomorphisms, 26
Abtf , of torsion-free abelian groups andhomomorphisms, 141
AdjT , of adjunctions inducing a monad T andmaps of adjunctions, 164
F ↓G, the comma category for a pair of functors
F : D → C and G : E → C, 22
Field, of fields and homomorphisms, 4
FieldEF , of intermediate fields F ⊂ K ⊂ E andhomomorphisms fixing F, 21
Fin, of finite sets and functions, 8Fin∗ , of pointed finite sets andbasepoint-preserving functions, equivalently
the opposite of Segal’s Γ, 15Finiso , of finite sets and bĳections, 8
FinMetric, of finite metric spaces and distance
non-increasing functions, 16
Finmono , of finite sets and injections, 105

Affine, of affine planes and bĳections that preserveand reflect incidence, 36
Ban, of real Banach spaces and continuous linear

maps, 26
BG, a group (or monoid) G regarded as a category,with one object and elements of the group G
as endomorphisms, 5
Card, the discrete category of cardinals, 138
CAT, of locally small categories and functors, 20
Cat, of small categories and functors, 20
C/c, the slice of a category C over an object c, 8
c/C, the slice of a category C under an object c, 8
cgHaus, of compactly generated Hausdorff spaces

Γ, Segal’s category, equivalent to the opposite ofFin∗ , 35
Graph, of graphs and homomorphisms, 4
GrModR , of Z-graded R-modules and gradedhomomorphisms, 14Group, of groups and homomorphisms, 4Groupepi , of groups and epimorphisms, 23Groupiso , of groups and isomorphisms, 22Groupoid, of groupoids and functors, 14

and continuous maps, 130
cHaus, of compact Hausdorff spaces andcontinuous maps, 140
ChR , of chain complexes of R-modules and chain

maps, 4
CJ , of functors J → C and natural transformations,44
Cluster, of clusters and refinements, 16
CMonoid, of commutative monoids andhomomorphisms, 119

Haus, of Hausdorff spaces and continuous maps,

Cop , the opposite of a category C, 9
CRing, of commutative unital rings and129
Htpy, of spaces and homotopy class of continuous

homomorphisms, 8
CT , the category of algebras for a monad T on acategory C, 160
CT , the Kleisli category for a monad T on acategory C, 162
C × D, the product of categories C and D, 19

maps, 5Htpy∗ , of based spaces and basepoint-preserving

homotopy class of based continuous maps, 5
HtpyCW , of CW complexes and homotopy classesof continuous maps, 54
J/ , the category obtained from J by freely∆, of finite non-empty ordinals andorder-preserving maps, 18∆+ , of finite ordinals and order-preserving maps,
193
DirGraph, of directed graphs andhomomorphisms, 4adjoining an initial object, 76

J. , the category obtained from J by freely

adjoining a terminal object, 76
Lattice, of lattices and lattice morphisms, 174
229

230

CATALOG OF CATEGORIES

Man, of smooth manifolds and smooth functions, 4Man∗ , of pointed smooth manifolds and pointed

smooth functions, 15
MatR , of positive integers and matrices withcoefficients in the unital ring R, 4
Meas, of measurable spaces and measurable
functions, 4Measure, of measure spaces and equivalenceclasses of measurable functions, 5ModelT , of models for a set T of sentences in a
first-order language and homomorphisms, 4
ModR , of left R-modules and homomorphisms, 4
R Mod, of right R-modules and homomorphisms,
20
Monoid, of monoids and homomorphisms, 20n, the ordinal n regarded as a category, 5
OG , of subgroups of a group G represented as left
G-sets of left cosets and G-equivariant maps,
21
ω, the ordinal ω regarded as a category, 5O(X), the poset of open subsets of a space X underinclusion, 18(P, ≤), a poset (or preorder) regarded as a category,with elements of P as objects and with amorphism x → y if and only if x ≤ y, 5
PCluster, of persistent clusters and refinements, 16
Poset, of partially-ordered sets andorder-preserving functions, 4
Proj| , of projective planes with a distinguished line
and bĳections that preserve and reflect
incidence, 36
rDirGraph, of reflexive directed graphs andhomomorphisms, 177
Ring, of unital rings and homomorphisms, 4, 41
Rng, of non-unital rings and homomorphisms, 8,

