An untitled book project
about symmetric spectra

This document is a preliminary and incomplete version of what may one day become abook about symmetric spectra. It probably contains an embarrassing number of typos
and inconsistencies, and hopefully not too many actual mistakes. I intend to post
updates regularly, so you may want to check my homepage for newer versions. I am
interested in feedback.

Stefan Schwede
Mathematisches Institut
Universität Bonn, Germany

schwede@math.uni-bonn.dewww.math.uni-bonn.de/people/schwede
v2.4 / July 12, 2007

Contents
Introduction

2

Chapter I. Basic definitions and examples
1. Symmetric spectra
2. Examples
3. Smash product4. Homotopy groups, M-modules and semistability
Exercises
History and credits

7
7
14
41
50
79
82

Chapter II. The stable homotopy category
1. Injective Ω-spectra
2. Additive structure
3. Triangulated structure
4. Stable equivalences
5. Derived smash product
Exercises
History and credits

85
85
89
93
98
112
122
123

Chapter III. Model structures
1. Level model structures
2. Stable model structures
3. Model structures for modules
Exercises
History and credits

125
125
131
133
136
136

Appendix A.
1. Tools from model category theory
2. Triangulated categories

139
139
145

Bibliography

149

Index

151

1

2

CONTENTS

Introduction
This textbook is an introduction to the modern foundations of stable homotopy theory and ‘algebra’over structured ring spectra, based on symmetric spectra. We begin with a quick historical review andattempt at motivation.
A crucial prerequisite for spectral algebra is an associative and commutative smash product on agood point-set level category of spectra, which lifts the well-known smash product pairing on the homotopy
category. The first construction of what is now called ‘the stable homotopy category’, including its symmetricmonoidal smash product, is due to Boardman [4] (unpublished); accounts of Boardman’s construction
appear in [65], [62] and [2, Part III] (Adams has to devote more than 30 pages to the construction andformal properties of the smash product).
To illustrate the drastic simplification that occurred in the foundations in the mid-90s, let us draw
an analogy with the algebraic context. Let R be a commutative ring and imagine for a moment that thenotion of a chain complex (of R-modules) has not been discovered, but nevertheless various complicatedconstructions of the unbounded derived category D(R) of the ring R exist. Moreover, constructions of
the derived tensor product on the derived category exist, but they are complicated and the proof that thederived tensor product is associative and commutative occupies 30 pages. In this situation, you could talkabout objects A in the derived category together with morphisms A ⊗L
R A −→ A, in the derived category,
which are associative and unital, and possibly commutative, again in the derived category. This notion may
be useful for some purposes, but it suffers from many defects – as one example, the category of modules(under derived tensor product in the derived category), does not in general form a triangulated category.
Now imagine that someone proposes the definition of a chain complex of R-modules and shows thatby formally inverting the quasi-isomorphisms, one can construct the derived category. She also defines
the tensor product of chain complexes and proves that tensoring with suitably nice (i.e., homotopicallyprojective) complexes preserves quasi-isomorphisms. It immediately follows that the tensor product descends
to an associative and commutative product on the derived category. What is even better, now one cansuddenly consider differential graded algebras, a ‘rigidified’ version of the crude multiplication ‘up-to-chain
homotopy’. We would quickly discover that this notion is much more powerful and that differential gradedalgebras arise all over the place (while chain complexes with a multiplication which is merely associative
up to chain homotopy seldom come up in nature).
Fortunately, this is not the historical course of development in homological algebra, but the development
in stable homotopy theory was, in several aspects, as indicated above. In the stable homotopy category
people could consider ring spectra ‘up to homotopy’, which are closely related to multiplicative cohomology
theories. However, the need and usefulness of ring spectra with rigidified multiplications soon became
apparent, and topologists developed different ways of dealing with them. One line of approach uses operads
for the bookkeeping of the homotopies which encode all higher forms of associativity and commutativity,
and this led to the notions of A∞ - respectively E∞ -ring spectra. Various notions of point-set level ring
spectra had been used (which were only later recognized as the monoids in a symmetric monoidal model
category). For example, the orthogonal ring spectra had appeared as I∗ -prefunctors in [42], the functors
with smash product were introduced in [6] and symmetric ring spectra appeared as strictly associative ring
spectra in [22, Def. 6.1] or as FSPs defined on spheres in [23, 2.7].
At this point it had become clear that many technicalities could be avoided if one had a smash producton a good point-set category of spectra which was associative and unital before passage to the homotopy
category. For a long time no such category was known, and there was even evidence that it might notexist [32]. In retrospect, the modern spectra categories could maybe have been found earlier if Quillen’sformalism of model categories [46] had been taken more seriously; from the model category perspective, one
should not expect an intrinsically ‘left adjoint’ construction like a smash product to have a good homotopicalbehavior in general, and along with the search for a smash product, one should look for a compatible notionof cofibrations.
In the mid-90s, several categories of spectra with nice smash products were discovered, and simultaneously, model categories experienced a major renaissance. Around 1993, Elmendorf, Kriz, Mandell and May

INTRODUCTION

3

introduced the S-modules [19] and Jeff Smith gave the first talks about symmetric spectra; the details of
the model structure were later worked out and written up by Hovey, Shipley and Smith [25]. In 1995, Lydakis [34] independently discovered and studied the smash product for Γ-spaces (in the sense of Segal [55]),
and a little later he developed model structures and smash product for simplicial functors [35]. Except for
the S-modules of Elmendorf, Kriz, Mandell and May, all other known models for spectra with nice smashproduct have a very similar flavor; they all arise as categories of continuous (or simplicial), space-valued
functors from a symmetric monoidal indexing category, and the smash product is a convolution product(defined as a left Kan extension), which had much earlier been studied by the category theorist Day [15].
This unifying context was made explicit by Mandell, May, Schwede and Shipley in [39], where another example, the orthogonal spectra were first worked out in detail. The different approaches to spectra categories
with smash product have been generalized and adapted to equivariant homotopy theory [16, 37, 38] andmotivic homotopy theory [17, 26, 27].
Why symmetric spectra? The author is a big fan of symmetric spectra; two important reasons arethat symmetric spectra are easy to define and require the least amount of symmetry among the models of thestable homotopy category with smash product. A consequence of the second point is that many interestinghomotopy types can be written down explicitly and in closed form. We give examples of this in Section I.2,
right after the basic definitions, among these are the sphere spectrum, suspension spectra, Eilenberg-Mac
Lane spectra, Thom spectra such as M O, M SO and M U , topological K-theory and algebraic K-theory
spectra.
Another consequence of the second point is that whenever someone writes down or constructs a model
for a homotopy type in one of the other worlds of spectra, then we immediately get a model as a symmetric
spectrum by applying one of the ‘forgetful’ functors from spectra with more symmetries which we recall
in Section I.2.3. In fact, symmetric spectra have a certain universal property (see Shipley’s paper [57]),
making them ‘initial’ among stable model categories with a compatible smash product.
There are already good sources available which explain the stable homotopy category, and there aremany research papers and at least one book devoted to structured ring spectra. However, my experience isthat for students learning the subject it is hard to reconcile the treatment of the stable homotopy category
as given, for example, in Adams’ notes [2], with the more recent model category approaches to, say, Smodules or symmetric spectra. So one aim of this book is to provide a source where one can learn about
the triangulated stable homotopy category and about stable model categories and a good point-set levelsmash product with just one notion of what a spectrum is.
The monograph [19] by Elmendorf, Kriz, Mandell and May develops the theory of one of the competing
frameworks, the S-modules, in detail. It has had a big impact and is widely used, for example because
many standard tools can simply be quoted from that book. The theory of symmetric spectra is by now
highly developed, but the results are spread over many research papers. The aim of this book is to collectbasic facts in one place, thus providing an alternative to [19].
Prerequisites. As a general principle, I assume knowledge of basic algebraic topology and unstablehomotopy theory. I will develop in parallel the theory of symmetric spectra based on topological spaces(compactly generated and weak Hausdorff) and simplicial sets. Whenever simplicial sets are used, I assume
basic knowledge of simplicial homotopy theory, as found for example in [21] or [41]. However, the useof simplicial sets is often convenient but hardly ever essential, so not much understanding is lost by justthinking about topological spaces throughout.
On the other hand, no prior knowledge of stable homotopy theory is assumed. In particular, we define
the stable homotopy category using symmetric spectra and develop its basic properties from scratch.
From Chapter III on I will freely use the language of Quillen’s model categories and basic results ofhomotopical algebra. The original source is [46], a good introduction is [18], and [24] is a thorough,extensive treatment.
Organization. We organize the book into chapters, each chapter into sections and some sections into
subsections. The numbering scheme for referring to definitions, theorems, examples etc. is as follows. If
we refer to something in the same chapter, then the reference number consists only of the arabic section
number and then a running number for all kind of environments. If the reference is to another chapter,4

CONTENTS

then we add the roman number of the chapter in front. So ‘Lemma 3.14’ refers to a Lemma in Section 3 ofthe same chapter, with running number 14, while ‘Example I.2.21’ is an example from the second sectionof the first chapter, with running number 21.
In the first chapter we introduce the basic concepts of a symmetric spectrum and symmetric ring
spectrum and then, before developing any extensive theory, discuss lots of examples. There is a section on
the smash product where we concentrate on its formal properties, leaving the homotopical properties of thesmash product to a later chapter. One of the few points where symmetric spectra are more complicatedthan other frameworks is that the usual homotopy groups can be somewhat pathological. So we spend thelast section of the first chapter on the structure of homotopy groups and the notion of semistable symmetric
spectra.
The second chapter is devoted to the stable homotopy category. We define it as the homotopy categoryof injective Ω-spectra, based on simplicial sets. Model categories will not be discussed explicitly until
Chapter III, but the justification behind this definition is that there is a stable model structure (the injective
stable model structure) in which every symmetric spectrum is cofibrant and where the fibrant objects are
precisely the injective Ω-spectra. We develop some basic theory around the stable homotopy category, suchas the triangulated structure, derived smash product, homotopy (co-)limits, Postnikov sections, localizationand completion, and discuss the relationship to the Spanier-Whitehead category and generalized cohomology
theories.
In Chapter III model structures enter the scene. We start by establishing the various level model
structures (projective, flat, injective, and their positive versions) for symmetric spectra, and then discuss
the associated, more important, stable model structures. We also develop the model structures for modules
over a fixed symmetric ring spectrum and for algebras over an operad of simplicial sets. The latter includes
the stable model structures for symmetric ring spectra and for commutative symmetric ring spectra.
Each chapter has a section containing exercises.
As a general rule, I do not attribute credit for definitions and theorems in the body of the text. Instead,
there is a section ‘History and credits’ at the end of each chapter, where I summarize, to the best of myknowledge, who contributed what. Additions and corrections are welcome.
Two philosophical points. (i) We make isomorphisms explicit. Thus we avoid phrases like ‘thecanonical isomorphism’ unless the isomorphism we have in mind has previously been defined. The main
reason for this is the author’s experience that what seems canonical to the expert may often not be clear
to a newcomer. Another reason is that here and there, one can get sign trouble if one is not careful about
choices of isomorphisms. A disadvantage is that we have to introduce lots of symbols or numbers to referto the isomorphisms.
(ii) We sometimes give two (and occasionally even three) proofs or constructions when they are sufficiently different and shed light on the theorem or notion under consideration. While this is logicallyredundant, we think that to understand a mathematical phenomenon it is good to see it from as manydifferent angles as possible.
Some conventions. Let us fix some terminology and enact several useful conventions. We think thatsome slight abuse of language and notation can often make statements more transparent, but when we allow
ourselves such imprecision we feel obliged to state them clearly here, at the risk of being picky.
We denote by T the category of pointed, compactly generated, weak Hausdorff topological spaces.
A map between topological spaces always refers to a continuous map, unless explicitly stated otherwise.
Similarly, an action of a group on a space refers to a continuous action.
It will be convenient to define the n-sphere S n as the one-point compactification of n-dimensional
euclidian space Rn , with the point at infinity as the basepoint.
For n ≥ 0, the symmetric group Σn is the group of bijections of the set {1, 2, . . . , n}; in particular,
Σ0 consists only of the identity of the empty set. It will often be convenient to identify the productgroup Σn × Σm with the subgroup of Σn+m of those permutations which take the sets {1, . . . , n} and{n + 1, . . . , n + m} to themselves. Whenever we do so, we implicitly use the monomorphism
Σn × Σm −→ Σn+m ,(τ, κ) 7→ τ × κ

INTRODUCTION

5given by(τ (i)for 1 ≤ i ≤ n,κ(i − n) + n for n + 1 ≤ i ≤ n + m.
We let the symmetric group Σn act from the left on Rn by permuting the coordinates, i.e.,γ(x1 , . . . , xn ) = (xγ −1 (1) , . . . , xγ −1 (n) ). This action compactifies to an action on S n which fixes the basepoint. The canonical linear isomorphism(τ × κ)(i) =Rn × Rm −→ Rn+m ,nm((x1 , . . . , xn ), (y1 , . . . , xm )) 7→ (x1 , . . . , xn , y1 , . . . , xm )induces a homeomorphism S ∧ S −→ S n+m which is equivariant with respect to the action of the groupΣn × Σm , acting on the target by restriction from Σn+m .
The topological spaces we consider are usually pointed, and we use the notation πn X for the n-th
homotopy group with respect to the distinguished basepoint, which we do not record in the notation.
Limits and colimits. Limits and colimits in a category are hardly ever unique, but the universal
property which they enjoy makes then ‘unique up to canonical isomorphism’. We want to fix our language
for talking about this unambiguously. We recall that a colimit of a functor F : I −→ C is a pair (F̄ , ϕ)consisting of an object F̄ of C and a natural transformation ϕ : F −→ cF̄ from F to the constant functor
with value F̄ which is initial among all natural transformations from F to constant functors. We often
follow the standard abuse of language and call the object F̄ a colimit, or even the colimit, of the functor F
and denote it by colimI F . When we need to refer to the natural transformation ϕ which is part of the dataof a colimit, we refer to the component ϕi : F (i) −→ colimI F at an object i ∈ I as the canonical morphismfrom the object F (i) to the colimit. Dually for limits.
Acknowledgments. A substantial part of this book was written during a sabbatical semester takenby the author at the Massachusetts Institute of Technology in the fall 2006, where I could also try out
some of the contents of this book in a graduate course. I am grateful to Haynes Miller for the invitationthat made this possible. I would like to thank the following people for helpful comments, corrections andimprovements: Daniel Davis, Moritz Groth, Lars Hesselholt, Jens Hornbostel, Katja Hutschenreuter, Tyler
Lawson, Steffen Sagave and Brooke Shipley.

CHAPTER I

Basic definitions and examples
1. Symmetric spectra
Definition 1.1. A symmetric spectrum consists of the following data:• a sequence of pointed spaces Xn for n ≥ 0• a basepoint preserving continuous left action of the symmetric group Σn on Xn for each n ≥ 0• based maps σn : Xn ∧ S 1 −→ Xn+1 for n ≥ 0.
This data is subject to the following condition: for all n, m ≥ 0, the composite(1.2)

Xn ∧ S mσn ∧ Id/ Xn+1 ∧ S m−1

σn+1 ∧Id/···

σn+m−2 ∧Id/ Xn+m−1 ∧ S 1

σn+m−1/ Xn+mis Σn × Σm -equivariant. We often denote this composite map by σ m . Here the symmetric group Σm acts
by permuting the coordinates of S m , and Σn × Σm acts on the target by restriction of the Σn+m -action.
We refer to the space Xn as the nth level of the symmetric spectrum X.
A morphism f : X −→ Y of symmetric spectra consists of Σn -equivariant based maps fn : Xn −→ Yn
for n ≥ 0, which are compatible with the structure maps in the sense that fn+1 ◦ σn = σn ◦ (fn ∧ IdS 1 ) for
all n ≥ 0. The category of symmetric spectra is denoted by SpΣ .
Definition 1.3. A symmetric ring spectrum R consists of the following data:• a sequence of pointed spaces Rn for n ≥ 0• a basepoint preserving continuous left action of the symmetric group Σn on Rn for each n ≥ 0• Σn × Σm -equivariant multiplication maps
µn,m : Rn ∧ Rm −→ Rn+mfor n, m ≥ 0, and• two unit maps
ι0 : S 0 −→ R0
This data is subject to the following conditions:(Associativity) The squareRn ∧ Rm ∧ R pµn,m ∧Idandι1 : S 1 −→ R1 .

Id ∧µm,p/ Rn ∧ Rm+p
µn,m+p

Rn+m ∧ Rpµn+m,p/ Rn+m+p

commutes for all n, m, p ≥ 0.
(Unit) The two composites
Rn ∼= Rn ∧ S 0

Id ∧ι0/ Rn ∧ R0

µn,0

/ Rn

Rn ∼
= S 0 ∧ Rn

ι0 ∧Id/ R0 ∧ R n

µ0,n

/ Rn

are the identity for all n ≥ 0.
7

8

I. BASIC DEFINITIONS AND EXAMPLES(Centrality) The diagram
Id ∧ι1Rn ∧ S 1µn,1/ Rn ∧ R1/ Rn+1χn,1τS 1 ∧ Rn

ι1 ∧Id/ R1 ∧ R nµ1,n/ R1+ncommutes for all n ≥ 0. Here χn,m ∈ Σn+m denotes the shuffle permutation which moves the first nelements past the last m elements, keeping each of the two blocks in order; in formulas,(i + m for 1 ≤ i ≤ n,
χn,m (i) =i−n
for n + 1 ≤ i ≤ n + m.
A symmetric ring spectrum R is commutative if the squareRn ∧ Rm
µn,m

twist



Rn+m

/ Rm ∧ Rn


χn,m

µm,n

/ Rm+ncommutes for all n, m ≥ 0.
A morphism f : R −→ S of symmetric ring spectra consists of Σn -equivariant based maps fn : Rn −→Sn for n ≥ 0, which are compatible with the multiplication and unit maps in the sense that fn+m ◦ µn,m =µn,m ◦ (fn ∧ fm ) for all n, m ≥ 0, and f0 ◦ ι0 = ι0 and f1 ◦ ι1 = ι1 .
Definition 1.4. A right module M over a symmetric ring spectrum R consists of the following data:• a sequence of pointed spaces Mn for n ≥ 0• a basepoint preserving continuous left action of the symmetric group Σn on Mn for each n ≥ 0,and
• Σn × Σm -equivariant action maps αn,m : Mn ∧ Rm −→ Mn+m for n, m ≥ 0.
The action maps have to be associative and unital in the sense that the following diagrams commute
Mn ∧ R m ∧ R p
αn,m ∧Id

Id ∧µm,p/ Mn ∧ R 0
Mn ∼= Mn ∧ S 0
TTTTT
TTTTT
αn,0
TTT
TTTTT Mn

/ Mn ∧ Rm+p

Id ∧ι0αn,m+p

Mn+m ∧ Rp

αn+m,p/ Mn+m+p

for all n, m, p ≥ 0. A morphism f : M −→ N of right R-modules consists of Σn -equivariant based maps
fn : Mn −→ Nn for n ≥ 0, which are compatible with the action maps in the sense that fn+m ◦ αn,m =
αn,m ◦ (fn ∧ Id) for all n, m ≥ 0. We denote the category of right R-modules by mod- R.
The k-th homotopy group of a symmetric spectrum X is defined as the colimit
πk X = colimn πk+n Xn
taken over the maps
(1.5)

−∧S 1

πk+n Xn −−−−−→ πk+n+1 Xn ∧ S 1(σn )∗−−−−−→ πk+n+1 Xn+1 .

Homotopy groups of symmetric spectra are abelian groups, and for symmetric ring spectra they often
form graded rings. More precisely, the underlying symmetric spectrum of R has to be ‘semistable’ (seeTheorem 4.44) for π∗ R to form a graded ring, see Theorem 4.54. In general the homotopy groups support
a more sophisticated algebraic structure, namely an algebra over the ‘injection operad’.