41
SET, of “large” sets and functions, 59
Set, of sets and functions, 4Set∗ , of pointed sets and basepoint-preserving

functions, 4Set∂ , of sets and partially-defined functions, 21
ShvX , of sheaves (of sets) on a space X and naturaltransformations, 141skC, the skeleton of the category C, 34
TG X, the translation groupoid for a G-set X, 34
Top, of spaces and continuous maps, 4Top∗ , of pointed spaces and basepoint-preserving

continuous maps, 4

pcTop∗ , of path-connected based spaces and based

continuous maps, 33
GVectB
k , of G-representations and equivariant

linear maps, 192

Vectbasisk , of finite-dimensional k-vector spaces

with a chosen basis and linear maps, 30
Vectfdk , of finite-dimensional k-vector spaces andlinear maps, 30
Vectk , of k-vector spaces and linear maps, 4

Glossary of Notation=, xvi⇒, xvi·, 3
B, xvi, xvi, 7d, xvia, 117
, xvi7→, xvi
, 11⇒, xvi, xvi, 162', 30→, xvi
, 11

R

F, 66

f ∗ , 10f∗ , 10F ↓G, 22(−)[ , 116
f : X → Y, 3
Hom(X, Y), 7
Hom(A, B), 130
Hom(V, W), 63
id, 6
1X , 3I, 29
J/ , 76
J. , 76

0, 5
1, 5
2, 5

Kn , 49
LanK F, 190
LF, 207
LF, 206
lim F, 75
limJ F, 76

An , 63
α : F ⇒ G, 23
α : F  G, 24
BG, 5

Map(X, Y), 130

C(X, Y), 7
C(c, −), 19
C/c, 8c/C, 8C(−, c), 19
CJ , 44[n], 18n, 5
OG , 21
ω, 5
L
, 83
⊗, 63
O(X), 18

cod, 6colim F, 75
colimJ F, 76
Cop , 9`j∈J , 81coskn , 199
CT , 160
CT , 162

Qj∈J , 77

y, 79
p, 81
RanK F, 190
RF, 207RF, 206∆, 74∆+ , 193∆, 18
Dn , 73
dom, 6(−)] , 116
skn , 199
skC, 34
231

232

S n , 73t, 81
Sym(G), 61
TG X, 34

×, 77
V ∗ , 17

GLOSSARY OF NOTATION

Indexalgebraic theory
category of models, 114
model of -, 175
apex
of a cone, 74
arrow, xi, see also morphism, 6
atomic, 120
automorphism, 7
group, 22
axiom
in a first-order language, 127
axiom of choice, 122-category, 46
2-functor, 47, 135
2-of-6 property, 205
abelian category, 139, 223
abelian group
free -, 119abelianization, 141
absolute
colimit, 170
Kan extension, 208
abstract nonsense, x, 36
accessible functor, 150
additive functor, 140
adjoint
left vs right -, 116
lower -, 118
upper -, 118adjoint functor, 116
multivariable -, 131
mutually left or right -, 126
two-variable -, 129
adjoint functor theorem, 144
general -, 146
special -, 148
adjunct, 116
adjunction, 116, 123
as a Kan extension, 210
counit, 123from a monad, 162, 163
fully-specified -, 124
monadic -, 120, 167morphism, 126
multivariable -, 131
triangle identity, 123
two-variable -, 129
unit, 122
affinelinear combination, 159
space, 63, 159
algebra, 37
canonical presentation, 169
category of -, 160
for an endofunctor, see also coalgebra
free -, 162
homomorphism, 160

based object, 67
basepoint, 4
Beck–Chevalley condition, 177bifunctor, 19
bilinear, 63
box topology
vs product topology, 77
Brouwer fixed point theorem, 15
cardinalarithmetic, 138
inaccessible, 6
regular, 6, 150
strong limit, 6
cartesian closed category, 129
categorification, xiiof a commutative monoid, 220of clustering algorithms, 16of the fundamental theorem of Galois theory, 21of the Jones polynomial, xiiof the natural numbers, 28of the orbit-stabilizer theorem, 34
category, 32-, 46abelian -, 139, 223
arrows only definition, 3
cardinality of -, 110
cartesian closed -, 129
center of -, 48
chaotic -, 36
cocomplete -, 85, 99
comma -, 22complete -, 85, 99
233