1. SYMMETRIC SPECTRA

9Remark 1.6. We have stated the axioms for symmetric ring spectra in terms of a minimal amount ofdata and conditions. Now we put these conditions into perspective. We consider a symmetric ring spectrum
R.
(i) It will be useful to have the following notation for iterated multiplication maps. For natural
numbers n1 , . . . , ni ≥ 0 we denote byµn1 ,...,ni : Rn1 ∧ . . . ∧ Rni −→ Rn1 +···+ni
the map obtained by composing multiplication maps smashed with suitable identity maps; by
associativity, the parentheses in the multiplications don’t matter. More formally we can define
the iterated multiplication maps inductively, settingµn1 ,...,ni = µn1 ,n2 +···+ni ◦ (IdRn1 ∧µn2 ,...,ni ) .
(ii) We can define higher-dimensional unit maps ιm : S m −→ Rm for m ≥ 2 as the compositeµ1,...,1

ι ∧...∧ι

1
1−−−−→
R1 ∧ . . . ∧ R1 −−−−→ Rm .
S m = S 1 ∧ . . . ∧ S 1 −−

Centrality then implies that ιm is Σm -equivariant, and it implies that the diagram
Rn ∧ S m

Id ∧ιm/ Rn ∧ Rm

µn,m

χn,m

twist


S m ∧ Rn

/ Rn+mιm ∧Id/ Rm ∧ Rn

µm,n/ Rm+ncommutes for all n, m ≥ 0, generalizing the original centrality condition.
(iii) As the terminology suggests, the symmetric ring spectrum R has an underlying symmetric
spectrum. In fact, the multiplication maps µn,m make R into a right module over itself, andmore generally, every right R-module M has an underlying symmetric spectrum as follows.
We keep the spaces Mn and symmetric group actions and define the missing structure maps
σn : Mn ∧ S 1 −→ Mn+1 as the composite αn,1 ◦ (IdMn ∧ι1 ). Associativity implies that the iterated
structure map σ m : Mn ∧ S m −→ Mn+m equals the composite
Id ∧ι

αn,m

Mn ∧ S m −−−−m→ Mn ∧ Rm −−−→ Mn+m .
So the iterated structure map is Σn × Σm -equivariant by part (ii) and the equivariance hypothesis
on αn,m , and we have in fact obtained a symmetric spectrum.
The forgetful functors which associates to a symmetric ring spectrum or module spectrum
its underlying symmetric spectrum have left adjoints. We will construct the left adjoints in Example 3.10 below after introducing the smash product of symmetric spectra. The left adjoints
associate to a symmetric spectrum X the ‘free R-module’ X ∧ R respectively the ‘free symmetricring spectrum’ T X generated by X, which we will refer to it as the tensor algebra.
(iv) If the symmetric ring spectrum R is commutative, then centrality is automatic.
(v) Using the internal smash product of symmetric spectra introduced in Section 3, we can identify
the ‘explicit’ definition of a symmetric ring spectrum which we just gave with a more ‘implicit’
definition of a symmetric spectrum R together with morphisms µ : R ∧ R −→ R and ι : S −→ R
(where S is the sphere spectrum, see Example 2.1) which are suitably associative and unital. The‘explicit’ and ‘implicit’ definitions of symmetric ring spectra coincide in the sense that they define
isomorphic categories, see Theorem 3.8.
We will often use a variation on the notions of symmetric spectrum and symmetric ring spectrum where
topological spaces are replaced by simplicial sets. We can go back and forth between the two concepts using
the adjoint functors of geometric realization and singular complex, as we explain below.
Definition 1.7 (Symmetric spectra of simplicial sets). A symmetric spectrum of simplicial sets consistsof the following data:

10

I. BASIC DEFINITIONS AND EXAMPLES• a sequence of pointed simplicial sets Xn for n ≥ 0• a basepoint preserving simplicial left action of the symmetric group Σn on Xn for each n ≥ 0• pointed morphisms σn : Xn ∧ S 1 −→ Xn+1 for n ≥ 0,such that for all n, m ≥ 0, the composite
Xn ∧ S mσn ∧ Id/ Xn+1 ∧ S m−1

σn+1 ∧Id/···

σn+m−2 ∧Id/ Xn+m−1 ∧ S 1

σn+m−1/ Xn+mis Σn ×Σm -equivariant. Here S 1 denotes the ‘small simplicial circle’ S 1 = ∆[1]/∂∆[1] and S m = S 1 ∧. . .∧S 1
is the mth smash power, with Σm permuting the factors.
We similarly define a symmetric ring spectrum of simplicial sets by replacing ‘space’ by ‘simplicial set’in Definition 1.3, while also replacing the topological circle S 1 by the simplicial circle S 1 = ∆[1]/∂∆[1] andreplacing S m by the m-fold smash power S m = S 1 ∧ . . . ∧ S 1 .
As we already mentioned we can apply the adjoint functors ‘geometric realization’, denoted | − |, and‘singular complex’, denoted S, levelwise to go back and forth between topological and simplicial symmetric
spectra. We use that geometric realization is a ‘strong symmetric monoidal’ functor, i.e., there is natural,
unital, associative and commutative homeomorphism(1.8)
rA,B : |A| ∧ |B| ∼= |A ∧ B|
for pointed simplicial sets A and B. Indeed, the canonical continuous map |A × B| −→ |A| × |B| isa homeomorphism (since we work in the category of compactly generated topological spaces) and thehomeomorphism rA,B is gotten from there by passing to quotients.
We already allowed ourselves the freedom to use the same symbols for the topological and simplicial
spheres. The justification is that the geometric realization of the simplicial S m is homeomorphic to the
topological S m . To be completely explicit, we choose a homeomorphism h : S 1 −→ |S 1 | [choose one...] andthen obtain a Σm -equivariant homeomorphism as the composite
rS 1 ,...,S 1

h(m)

Sm ∼
= S 1 ∧ · · · ∧ S 1 −−−→ |S 1 | ∧ · · · ∧ |S 1 | −−−−−−→ |S 1 ∧ · · · ∧ S 1 | = |S m | .
Now we can finally define the adjoint functors ‘geometric realization’ and ‘singular complex’ for symmetric
spectra.
If Y is a symmetric spectrum of simplicial sets we define a symmetric spectrum |Y | of topological spaces
by |Y |n = |Yn | with structure maps
Id ∧h

rY

,S 1

|σn |

n
|Yn | ∧ S 1 −−−→ |Yn | ∧ |S 1 | −−−−→ |Yn ∧ S 1 | −−→ |Yn+1 | .

Commutativity of the isomorphism (1.8) guarantees that the equivariance condition for the iterated structure map σ m is inherited by the realization |Y |.
Adjoint to the homeomorphism (1.8) is a ‘lax symmetric monoidal’ transformation of pointed simplicial
sets, i.e., a natural, unital, associative and commutative morphism S(X) ∧ S(Y ) −→ S(X ∧ Y ) for pointed
spaces X and Y . So if X is a symmetric spectrum of topological spaces, then we get a symmetric spectrum
S(X) of simplicial sets by S(X)n = S(Xn ) with structure map
Id ∧ĥ

S(σn )

S(Xn ) ∧ S 1 −−−→ S(Xn ) ∧ S(S 1 ) −→ S(Xn ∧ S 1 ) −−−−→ S(Xn+1 ) .
Here ĥ : S 1 −→ S(S 1 ) is the morphism of pointed simplicial sets which is adjoint to the inverse homeomorphism h−1 : |S 1 | −→ S 1 . We use the adjunction unit and counit between | − | and S levelwise to
make geometric realization and singular complex into adjoint functors between topological and simplicial
symmetric spectra.
Geometric realization and singular complex are lax symmetric monoid functors with respect to thesmash products of pointed spaces and pointed simplicial sets (geometric realization is even strong symmetricmonoidal, i.e., commutes with the smash product up to homeomorphism). So both constructions preserve
multiplications, so they take ring spectra to ring spectra and preserve commutativity.
The homotopy groups of a symmetric spectrum based on simplicial sets Y are defined as the homotopy
groups of the geometric realization |Y |.

1. SYMMETRIC SPECTRA

11

Now we introduce an important class of symmetric spectra.
Definition 1.9. A symmetric spectrum of topological spaces X is an Ω-spectrum if for all n ≥ 0 themap σ̃n : Xn −→ ΩXn+1 which is adjoint to the structure map σn : Xn ∧ S 1 → Xn+1 is a weak homotopy
equivalence. The symmetric spectrum X is a positive Ω-spectrum if the map σ̃n : Xn −→ ΩXn+1 is a weakequivalence for all positive values of n (but not necessarily for n = 0).
A symmetric spectrum of simplicial sets Y is an Ω-spectrum respectively positive Ω-spectrum if thegeometric realization |Y | is an Ω-spectrum, respectively positive Ω-spectrum, of topological spaces.
A symmetric spectrum of simplicial sets Y is thus an Ω-spectrum if and only if for all n ≥ 0 the map|Yn | −→ Ω|Yn+1 | which is adjoint to the composite
∼
=

|σn |

|Yn | ∧ S 1 −−→ |Yn ∧ S 1 | −−→ |Yn+1 |is a weak homotopy equivalence. Our definition of ‘Ω-spectrum’ differs slightly from other sources in that
we do not require that each simplicial set Yn has to be a Kan complex. If Y is a symmetric spectrum ofsimplicial sets in which all the Yn ’s are Kan, then the natural maps |ΩYn | −→ Ω|Yn | adjoint to|evaluate||ΩYn | ∧ S 1 −→ |(ΩYn ) ∧ S 1 | −−−−−−→ |Yn |are weak equivalences, and so Y is an Ω-spectrum in our sense if and only if the morphisms of simplicial
sets σ̃n : Yn −→ ΩYn+1 adjoint to the structure maps are weak equivalences.
For every Ω-spectrum X and all k, n ≥ 0, the canonical map πk Xn −→ πk−n X is a bijection. Indeed,
the homotopy groups of ΩXn+1 are isomorphic to the homotopy groups of Xn+1 , shifted by one dimension.
So the colimit system which defines πk−n X is isomorphic to the colimit systemπk Xn −→ πk (ΩXn+1 ) −→ πk (Ω2 Xn+2 ) −→ · · · ,(1.10)

where the maps in the system are induced by the maps σ̃n adjoint to the structure maps. In an Ω-spectrum,
the maps σ̃n are weak equivalences, so all maps in the sequence (1.10) are bijective, hence so is the map
from each term to the colimit πk−n X.
Several examples of Ω-spectra will come up in the next section, for example Eilenberg-Mac Lane spectra(Example 2.7) and spectra arising from very special Γ-spaces by evaluation on spheres (Example 2.39).
Examples which arise naturally as positive Ω-spectra are the spectra of topological K-theory (Example 2.10)
and algebraic K-theory K(C) (Example 2.11) and spectra arising from special (but not necessarily very
special) Γ-spaces by evaluation on spheres. The Ω-spectra with the additional property of being ‘injective’
form the objects of the stable homotopy category (see Chapter II).
Remark 1.11 (Coordinate free symmetric spectra). There is an equivalent definition of symmetric
spectra which is, in a certain sense, ‘coordinate free’; the reason for calling this ‘coordinate free’ will
hopefully become clear after our discussion of orthogonal spectra in Example 2.40.
If A is a finite set we denote by RA the set of functions from A to R with pointwise structure as a
R-vector space. We let S A denote the one-point compactification of RA , a sphere of dimension equal to thecardinality of A. A coordinate free symmetric spectrum consists of the following data:• a pointed space XA for every finite set A• a based continuous map α∗ : XA ∧ S B−α(A) −→ XB for every injective map α : A −→ B of finite
sets, where B − α(A) is the complement of the image of α.
This data is subject to the following conditions:• (Unitality) For every finite set A, the composite(IdA )∗XA ∼= XA ∧ S ∅ −−−−→ XA

is the identity.