234

concrete -, 4, 42
connected -, 33, 91
discrete -, 5
Eilenberg–Moore -, 160
enriched -, 223
essentially small -, 35filtered -, 113
free -, 120
functor -, 44–45
homotopical -, 205
homotopy -, 205
indexing -, 38internal -, 89
isomorphism of -, 20
Kleisli -, 162
locally presentable -, 150
locally small -, 7monoidal -, 154, 219
naming convention, 3
nerve of -, 142, 215
of algebras, 160of categories, 20of elements, 66, 68, 102of models for an algebraic theory, 114
opposite -, 9product of -, 19reflective sub-, 140, 142, 158, 167
skeletal -, 34
slice -, 8, 67
small -, 6symmetric monoidal -, 219
terminal -, 78
category theory
birth of -, 1–2
Cayley’s theorem, 61
cell complex, 73
centerof a category, 48
of a group, 22
chain complex, 4boundary, 14
cycle, 14
homology, 14
chain rule, 14
chaotic category, 36classifying spaceof a group, 54
closure, 158left or right -, 129
closure operator, 158, 175
clustering algorithm, 16co-, see also -, 202
coalgebra, 162
for an endofunctor, 44
terminal, 44
coarsest, 77
cocomplete
category, 85, 99

INDEX

cocone, 74
cocontinuous functor, 99
codomain, 3
coequalizer, 81
diagram, 81
split -, 169
U-split -, 170
cograph, xii
coherence, 219
cohomology, 1
coinduced representation, 192, 196, 201
cokernel, 82, 139
colimit, 73, 75, 76
absolute -, 170
as a coequalizer between coproducts, 96
as a Kan extension, 210
commutes with colimits, 111cone, 75, 76
connected -, 91
diagram, 76, 204
filtered -, 114
functor, 106, 136
in a functor category, 93
non-strictness of -, 106, 109
representable universal property, 96
sequential -, 81
uniqueness of -, 76
universal -, 222
universal property of -, 82
comma category, 22commutative
cube, 40
diagram, 38
rectangle, 39
square, 24, 38
commutator subgroup, 22, 141
commute, 36comonad, see also monad, 158
complete
category, 85, 99
lattice, 103, 110
composite morphism, 3
concrete category, 42cone, 74
colimit -, 75, 76
legs of -, 74
limit -, 75, 76
nadir of -, 74
over a category, 76, 105, 203
over a diagram, 74
summit of -, 74
under a category, 76, 105, 203
under a diagram, 74
configuration space, 101, 105connected category, 91
conservative functor, 180
continuousfunction, 18, 175

INDEX

functor, 99
contractible groupoid, 62
contravariant functor, 17
copower, 96
coproduct, 81
disjoint -, 222
universal property of -, 82
coskeletonn-, 199
cotensor, 94
counitof an adjunction, 123
covariant functor, 17
coYoneda lemma, 212
create, 90
strictly -, 91
currying, 54, 129
decategorification, 28
deformation, 207
dense subcategory, 213density theorem, 212–213
dependent
product, 67
sum, 67
derivative
functoriality of -, 14
total -, 14derived functor, 206, 207
point-set -, 207
total -, 206
is an absolute Kan extension, 208
via a deformation, 207
diagram, 36, 38, 74, 120
category, 44
colimit -, 76, 204
commutative -, 36, 38
connected -, 91
equalizer -, 78
finite -, 99
limit -, 76, 204
shape of -, 38, 74
small -, 85, 110
direct image, 118, 138
direct limit, 81
direct sum, 83
directed graph, 199
discrete category, 5
discrete dynamical system, 50
discrete fibration, 72
domain, 3
dual
basis, 23of a categorical statement, 10
theorem, 10
vector space, 17, 23
dualityin projective plane geometry, 2235

Eilenberg–MacLane space, 54
Eilenberg–Moore category, 160
colimits in -, 181, 184
isomorphisms in -, 180
limits in -, 181
universal property, 164
element
category of -, 66, 68
generalized -, 98
universal -, 66, 68
elementary topos, 177
embedding, 31
full -, 31
endofunctor, 13morphism, 7enriched category, 223
epi, see also epimorphism
epic, 11
epimorphism, 11
split -, 12
use in commutative diagrams, 43
equalizer, 78
diagram, 78
equational witness, 19
equivalence of categories, 30, 31
composite -, 30, 35
equivalence relation, 103–104
effective -, 222
equivariant map, 25
essential image, 35, 143
essentially, 35
discrete, 35
small, 35
surjective, 31
evaluation map, 65, 154, see also counit
evil, 35
exact
functor, 223
left or right -, 139
sequence, 1, 223
existential quantification, 118
extension of scalars, 120
extranatural transformation, 28
faithful, 30, 140
fiber, 79
fiber product, 79
fiber space, 105
fibration
discrete left or right -, 72
filteredcategory, 113
colimit, 114
finest, 77
finitaryfunctor, 175
monad, 175, 184236

finite colimit
from pushouts and an initial object, 98–99
finite limit
from pullbacks and a terminal object, 98–99
fixed point, 87
functor, 198
forgetful functor, 13
fork, 81
formal sum, 115
four color theorem, 49
free, 53, 119abelian group, 52, 119
action, 56, 63
algebra, 162
algebra morphism, 162
category, 120
group, 14, 52, 119, 123
module, 119monoid, 119
product, 183
vector space, 115, 119
Frobenius reciprocity, ix, 120
full, 30, 140embedding, 31
functioncograph of -, xii
continuous -, 18graph of -, xii
partial -, 21
space, 130
functor, 132-, 135
accessible -, 150
additive -, 140
adjoint -, 116bi-, 19
category, 44–45
hom-set as limit, 89
closed -, 129
cocontinuous -, 99
conservative -, 180constant -, 74
continuous -, 99
contravariant -, 17
covariant -, 17
endo-, 13
essentially surjective -, 31
exact -, 223
faithful -, 30, 140
finitary -, 175
forgetful -, 13full -, 30, 140
homotopical -, 205
left or right exact -, 139
localization -, 205
monadic -, 167morphism, 22
preserves isomorphisms, 18

INDEX

representable -, 51
represented -, 19
two-sided represented -, 20, 60
functoriality, 13, 17
in group theory, 22–23
of limits and colimits, 106
fundamental group, 14, 33–34
fundamental groupoid, 7, 14, 33, 221
G-action, 19
G-equivariant, 25
G-object, 18, 192
G-representation, 18
coinduced -, 192, 196, 201
induced -, 192, 196, 201
G-set, 18
G-space, 18
Galois connection
antitone -, 127
monotone -, 118
Galois extension, 21
Galois group, 21
Galois theory
fundamental theorem -, 21
generalized element, 98
generator
for a category, see also separator
for an algebra, 169
geometric realization, 216
global, 31
gluing, 73
graphcomplete -, 49of a function, xii
vertex coloring, 49, 67
Grothendieck topos, 222
image factorization in a -, 105
Grothendieck universe, 6
group, 7
as a one-object category, 5classifying space -, 54
completion, 119
extension, 1
free -, 14, 52, 119, 123
free abelian -, 52
free product of -, 183
opposite -, 10presentation, 169
ring, 65, 119
torsion sub-, 26
group action, 18
free -, 56, 63
transitive -, 63
groupoid, 7connected -, 33
contractible -, 62
maximal -, 8, 121
guess and check, 83