12

I. BASIC DEFINITIONS AND EXAMPLES• (Associativity) For every pair of composable injections α : A −→ B and β : B −→ C the diagram
XA ∧ S B−α(A) ∧ S C−β(B)Id ∧β !

/ X ∧ S C−β(α(A))
A

α∗ ∧Id
XB ∧ S C−β(B)

(βα)∗

β∗


/ XC

∼
commutes. In the top vertical map we use the homeomorphism β ! : S B−α(A) ∧ S C−β(B) =S C−β(α(A)) which is one-point compactified from the linear isomorphism RB−α(A) × RC−β(B) ∼=RC−β(α(A)) which uses β on the basis elements indexed by B − α(A) and the identity on the basis
elements indexed by C − β(B).
A coordinate free symmetric spectrum X gives rise to a symmetric spectrum in the sense of Definition 1.1
as follows. For this we let n = {1, . . . , n} denote the ‘standard’ set with n elements and identify Rn with
Rn and S n with S n .
We set Xn = Xn . A permutation γ ∈ Σn acts on Xn as the compositeγ∗Xn ∼= Xn ∧ S ∅ −→ Xn .

The associativity condition in particular shows that this is in fact an associative action. We define thestructure map σn : Xn ∧ S 1 −→ Xn+1 as the mapι∗ : Xn ∧ S 1 ∼= Xn ∧ S {n+1} −→ Xn+1
where ι : n −→ n + 1 is the inclusion, and the homeomorphism S {n+1} ∼= S 1 arises from the linear{n+1} ∼ 1
isomorphism R= R respecting the preferred bases.
The associativity condition implies that the iterated structure map σ m : Xn ∧ S m −→ Xn+m equals
the composite
ιm∗∼ Xn ∧ S {n+1,...,n+m} −Xn ∧ S m =−→ Xn+mwhere ιm : n −→ n + m is the inclusion. The equivariance property is seen as follows: for γ ∈ Σn we haveιm ◦ γ = (γ × 1) ◦ ιm , and associativity for this injection n −→ n + m amounts to Σn × 1-equivariance. Forτ ∈ Σm we have ιm = (1 × τ ) ◦ ιm , and associativity for this injection n −→ n + m amounts to 1 × Σm equivariance. Exercise 5.1 shows that this ‘forgetful’ functor from coordinate free symmetric spectra tosymmetric spectra is an equivalence of categories. [Say how ring spectra are formulated in this language]
Remark 1.12 (Manipulations rules for coordinates). Natural numbers occurring as levels of a symmetric
spectrum or as dimensions of homotopy groups are really placeholders for sphere coordinates. The role ofthe symmetric group actions on the spaces of a symmetric spectrum is to keep track of how such coordinates
are shuffled. Permutations will come up over and over again in constructions and results about symmetric
spectra, and there is a very useful small set of rules which predict when to expect permutations. I recommend
being very picky about the order in which dimensions or levels occur when performing constructions withsymmetric spectra, as this predicts necessary permutations and helps to prevent mistakes. Sometimes
missing a permutation just means missing a sign; in particular missing an even permutation may not haveany visible effect. But in general the issue is more serious; for symmetric spectra which are not semistable,
missing a permutation typically misses a nontrivial operation.
A first example of this are the centrality and commutativity conditions for symmetric ring spectra,
which use shuffle permutations χn,1 and χn,m . A good way of remembering when to expect a shuffle is tocarefully distinguish between indices such as n + m and m + n. Of course these two numbers are equal,
but the fact that one arises naturally instead of the other reminds us that a shuffle permutation should be
inserted. A shuffle required whenever identifying n + m with m + n is just one rule, and here are somemore.
Main rule: When manipulating expressions which occur as levels of symmetric spectra or dimensionsof spheres, be very attentive for how these expressions arise naturally and when you use the basic rules

1. SYMMETRIC SPECTRA

13

of arithmetic of natural numbers. When using the basic laws of addition and multiplication of natural
numbers in such a context, add permutations according to the following rules (i)-(v).
(i) Do not worry about associativity of addition or multiplication, or the fact that 0 respectively 1
are units for those operations. No permutations are required.
(ii) Whenever using commutativity of addition as in n + m = m + n, add a shuffle permutation
χn,m ∈ Σn+m .
(iii) Whenever using commutativity of multiplication as in nm = mn, add a multiplicative shuffleχ×n,m ∈ Σnm defined by
χ×n,m (j + (i − 1)n) = i + (j − 1)mfor 1 ≤ j ≤ n and 1 ≤ i ≤ m.
(iv) Do not worry about left distributivity as in p(n + m) = pn + pm. No permutation is required.
(v) Whenever using right distributivity as in (n + m)q = nq + mq, add the permutation
××(χ×
q,n × χq,m ) ◦ χn+m,q ∈ Σ(n+m)q .

Rule (v) also requires us to throw in permutations whenever we identify a product nq with an iterated sum
q + · · · + q (n copies) since we use right distributivity in the process. However, no permutations are needed
when instead identifying nq with a sum of q copies of n, since that only uses left distributivity.
The heuristic rules (i) through (v) above are a great help in guessing when to expect coordinate orlevel permutations when working with symmetric spectra. But the rules are more than heuristics, and are
based on the following rigorous mathematics. Typically, there are ‘coordinate free’ constructions in the
background (compare Remark 1.11) which are indexed by finite sets A which are not identified with anyof the standard finite sets n = {1, . . . , n}. The outcome of such constructions may naturally be indexed
by sets which are built by forming disjoint unions or products. The permutations arise because in contrast
to the arithmetic rules for + and ·, their analogues for disjoint union and cartesian product of sets only
holds up to isomorphism, and one can arrange to make some, but not all, of the required isomorphisms be
identity maps.
In more detail, when we want to restrict a ‘coordinate free’ construction to symmetric spectra, we
specialize to standard finite sets n; however, if the coordinate free construction involves disjoint union orcartesian product, we need to identify the unions or products of standard finite sets in a consistent way
with the standard finite set of the same cardinality. A consistent way to do that amounts to what is calleda structure of bipermutative category on the category of standard finite sets. So we define binary functors+ and · on standard finite sets resembling addition and multiplication of natural numbers as closely as
possible.
We let Fin denote the category of standard finite sets whose objects are the sets n for n ≥ 0 and whosemorphisms are all set maps. We define the sum functor + : Fin × Fin −→ Fin by addition on objects andby ‘disjoint union’ on morphisms. More precisely, for morphisms f : n −→ n0 and g : m −→ m0 we definef + g : n + m −→ n0 + m0 by(f (i)if 1 ≤ i ≤ n, and(f + g)(i) =g(i − n) + n0 if n + 1 ≤ i ≤ n + m.
This operation is strictly associative and the empty set 0 is a strict unit. The symmetry isomorphism isthe shuffle map χn,m : n + m −→ m + n.
We define the product functor · : Fin × Fin −→ Fin by multiplication on objects and by ‘cartesian
product’ on morphisms. To make sense of this we have to linearly order the product of the sets n and m.
There are two choices which are more obvious than others, namely lexicographically with either the first or
the second coordinate defined as the more important one. Both choices work fine, and we will prefer the firstcoordinate. More precisely, for morphisms f : n −→ n0 and g : m −→ m0 we define f · g : n · m −→ n0 · m0
by(f · g)(j + (i − 1)n) = f (j) + (g(i) − 1)n014

I. BASIC DEFINITIONS AND EXAMPLES

for 1 ≤ j ≤ n and 1 ≤ i ≤ m. The product · is also strictly associative and the set 1 is a strict unit. The
commutativity isomorphism is the multiplicative shuffle χ×
nm : n · m −→ m · n.
This choice of ordering the product of n and m has the effect of making n · m ‘naturally’ the same asn + · · · + n (m copies), because we havef · Idm = f + · · · + f(m copies).

Since p·k ‘is’ p+· · ·+p (k times), we can take the left distributivity isomorphism p·(n+m) = (p·n)+(p·m)
as the identity (compare rule (iv)).
In contrast, Idn ·g is in general not equal to g + · · · + g (n copies), but rather we have×
Idn ·g = χ×
m0 ,n (g + · · · + g)χn,m

for a morphism g : m −→ m0 . However, then right distributivity isomorphism cannot be taken as theidentity; since the coherence diagramq · (n + m)

χ×
q,n+m

right dist.

left dist.

q·n+q·m/ (n + m) · q

×χ×q,n +χq,m
/ n·q+m·q

is supposed to commute, we are forced to define the right distributivity isomorphism (n + m) · q ∼= (n · q) +××(m · q) as (χ×q,n × χq,m ) ◦ χn+m,q , which explains rule (v) above.
2. Examples
In this section we give examples of symmetric spectra and symmetric ring spectra, which we havegrouped into three subsections. Section 2.1 contains basic examples of important stable homotopy types
which one can write down in closed form as a symmetric spectrum. We discuss in particular the sphere spectrum (2.1), suspension spectra (2.6), Eilenberg-Mac Lane spectra (2.7), Thom spectra (2.8, 2.9 and 2.42),
topological K-theory (2.10) and algebraic K-theory spectra (2.11).
In Section 2.2 we discuss constructions which produce new symmetric spectra and ring spectra fromold ones, or from a symmetric spectrum and a space. We define free (2.12) and semifree symmetric spectra (2.13), limits and colimits (2.14), smash product with a space (2.15), suspension (2.16), shift (2.18),
twisted smash product with a Σm -space (2.20), function spectra (2.22), loop spectra (2.23), mapping
spaces (2.24), internal Hom spectra (2.25), endomorphism ring spectra (2.26), mapping telescope and diagonal of a sequence (2.27), smash product with an I-space (2.31), monoid ring spectra (2.32), ring spectrafrom multiplicative I-spaces (2.33), matrix ring spectra (2.34), inverting an integer (2.35) or an element in
π0 (2.36) of a symmetric ring spectrum and adjoining roots of unity to a symmetric ring spectrum (2.37).
In Section 2.3 we review other kinds of spectra and various forgetful functors between them. We
discuss continuous functors (2.38), Γ-spaces (2.39), orthogonal spectra (2.40), unitary spectra (2.41) andS-modules (2.43). The main point here is that an object in any of these other categories also gives rise toa symmetric spectrum.
2.1. Basic examples.
Example 2.1 (Sphere spectrum). The symmetric sphere spectrum S is given by Sn = S n , where thesymmetric group permutes the coordinates and σn : S n ∧ S 1 −→ S n+1 is the canonical isomorphism.
This is a commutative symmetric ring spectrum with identity as unit map and the canonical isomorphism
S n ∧S m −→ S n+m as multiplication map. The sphere spectrum is the initial symmetric ring spectrum: if R
is any symmetric ring spectrum, then a unique morphism of symmetric ring spectra S −→ R is given by thecollection of unit maps ιn : S n −→ Rn (compare 1.6 (ii)). Being initial, the sphere spectrum plays the sameformal role for symmetric ring spectra as the integers Z play for rings. This motivates the notation ‘S’ using
the \mathbb font. The category of right S-modules is isomorphic to the category of symmetric spectra, via

2. EXAMPLES

15

the forgetful functor mod- S −→ SpΣ (see Remark 1.6 (ii)). Indeed, if X is a symmetric spectrum then theassociativity condition shows that there is at most one way to define action maps
αn,m : Xn ∧ S m −→ Xn+m ,namely as the iterated structure map σ m , and these do define the structure of right S-module on X.
The homotopy group πk S = colimn πk+n S n is called the k-th stable homotopy group of spheres, or thek-th stable stem. Since S n is (n − 1)-connected, the group πk S is trivial for negative values of k. The
degree of a self-map of a sphere provides an isomorphism π0 S ∼= Z.
For k ≥ 1, the homotopy group πk S is finite. This is a direct consequence of Serre’s calculation of
the homotopy groups of spheres modulo torsion, which we recall without giving a proof, and Freudenthal’ssuspension theorem.
Theorem 2.2 (Serre). Let m > n ≥ 1. Then((finite group) ⊕ Z
nπm S =(finite group)

if n is even and m = 2n − 1else.

Thus for k ≥ 1, the stable stem πks = πk S is finite.
As a concrete example, we inspect the colimit system defining π1 S, the first stable homotopy group of
spheres. Since the universal cover of S 1 is the real line, which is contractible, the theory of covering spaces
shows that the groups πn S 1 are trivial for n ≥ 2. The Hopf map
proj.

η : S 3 ⊆ C2 − {0} −−−→ CP 1 ∼= S2
is a locally trivial fibre bundle with fibre S 1 , so it gives rise to long exact sequence of homotopy groups. Since
the fibre S 1 has no homotopy above dimension one, the group π3 S 2 is free abelian of rank one, generated by
the class of η. By Freudenthal’s suspension theorem the suspension homomorphism − ∧ S 1 : π3 S 2 −→ π4 S 3
is surjective and from π4 S 3 on the suspension homomorphism is an isomorphism. So the first stable stem
π1s is cyclic, generated by the image of η, and its order equals the order of the suspension of η. On the
one hand, η itself is stably essential, since the Steenrod operation Sq 2 acts non-trivially on the mod-2
cohomology of the mapping cone of η, which is homeomorphic to CP 2 .
On the other hand, twice the suspension of η is null-homotopic. To see this we consider the commutativesquare(x, y)_

S3(x̄, ȳ)


S3

η

η

/ CP 1[x : y]_/ CP 1[x̄ : ȳ]in which the vertical maps are induced by complex conjugation in both coordinates of C2 . The left vertical
map has degree 1, so it is homotopic to the identity, whereas complex conjugation on CP 1 ∼= S 2 has degree−1. So (−1) ◦ η is homotopic to η. Thus the suspension of η is homotopic to the suspension of (−1) ◦ η,
which by the following lemma is homotopic to the negative of η ∧ S 1 .
Lemma 2.3. Let Y be a pointed space, m ≥ 0 and f : S m −→ S m a based map of degree k. Then forevery homotopy class x ∈ πn (Y ∧ S m ) the classes (IdY ∧f )∗ (x) and k · x become equal in πn+1 (Y ∧ S m+1 )after one suspension.
Proof. Let dk : S 1 −→ S 1 be any pointed map of degree k. Then the maps f ∧ S 1 , S m ∧ dk :−→ S m+1 have the same degree k, hence they are based homotopic. Suppose x is represented bySnϕ : S −→ Y ∧ S m . Then the suspensions of (Y ∧ f )∗ (x) is represented by (Y ∧ f ∧ S 1 ) ◦ (ϕ ∧ S 1 ) which
is homotopic to (Y ∧ S m ∧ dk ) ◦ (ϕ ∧ S 1 ) = (ϕ ∧ S 1 ) ◦ (S n ∧ dk ). Precomposition with the degree k map
S n ∧ dk of S n+1 induces multiplication by k, so the last map represents the suspension of k · x.

m+1

16

I. BASIC DEFINITIONS AND EXAMPLES

The conclusion of Lemma 2.3 does not in general hold without the extra suspension, i.e., (Y ∧ f )∗ (x)need not equal (−1)k x in πn (Y ∧ S m ): as we showed above, (−1) ◦ η is homotopic to η, which is not
homotopic to −η since η generates the infinite cyclic group π3 S 2 .
As far as we know, the stable homotopy groups of spheres don’t follow any simple pattern. Muchmachinery of algebraic topology has been developed to calculate homotopy groups of spheres, both unstable
and stable, but no one expects to ever get explicit formulae for all stable homotopy groups of spheres. The
Adams spectral sequence based on mod-p cohomology and the Adams-Novikov spectral sequence based on
M U (complex cobordism) or BP (the Brown-Peterson spectrum at a fixed prime p) are the most effective
tools for explicit calculations as well as for discovering systematic phenomena.
Example 2.4 (Multiplication in stable stems). The stable stems π∗s = π∗ S form a graded commutative
ring which acts on the homotopy groups of every other symmetric spectrum X. We denote the action
simply by a ‘dot’· : πk X × πl S −→ πk+l X;
the definition is essentially straightforward, but there is one subtlety in showing that the product is welldefined. I repeat an earlier warning. It is tempting to try to define a product on the homotopy groups
of a symmetric ring spectrum R in a similar fashion, by smashing representatives and shuffling sphere
coordinates into their natural order. This will indeed give an associative product in many natural cases,
namely whenever the underlying symmetric spectrum of R is ‘semistable’, see Theorem 4.54. However, if
R is not semistable, then smashing of representatives does not descend to a well-defined product on stablehomotopy groups! In that case the algebraic structure that the homotopy groups of R enjoy is more subtle.
Suppose f : S k+n −→ Xn and g : S l+m −→ S m represent classes in πk X respectively πl S. Then weagree that the composite(2.5)

Id ∧χl,n ∧Id

σm

f ∧g

S k+l+n+m −−−−−−−→ S k+n+l+m −−−−→ Xn ∧ S m −−−→ Xn+m

represents the product of [f ] and [g]. The shuffle permutation χl,n is predicted by the principle that all
natural number must occur in the ‘natural order’ compare Remark 1.12. If we simply smash f and g thedimension of the sphere of origin is naturally (k + n) + (l + m), but in order to represent an element of
πk+l X they should occur in the order (k + l) + (n + m), whence the shuffle permutation (which here simply
introduces the sign (−1)ln ).
We check that the multiplication is well-defined. If we replace g : S l+m −→ S m by its suspensiong ∧ S 1 , then the composite (2.5) changes to its suspension, composed with the structure map σn+m :Xn+m ∧ S 1 −→ Xn+m+1 . So the resulting stable class is independent of the representative g of thestable class in πl S. Independence of the representative for πk X is slightly more subtle. If we replacef : S k+n −→ Xn by the representative σn ◦ (f ∧ S 1 ) : S k+n+1 −→ Xn+1 , then the composite (2.5) changes
to σ 1+m (f ∧ Id ∧g)(Id ∧χl,n+1 ∧ Id), which is the lower left composite in the commutative diagram
Id ∧χl,n ∧χ1,m

f ∧g∧Id/ S k+n+l+m+1
S k+l+n+1+m
TTTT
TTTT
TTT
Id ∧χl+m,1
Id ∧χl,n+1 ∧Id TTTTT*
S k+n+1+l+mf ∧Id ∧g

/ Xn ∧ S m+1


Id ∧χm,1/ Xn ∧ S 1+m

σ 1+m/ Xn+1+m

By Lemma 2.3 the map Id ∧χm,1 induces multiplication by (−1)m on homotopy groups after one suspension.
This cancels the sign coming from the shuffle factor χ1,m in the initial horizontal map. So the composite
is homotopic, after one suspension, to the composite σ 1+m (f ∧ g ∧ Id)(Id ∧χl,n ∧ Id), which represents thesame stable class as (2.5).
Now we verify that the dot product is biadditive. We only show the relation (x + x0 ) · y = x · y + x0 · y,and additivity in y is similar. Suppose as before that f, f 0 : S k+n −→ Xn and g : S l+m −→ S m represent
classes in πk X respectively πl S. Then the sum of f and f 0 in πk+n Xn is represented by the composite
pinchf ∨f 0S k+n −−−→ S k+n ∨S k+n −−−→ Xn .