INDEX

Haskell, 154, 163
Hilbert’s Nullstellensatz, 127
hom-set, 7
homology, 1, 14
Khovanov -, xii
homomorphismof algebras, 160
homotopicalcategory, 205
functor, 205
structureminimal -, 206
homotopy categoryof a simplicial set, 215
of spaces, 206
homotopy equivalence, 206
chain -, 206
homotopy invariant, 14
horizontal composition, 45
idempotent, 88
monad, 158, 167, 168
split -, 88
identity morphism, 3
image, 223
essential -, 143
factorization, 105
induced representation, ix, 192, 196, 201
infimum, 13, 103
universal property of -, 113
initialalgebra, see also terminal coalgebra
jointly weakly -, 146
object, 41, 51, 81
as a limit, 109
weakly -, 109, 146interior, 158
internal category, 89intersection, 148
invariant, 27
inverseleft -, 12
right -, 12
inverse image, 118, 138
inverse limit, 80
isomorphic, 7
representably -, 62
isomorphism, 7natural -, 24
representable -, 61
Jacobian matrix, 14
Jones polynomial, xii
Kan extension, 190
absolute -, 208
along a fully faithful functor, 203
defining a limit or colimit, 210
defining a monad, 214237

defining an adjunction, 210
existence of -, 196
formula, 194
pointwise -, 201, 202preservation of -, 200
kernel, 139
pair, 103
kernel operator, 158
Khovanov homology, xii
Klein’s Erlangen program, 36
Kleisli category, 162, 165
universal property, 164
lattice, 174
complete -, 103, 110
semi-, 174
Lawvere theory, 179
model of -, 179
left adjointpreserves colimits, 138
left exact functor, 139
left Kan extension
colimit formula, 194
lim inf, 113
lim sup, 113
limit, 73, 75
as a Kan extension, 210
as an equalizer between products, 96
commutes with limits, 111cone, 75, 76
connected -, 91
diagram, 76, 204
finite -, 114
functor, 106, 136
in a functor category, 93
in the category of sets, 85–89
non-strictness of -, 106, 109of a sequence, 113
representable universal property of -, 94
uniqueness of -, 76
universal property of -, 82
linear algebra
abstract vs concrete, 30, 33
linear functional, 17
local, 31
local object, 143
localization, 140, 143
functor, 205
in ring theory, 141
locally presentable category, 150
locally small, 7loop, 53
space, 131
free -, 131map, xi, see also morphism, 6Markovchain, 163
kernel, 163

238

measurable
function, 157
space, 157
middle four interchange, 46
modulefree -, 119
monad, 154
as a Kan extension, 214
as a monoid, 154
codensity -, 214
continuation -, 157
double dual -, 157
double power set -, 157
finitary -, 175, 184
free abelian group -, 156
free commutative monoid -, 157
free group -, 156
free module -, 156
free monoid -, 156
free vector space -, 156
from an adjunction, 155
Giry -, 157, 163, 215
idempotent -, 158, 167, 168
list -, 156
maybe -, 155
ultrafilter -, 156, 158
monadic
adjunction, 167
functor, 167
strictly -, 167
monadicity theorem, 173, 177
monic, 11mono, see also monomorphism
monoid, 5, 37, 161, 174
as a one-object category, 5
free -, 119
in a monoidal category, 154
topological -, 37
monoidal category, 154, 219
symmetric -, 219
monoidal product, 219
monomorphism, 11
split -, 12
use in commutative diagrams, 43morphismcomposite -, 3
etymology, xi
identity -, 3
in a category, 3
not always a function, 6
opposing pair of -, xvi
parallel pair of -, xvi
multiplicationof a monad, 154
nadirof a cone, 74
natural, 1–2