2. EXAMPLES

17

In the square1∧χl,n ∧1

S k+l+n+m/ S k+n+l+m

pinch


S k+l+n+m ∨S k+l+n+m (1∧χl,n ∧1)∨(1∧χl,n

WWWWW
WWWWW(f +f 0 )∧g
WWWWW
pinch∧Id
WWWWW
WWWW+

/ Xn ∧ S m(S k+n ∨S k+n ) ∧ S l+mO(f ∨f 0 )∧g gggggg3
ggggg
∼
=
ggggg 0gggg
(f∧g)∨(f∧g)
gggg/ S k+n+l+m ∨S k+n+l+m∧1)

the right part commutes on the nose and the left square commutes up to homotopy. After composing with
the iterated structure map σ m : Xn ∧ S m −→ Xn+m , the composite around the top of the diagram becomes(f + f 0 ) · g, whereas the composite around the bottom represents f g + f 0 g. This proves additivity of thedot product in the left variable.
If we specialize to X = S then the product provides a biadditive graded pairing · : πk S × πl S −→ πk+l S
of the stable homotopy groups of spheres. We claim that for every symmetric spectrum X the diagram/ πk+l X × πj S

·×Id

πk X × πl S × πj S
Id ×·
πk X × πl+j S·/ πk+l+j X·

commutes, so the product on the stable stems and the action on the homotopy groups of a symmetric
spectrum are associative. After choosing representing maps S k+n −→ Xn , S l+m −→ S m and S j+q −→ S q
and unraveling all the definitions, this associativity ultimately boils down to the equality(k × χl,n × m × j × q) ◦ (k × l × χj,n+m × q) = (k × n × l × χj,m × q) ◦ (k × χl+j,n × m × q)
in the symmetric group Σk+l+j+q+n+m and commutativity of the squareXn ∧ S m ∧ S q
Id ∧∼=/ Xn+m ∧ S q

σ m ∧Id

Xn ∧ S m+q

σq/ Xn+m+q

σ m+q

Finally, the multiplication in the homotopy groups of spheres is commutative in the graded sense, i.e.,
we have xy = (−1)kl yx for x ∈ πk S and y ∈ πl S. Indeed, for representing maps f : S k+n −→ S n andg : S l+m −→ S m the square
S k+l+n+m
χk,l ×χn,m

Id ∧χl,n ∧Idf ∧g/ S k+n+l+m/ S n+mχn,m

S l+k+m+n

Id ∧χk,m ∧Id/ S l+m+k+ng∧f/ S m+n

commutes. The two vertical coordinate permutations induce the signs (−1)kl+nm respectively (after one
suspension) (−1)nm on homotopy groups. Since the upper horizontal composite represents xy and the lower
composite represents yx, this proves the relation xy = (−1)kl yx.
The following table gives the stable homotopy groups of spheres through dimension 8:nπn S
generator

0
Z
ι

1
Z/2
η

2
Z/2
η2

3
Z/24
ν

4
0

5
0

6
Z/2
ν2

7
8
Z/240 (Z/2)2
σ
ησ, ε

18

I. BASIC DEFINITIONS AND EXAMPLES

Here ν and σ are the Hopf maps which arises unstably as fibre bundles S 7 −→ S 4 respectively S 15 −→ S 8 .
The element ε in the 8-stem can be defined using Toda brackets (see Construction 4.71) as ε = ησ +hν, η, νi.
The table records all multiplicative relations in this range except for η 3 = 12ν. A theorem of Nishida’s saysthat every homotopy element of positive dimension is nilpotent.
Example 2.6 (Suspension spectra). Every pointed space K gives rise to a suspension spectrum Σ∞ K
via
(Σ∞ K)n = K ∧ S n∼=→ K ∧ S n+1 ; we then havewith structure maps given by the canonical isomorphism (K ∧ S n ) ∧ S 1 −∞ 0∼
S = Σ S . The homotopy group
πks K = πk (Σ∞ K) = colimn πk+n (K ∧ S n )is called the kth stable homotopy group of K.
Since K ∧ S n is (n − 1)-connected, the suspension spectrum Σ∞ K is connective. The Freudenthal
suspension theorem implies that for every suspension spectrum, the colimit system for a specific homotopy
group always stabilizes. A symmetric spectrum X is isomorphic to a suspension spectrum (necessarily thatof its zeroth space X0 ) if and only if every structure map σn : Xn ∧ S 1 −→ Xn+1 is a homeomorphism.
Example 2.7 (Eilenberg-Mac Lane spectra). For an abelian group A, the Eilenberg-Mac Lane spectrum
HA, based on simplicial sets, is defined by(HA)n = A ⊗ Z̃[S n ] ,i.e., the underlying simplicial set of the dimensionwise tensor product of A with the reduced free simplicial
abelian group generated by the simplicial n-sphere. The symmetric group acts by permuting the smash
factors of S n . The geometric realization of (HA)n is an Eilenberg-Mac Lane space of type (A, n), i.e.,
it has only one non-trivial homotopy group in dimensions n, which is isomorphic to A. The loop spaceof the next space (HA)n+1 is also an Eilenberg-Mac Lane space of type (A, n), and in fact the map
σ̃n : (HA)n −→ Ω(HA)n+1 adjoint to the structure map is a weak equivalence for all n ≥ 0. In otherwords, HA is an Ω-spectrum.
It follows that the homotopy groups of the symmetric spectrum HA are concentrated in dimension
zero, where we have a natural isomorphism A = π0 (HA)0 ∼= π0 HA. If A is not just an abelian group but
also has a ring structure, then HA becomes a symmetric ring spectrum via the multiplication map(HA)n ∧ (HA)m = (A ⊗ Z[S n ]) ∧ (A ⊗ Z[S m ])−→ A⊗Z[S n+m ] = (HA)n+m
given by!
Xiai · xiXX∧bj · yj  7−→(ai · bj ) · (xi ∧ yj ) .
j

i,j

m

The unit maps S −→ (HA)m are given by the inclusion of generators.
We shall see in Example 3.11 below that the Eilenberg-Mac Lane functor H can be made into a lax
symmetric monoidal functor with respect to the tensor product of abelian groups and the smash product ofsymmetric spectra; this also explains why H takes rings (monoids in the category of abelian with respect
to tensor product) to ring spectra (monoids in the category of symmetric spectra with respect to smashproduct).
Example 2.8 (Real cobordism spectra). We define a commutative symmetric ring spectrum M O whose
stable homotopy groups are isomorphic to the ring of cobordism classes of closed smooth manifolds. We set
M On = EO(n)+ ∧O(n) S n ,the Thom space of the tautological vector bundle EO(n) ×O(n) Rn over BO(n) = EO(n)/O(n). HereO(n) is the n-th orthogonal group consisting of Euclidean automorphisms of Rn . The space EO(n) is thegeometric realization of the simplicial space which in dimension k is the (k + 1)-fold product of copies of2. EXAMPLES

19

O(n), and where face maps are projections. Thus EO(n) is contractible and has a right action by O(n).
The right O(n)-action is used to form the orbit space M On , where we remember that S n is the one-pointcompactification of Rn , so it comes with a left O(n)-action.
The symmetric group Σn acts on O(n) by conjugation with the permutation matrices. Since the ‘E’construction is natural in topological groups, this induces an action of Σn on EO(n). If we let Σn act on
the sphere S n by coordinate permutations and diagonally on EO(n)+ ∧ S n , then the action descends to thequotient space M On .
The unit of the ring spectrum M O is given by the maps
Sn ∼= O(n)+ ∧O(n) S n −→ EO(n)+ ∧O(n) S n = M On
using the ‘vertex map’ O(n) −→ EO(n). There are multiplication maps
M On ∧ M Om −→ M On+m
∼ S n+m which is equivariant with respect to the groupwhich are induced from the identification S n ∧ S m =O(n)×O(m), viewed as a subgroup of O(n+m) by block sum of matrices. The fact that these multiplication
maps are associative and commutative uses that• for topological groups G and H, the simplicial model of EG comes with a natural, associative andcommutative isomorphism E(G × H) ∼= EG × EH;• the group monomorphisms O(n) × O(m) −→ O(n + m) by orthogonal direct sum are strictly
associative, and the following diagram commutes
O(n) × O(m)twistO(m) × O(n)/ O(n + m)conj. by χn,m

/ O(m + n)

where the right vertical map is conjugation by the permutation matrix of the shuffle permutation χn,m .
Essentially the same construction gives commutative symmetric ring spectra M SO representing oriented
bordism and M Spin representing spin bordism. For M SO this uses that conjugation of O(n) by a permutation matrix restricts to an automorphism of SO(n) and the block sum of two special orthogonaltransformations is again special. For M Spin it uses that the block sum pairing and the Σn -action uniquelylift from the groups SO(n) to their universal covers Spin(n).
We intend to discuss these and other examples of Thom spectra in more detail in a later chapter.
Example 2.9 (Complex cobordism spectra). The cobordism ring spectra M U and M SU representing
complex bordism, or symplectic bordism M Sp have to be handled slightly differently from real Thom
spectra such as M O in the previous example. The point is that M U and M SU are most naturally indexedon ‘even spheres’, i.e., one-point compactifications of complex vector spaces, and M Sp is most naturallyindexed on spheres of dimensions divisible by 4. However, a small variation gives M U , M SU and M Sp ascommutative symmetric ring spectra, as we shall now explain. The complex cobordism spectrum M U plays
an important role in stable homotopy theory because of its relationship to the theory of formal groups laws.
Thus module and algebra spectra over M U are important, and we plan to study these in some detail later.
We first consider the collection of pointed spaces M U with(M U )n = EU (n)+ ∧U (n) S 2n ,the Thom space of the tautological complex vector bundle EU (n) ×U (n) Cn over BU (n) = EU (n)/U (n).
Here U (n) is the n-th unitary group consisting of Euclidean automorphisms of Cn . The Σn -action arises
from conjugation by permutation matrices and the permutation of complex coordinates, similarly as in thecase of M O above.
There are multiplication maps
(M U )p ∧ (M U )q −→ (M U )p+q

20

I. BASIC DEFINITIONS AND EXAMPLES

which are induced from the identification Cp ⊕ Cq ∼= Cp+q which is equivariant with respect to the groupU (p) × U (q), viewed as a subgroup of U (p + q) by direct sum of linear maps. There is a unit map ι0 : S 0 −→(M U )0 , but instead of a unit map from the circle S 1 , we only have a unit map S 2 −→ (M U )1 . Thus we
do not end up with a symmetric spectrum since we only get structure maps (M U )n ∧ S 2 −→ (M U )n+1
involving the 2-sphere. In other words, M U has the structure of what could be called an ‘even symmetricring spectrum’ (M U is really a unitary ring spectrum, as we shall define in Example 2.41 below).
In order to get an honest symmetric ring spectrum we now use a general construction which turns acommutative monoid ΦR in the category of symmetric sequences into a new such monoid by appropriately
looping all the spaces involved. We set
Φ(R)n = map(S n , Rn )an let the symmetric group act by conjugation. Then the product of R combined with smashing maps gives
Σn × Σm -equivariant maps
Φ(R)n ∧ Φ(R)m = map(S n , Rn ) ∧ map(S m , Rm ) −→ map(S n+m , Rn+m ) = Φ(R)n+m
f ∧g

7−→

f · g = µn,m ◦ (f ∧ g) .