INDEX

natural isomorphism, 24
as an invertible natural transformation, 28
between diagrams, 107
natural numbers, 50
natural transformation, 23
components of -, 23composition, 44, 45
constant -, 74
extra-, 28
globular diagram for -, 24
invertible -, 28
nerveof a category, 142, 215
no cloning theorem, 26
objectin a category, 3
local -, 143
op-, 202
open cover, 91
opposing pair, xvi
opposite
category, 9group, 10
orbit, 34
orbit category, 21, 198
orbit-stabilizer theorem, 35
p-adic integers, 1, 80, 182
p-adic solenoid, 1, 101parallel pair, xvi, 78
pasting diagram, 45, 190
path, 53, 221
components, 107re-parameterization of -, 61
persistent cluster, 16
pointed set, 160
pointwise Kan extension, 199, 201, 202
poset, 5
as a category, 5
power, 94preorder, 5
as a category, 5presentation
for a group, 169
for an algebra, 169preserve, 90
presheaf, 18, 141
probability measure, 157, 215
finitely additive -, 215
product, 73, 77
dependent -, 67
fiber -, 79projection, 77
universal property of -, 82
projection
from a product, 77
propositional function, 118
Prüfer p-group, xiv, 1

INDEX

pullback, 78
pushout, 81
quasi-isomorphism, 206
quotient, 73
recursion, 50
reflect, 90
reflective subcategory, 140, 142, 158, 167
reflexive pair, 177
regular cardinal, 150
relation, 103
for an algebra, 169
representability theorem, 149
Freyd’s -, 149
representableisomorphism, 61
vs represented, 51
representable functor, 51
preserves limits, 94
representably
isomorphic, 62
representation, 51
characterizes its representing object, 62
uniqueness of -, 69
represented functor, 19
as a left Kan extension, 190
two-sided -, 20, 60
restriction of scalars, 120
retract, 12, 88
retraction, 12
Riesz representation theorem, 26
right adjointpreserves limits, 136
right exact functor, 139
right Kan extension
limit formula, 194
ring, 37
unital vs non-unital -, 143, 150
row operation, 60
Russell’s paradox, 6section, 12
semilatticemeet or join -, 174
separator
for a category, 147, 222
sequential colimit, 81
set theory
for category theory, 6
sheaf, 141, 222
condition, 91
sheafification, 141
Sierpinski space, 54
signature, 4, 127
simplicial set, 18, 141, 193
augmented -, 193
geometric realization of -, 216
homotopy category of -, 142, 215

239

skeleton, 34
n-, 199
slice category, 8, 67
small, 6, 110
diagram, 85
essentially -, 35
locally -, 7smash product, 131
solution set, 146
spaceconfiguration -, 101convenient category of -, 129
fiber -, 105
species, 108
split
coequalizer, 169
epimorphism, 12
monomorphism, 12
stabilizer, 34
Stone–Čech compactification, 140, 149
structure, 127
subcategory, 8full -, 31
subobject, 73, 103, 148classifier, 70
sum
dependent -, 67
summitof a cone, 74
supremum, 13, 103
universal property of -, 113
suspension
reduced -, 131
symmetric monoidal category, 219
syntax and semantics, 128
tensor, 96
tensor product, 63
associativity, 65
distributes over direct sums, 138
symmetry, 64
unit, 65
terminal
category, 78
coalgebra, 44, 222
object, 41, 51, 78
the, 62
topological space, see also space
topos
elementary -, 177
Grothendieck -, 222
torsion subgroup, 26
torsor, 63, 69
torus, 81
total derived functor, 206
total singular complex, 216
transitive action, 63translation groupoid, 34, 69

240transpose, 116
triangle identity, 123
triple, 177
tripleability theorem
precise -, 177
reflexive -, 177
twisted arrow category, 71
ultrafilter, 156
union, 82
unitof a monad, 154of an adjunction, 122unit interval, 221
unit object, 219
universal coefficient theorem, 1
universal cover
is not universal, 62
universal element, 62, 66
universal property, 51, 62, 66
as an initial or terminal object, 68
universal quantification, 118
vector spacedual -, 1, 17, 23
dual vs double dual, 23
free -, 115, 119vertical composition, 45
von Neumann hierarchy, 6
walkingarrow, 29, 53
isomorphism, 29, 53
weak equivalence, 205
weak homotopy equivalence, 206
whiskering, 46, 47
Yoneda embedding, 59
Yoneda lemma, 57–59, 217
corollary, 60
dualization of -, 212
generalization of -, 211–212
meaning of the phrase -, 60
Zariski topology, 127zero object, 43, 103, 139, 223

INDEX