Now we apply this construction to M U and obtain a commutative monoid M U = Φ(M U ) in the
category of symmetric sequences. We make M U into a symmetric ring spectrum via the unit map S 1 −→(M U )1 = map(S 1 , (M U )1 ) which is adjoint toι : S2 ∼= U (1)+ ∧U (1) S 2 −→ EU (1)+ ∧U (1) S 2 = (M U )1
using the ‘vertex map’ U (1) −→ EU (1). More precisely, we use the decomposition C = R · 1 ⊕ R · i to viewS 2 as the smash product of a ‘real’ and ‘imaginary’ circle, and then we view the source of the unit map
S 1 −→ (M U )1 = map(S 1 , (M U )1 ) as the real circle, and we think of the imaginary circle as parameterizing
the loop coordinate in the target (M U )1 . Since the multiplication of M U is commutative, the centrality
condition is automatically satisfied. Then the iterated unit map
S n −→ (M U )n = Ωn (M U )n
is given by(x1 , . . . , xn )7−→((y1 , . . . , yn ) 7→ µ(ι(x1 , y1 ), . . . , ι(xn , yn )))where µ : (M U )∧n
1 −→ (M U )n is the iterated multiplication map.
The homotopy groups of M U are given by
πk M U = colimn πk+n map(S n , (M U n )) ∼= colimn πk+2n (EU (n)+ ∧U (n) S 2n ) ;so by Thom’s theorem they are isomorphic to the ring of cobordism classes of stably almost complex kmanifolds. So even though the individual spaces M Un = map(S n , EU (n)+ ∧U (n) S 2n ) are not Thom spaces,
the symmetric spectrum which they form altogether has the ‘correct’ homotopy groups (and in fact, thecorrect stable homotopy type).
Essentially the same construction gives a commutative symmetric ring spectrum M SU . The symplectic
bordism and M Sp can also be handled similarly: it first arises as a commutative monoid M Sp in symmetricsequences with structure maps (M Sp)n ∧ S 4 −→ (M Sp)n+1 and a unit map S 4 −→ (M Sp)1 . If we apply
the construction Φ three times, we obtain a commutative symmetric ring spectrum M Sp = Φ3 (M Sp)
representing symplectic bordism.
Example 2.10 (Topological K-theory). We define the commutative symmetric ring spectrum KU ofcomplex topological K-theory. We set
KUn = hom(q n (C0 (Rn )), K(n) )where:• C0 (Rn ) is the C ∗ -algebra of continuous complex valued functions on Rn which vanish at infinity;

2. EXAMPLES

21• q n is the n-th iterate of a functor q which associates to a C ∗ -algebra its Cuntz algebra; qA is thekernel of the fold morphism A∗A −→ A, where the star is the categorical coproduct of C ∗ -algebras(a certain completion of the algebraic coproduct);• K is the C ∗ -algebra of compact operators on a fixed separable Hilbert space, and K(n) is the spatial
tensor product of n copies of K;• hom is the space of ∗-homomorphisms between two C ∗ -algebras with the subspace topology of thecompact open topology on the space of all continuous maps; the basepoint is the zero map.
Here C ∗ -algebras are not necessarily unital, and homomorphisms need not preserve units, if they exist.
There is an adjunction as follows: if K is a locally compact space and A and B are C ∗ -algebras, thenT (K̄, hom(A, B)) ∼= hom(A, C0 K ⊗ B)
where K̄ is the one-point compactification.
There is an action of the symmetric group on q n A, but it is not obvious from what we have saidso far [define it]. The symmetric group also acts on C0 (Rn ) by permutation of coordinates, on K(n) by
permutation of tensor factors, and on the mapping space KUn by conjugation.
In level 0 we have q 0 A = A, so q 0 (C0 (Rn )) = C, and K(0) = C. Thus we have KU0 = hom(C, C)
which consists of two elements, the zero and the identity homomorphism. So we define ι0 : S 0 :−→ KU0
as the homeomorphism which sends the basepoint to the zero homomorphism and the non-basepoint to
the identity of C. We have KU1 = hom(q(C0 (R)), K) [identify with infinite unitary group] and via theadjunction, the unit map ι1 : S 1 −→ KU1 corresponds to a C ∗ -homomorphism ι̂1 : q(C0 (R)) −→ C0 (R) ⊗ K
which is defined as the composite
q(C0 (R)) −→ C0 (R) −→ C0 (R) ⊗ K
where the first map is the restriction of the morphism 1 ∗ 0 : C0 (R) ∗ C0 (R) −→ C0 (R) to the kernel of thefold map and the second map sends f to f ⊗ e where e ∈ K is a fixed rank 1 projection. [this should be apositive Ω-spectrum. Necessary modifications to yield KO and KT . How about KSp ? C2 -action on KU ?]The Bott periodicity theorem says that there is a homotopy equivalence Ω2 BU ' Z × BU . It implies
that the homotopy ring of KU contains a unit in dimension 2, the Bott class u ∈ π2 KU . There is an
isomorphism of graded rings π∗ KU ∼= Z[u, u−1 ].
There is a variant, the symmetric spectrum of real topological K-theory KO. The real version has a
Bott-periodicity of order 8, i.e., there is a homotopy equivalenceΩ8 BO ' Z × BO ,which gives a unit β ∈ π8 KO.
The following table (and Bott periodicity) gives the homotopy groups of the spectrum KO:nπn KO
generator

0
Z
ι

1
2
Z/2 Z/2
η
η2

3
0

4
Z
ξ

5
0

6
0

7
0

8
Z
β

Here η is the Hurewicz image of the Hopf map, i.e., the image of the class η ∈ π1 S under the unique
homomorphism of ring spectra S −→ KO. There is a homomorphism of ring spectra KO −→ KU , the‘complexification map’, which is injective on homotopy groups in dimensions divisible by 4, and bijective indimensions divisible by 8. The elements ξ and β can be defined by the property that they hit 2u2 ∈ π4 KU
respectively u4 ∈ π8 KU under this complexification map. Thus there is the multiplicative relation ξ 2 = 4β.
Example 2.11 (Algebraic K-theory). There are various formalisms which associate to a category withsuitable extra structure an algebraic K-theory space. These spaces are typically infinite loop spaces ina natural way, i.e., they arise from an Ω-spectrum. One very general framework is Waldhausen’s S· construction which accepts categories with cofibrations and weak equivalences as input and which produces
symmetric spectra which are positive Ω-spectra.

22

I. BASIC DEFINITIONS AND EXAMPLES

We consider a category C with cofibrations and weak equivalences in the sense of Waldhausen [66]. For
any finite set Q we denote by P(Q) the power set of Q viewed as a poset under inclusions, and thus asa category. A Q-cube in C is a functor X : P(Q) −→ C. Such a Q-cube X is a cofibration cube if for all
S ⊂ T ⊂ Q the canonical map
colim
X(U ) −→ X(T )⊂S⊂U 6=Tis a cofibration in C. (The colimit on the left can be formed by iterated pushouts along cofibrations, so it
exists in C.)
We view the ordered set [n] = {0 < 1 < · · · < n} as a category. If n = {ns }s∈Q is a Q-tuple ofnon-negative integers, we denote by [n] the product category of the categories [ns ], s ∈ Q. For a morphismi → j in [n] and a subset U ⊂ Q we let (i → j)U be the new morphisms in [n] whose sth component is
is → js if s ∈ U and the identity is → is if s 6∈ U . Then for each morphism i → j in [n], the assignment
U 7→ (i → j)Udefines a Q-cube in the arrow category Ar[n].
For a finite set Q and a Q-indexed tuple n = {ns }s∈Q we define a category SnQ C as the full subcategoryof the category of functors from Ar[n] to C consisting of the functors
A : Ar[n] −→ C ,(i → j) 7→ Ai→jwith the following properties:(i) if some component is → js of i → j is an identity (i.e., if is = js for some s ∈ Q), then Ai→j = ∗ isthe distinguished zero object of C;(ii) for every pair of composable morphisms i → j → k the cube
U 7→ A(j→k)U ◦(i→j)
is a cofibration cube(iii) for every pair of composable morphisms i → j → k the squarecolim ⊂ A(j→k)U ◦(i→j)/ Ai→k∗/ Aj→kU 6=Q

is a pushout in C.
The category SnQ C depends contravariantly on [n], so that as [n] varies, we get a Q-simplicial category
Q
S· C. We can make S·Q C into a Q-simplicial object of categories with cofibrations and weak equivalences
as follows. A morphism f : A −→ A0 is a cofibration in SnQ C if for every pair of composable morphisms
i → j → k the induced map of Q-cubesU 7→ A(j→k)U ◦(i→j) −→ U 7→ A0(j→k)U ◦(i→j)
is a cofibration cube when viewed as a (|Q| + 1)-cube in C. A morphism f : A −→ A0 is a weak equivalencein SnQ C if for every morphism i −→ j in [n] the morphism fi−→j is a weak equivalence in C. If Q has one
element, the S·Q C is isomorphic to S· C as defined by Waldhausen [66]. If P ⊂ Q there is an isomorphismof Q-simplicial categories with cofibrations and weak equivalences
SQC ∼= S Q−P (S P C)···

[define]. So a choice of linear ordering of the set Q specifies an isomorphism of categories
S·Q C ∼
= S· · · · S· C
to the |Q|-fold iterate of the S· -construction. Note that the permutation group of the set Q acts on S·Q C
by permuting the indices.

2. EXAMPLES

23

Now we are ready to define the algebraic K-theory spectrum K(C) of the category with cofibrations
and weak equivalences C. (This is really naturally a coordinate free symmetric spectrum in the sense of
Remark 1.11.) It is the symmetric spectrum of simplicial sets with nth level given by{1,...,n}
K(C)n = N· wS·
C ,i.e., the nerve of the subcategory of weak equivalences in S·Q C for the special case Q = {1, . . . , n}. The{1,...,n}basepoint is the object of S·
C given by the constant functor with values the distinguished zero object.
{1,...,n}
The group Σn of permutations of the set {1, . . . , n}, acts on S·
C preserving weak equivalences, so it
acts on the simplicial set K(C)n . Note that K(C)0 is the nerve of the category wC of weak equivalences inC.
We still have to define the Σn × Σm -equivariant structure maps
K(C)n ∧ S m −→ K(C)n+m .
Consider a biexact functor ∧ : C × D −→ E between categories with cofibrations and weak equivalences. For
disjoint finite subsets Q and Q0 we obtain a biexact functor of (Q∪Q0 )-simplicial categories with cofibrations
and weak equivalences
00∧ : S·Q C × S·Q D −→ S·Q∪Q E
by assigning(A ∧ A0 )i∪i0 →j∪j 0 = Ai→j ∧ A0i0 →j .
We specialize to Q = {1, . . . , n} and Q0 = {n + 1, . . . , n + m}, restrict to weak equivalences and take nerves.
This yields a Σn × Σm -equivariant map K(C)n × K(D)m −→ K(E)n+m which factors as
K(C)n ∧ K(D)m −→ K(E)n+m .
These maps are associative for strictly associative pairings [explain].
The universal example of a category with cofibrations and weak equivalences which acts on any othersuch category is the category Γ of finite pointed sets n+ = {0, 1, . . . , n} with 0 as basepoint, and pointed
set maps. Here the cofibrations are the injective maps and the weak equivalences are the bijections. The‘smash product’ functor∧ : Γ × Γ −→ Γ , (n+ , m+ ) 7→ (nm)+is biexact and strictly associative so it makes the symmetric sequence {K(Γ)n }n≥0 into a strict monoid ofsymmetric sequences. Here we identify n+ ∧ m+ with (nm)+ using the lexicographic ordering. We define
a map of simplicial sets
ι0 : S 0 −→ K(Γ)0 = N· iΓ
by sending the basepoint to the identity map of the set 0+ and the non-basepoint to the identity map of
the set 1+ . The unit map
ι1 : S 1 −→ K(Γ)1 = N· iS·1 Γ
sends the non-degenerate 1-simplex of S 1 to the identity map of the set 1+ , which is a 1-simplex of thenerve of the category iS·1 Γ [check; remark that S −→ K(Γ) is a π∗ -isomorphism].
A biexact action∧ : C × Γ −→ C
is given by sending (A, n+ ) to a choice of n-fold coproduct of the object A. This makes K(C) into a module
over the symmetric ring spectrum K(Γ); in particular, the structure maps
Id ∧ι1
K(C)n ∧ S 1 −−−−→
K(C)n ∧ K(Γ)1 −→ K(C)n+1

make K(C) into a symmetric spectrum.

24

I. BASIC DEFINITIONS AND EXAMPLES

2.2. Constructions.
Example 2.12 (Free symmetric spectra). Given a pointed space K and m ≥ 0, we define a symmetric
spectrum Fm K which is ‘freely generated by K in level m’. So technically we construct a left adjoint
Fm : T −→ SpΣ to the forgetful functor which takes a symmetric spectrum X to the pointed space Xm .
The nth level of Fm K is given, as a pointed Σn -space byn−m(Fm K)n = Σ+.
n ∧1×Σn−m K ∧ S

Here 1 × Σn−m is the subgroup of Σn of permutations which fix the first m elements. The structure map
σn : (Fm K)n ∧ S 1 −→ (Fm K)n+1 is given by

n−m
n−m+1
Σ+
∧ S 1 −→ Σ+n ∧1×Σn−m K ∧ S
n+1 ∧1×Σn+1−m K ∧ S[τ, k ∧ x1 ∧ · · · ∧ xn−m ] ∧ xn−m+1 7−→ [τ, k ∧ x1 ∧ · · · ∧ xn−m ∧ xn−m+1 ]Free symmetric spectra generated in level zero are just suspension spectra, i.e., there is a natural isomorphism F0 K ∼= Σ∞ K.
We calculate the 0th stable homotopy group of the symmetric spectrum F1 S 1 . Explicitly, F1 S 1 is givenby1
n−1(F1 S 1 )n = Σ+.
n ∧1×Σn−1 S ∧ S
So (F1 S 1 )n is a wedge of n copies of S n and in the stable range, i.e., up to roughly dimensions 2n, thehomotopy groups of (F1 S 1 )n are a direct sum of n copies of the homotopy groups of S n . Moreover, inthe stable range, the map in the colimit system (1.5) is a direct summand inclusion into (n + 1) copies of
the homotopy groups of S n . Thus in the colimit, the stable homotopy groups of the symmetric spectrum
F1 S 1 are a countably infinite direct sum of copies of the stable homotopy groups of spheres. In particular,
F1 S 1 is not π∗ -isomorphic to the sphere spectrum S, whose zeroth homotopy group is a single copy of theintegers. Still, as we shall explain in Example 4.2 of Chapter II, F1 S 1 represents the same stable homotopy
type as the sphere spectrum S. More generally, the free symmetric spectrum Fn K is stably equivalent toan n-fold desuspension of the suspension spectrum of K.
Example 2.13 (Semifree symmetric spectra). There are somewhat ‘less free’ symmetric spectra startingfrom a pointed Σm -space L which we want to install in level m, and then fill in the remaining data of asymmetric spectrum in the freest possible way. In other words, we claim that the forgetful functor
Sp −→ Σm -T ,X 7−→ Xm

has a left adjoint which we denote Gm ; we refer to Gm L as the semifree symmetric spectrum generated by
L in level m. In level n we haven−m
(Gm L)n = Σ+.
n ∧Σm ×Σn−m L ∧ S

The structure map σn : (Gm K)n ∧ S 1 −→ (Gm K)n+1 is defined by the same tautological formula as in theprevious example. In fact every free symmetric spectrum is semifree, i.e., there is a natural isomorphism
Fm K ∼= Gm (Σ+m ∧ K).
Example 2.14 (Limits and colimits). The category of symmetric spectra has all limits and colimits,and they are defined levelwise. Let us be a bit more precise and consider a functor F : J −→ SpΣ from asmall category J to the category of symmetric spectra. Then we define a symmetric spectrum colimJ F inlevel n by(colimJ F )n = colimj∈J F (j)n ,the colimit being taken in the category of pointed Σn -spaces. The structure map is the composite
colim σJ n(colimj∈J F (j)n ) ∧ S 1 ∼= colimj∈J (F (j)n ∧ S 1 ) −−−−−−→ colimj∈J F (j)n+1 ;here we exploit that smashing with S 1 is a left adjoint, and thus the natural map colimj∈J (F (j)n ∧ S 1 ) −→(colimj∈J F (j)n ) ∧ S 1 is a homeomorphism, whose inverse is the first map above.

2. EXAMPLES

25

The argument for inverse limits is similar, but we have to use that structure maps can also be definedin the adjoint form. We can take(limJ F )n = limj∈J F (j)n ,and the structure map is adjoint to the composite
lim σ̂nlimj∈J F (j)n −−−J−−→
limj∈J Ω(F (j)n+1 ) ∼= Ω (limj∈J F (j)n+1 ) .

[Same for modules]
The inverse limit, calculated levelwise, of a diagram of symmetric ring spectra and homomorphisms
is again a symmetric ring spectrum. In other words, symmetric ring spectra have limits and the forgetful
functor to symmetric spectra preserves them. Symmetric ring spectra also have co-limits, but they are notpreserved by the forgetful functor.
Example 2.15 (Smash products with spaces). If K is pointed space and X a symmetric spectrum, we
define a new symmetric spectrum K ∧ X by smashing K levelwise with the terms of X, i.e., (K ∧ X)n =K ∧ Xn . The structure map does not interact with the space K, i.e., it is given by the composite
Id ∧σ(K ∧ X)n ∧ S 1 = K ∧ Xn ∧ S 1 −−−−−n→ K ∧ Xn+1 = (K ∧ X)n+1 .
For example, the spectrum K ∧ S is equal to the suspension spectrum Σ∞ K. [Same for modules]
Example 2.16 (Suspension). A special case of the previous construction is the suspension S 1 ∧ X of asymmetric spectrum X. So S 1 ∧ X is defined by applying the functor S 1 ∧ − levelwise, where the structuremaps do not interact with the new suspension coordinate.
We claim that suspension simply shifts the homotopy groups of a symmetric spectrum X. The maps
S 1 ∧ − : πk+n Xn −→ π1+k+n (S 1 ∧ Xn ) for varying n form part of a diagram(2.17)

S 1 ∧−

πk+n Xn−∧S 1r
πk+n+1 (Xn ∧ S 1 )(−1)k+n τ∗−∧S 1

π1+k+n+1 (S 1 ∧ Xn ∧ S 1 )(S 1 ∧σn )∗(σn )∗


πk+n+1 Xn+1

/ π1+k+n (S 1 ∧ Xn )S 1 ∧−

/ π1+k+n+1 (S 1 ∧ Xn+1 )whose outer rectangle commutes. This shows first of all that the maps S 1 ∧ − are compatible with thestabilization process for the homotopy groups of X respectively S 1 ∧ X, and thus induce a natural map
S 1 ∧ − : πk X −→ π1+k (S 1 ∧ X) .
We claim that this map is an isomorphism.
For this purpose we consider the diagonal dotted morphism which involves the twist isomorphismτ : Xn ∧ S 1 −→ S 1 ∧ Xn which interchanges the two factors in a smash product. This dotted morphism
makes the upper left triangle in diagram (2.17) commute, which implies that the map S 1 ∧ − : πk X −→π1+k (S 1 ∧ X) is injective.
However, the dotted morphism does not in general make the lower right portion of diagram (2.17)
commute as it stands, but it does after another suspension. Indeed, if we ignore the signs for a moment, then
the two ways from π1+k+n (S 1 ∧Xn ) to π1+k+n+1 (S 1 ∧Xn+1 ) send the class of a map α : S 1+k+n −→ S 1 ∧Xn

26

I. BASIC DEFINITIONS AND EXAMPLES

to the respective composites in the square/ S 1 ∧ Xn ∧ S 1
~~
~~~~
~
S 1 ∧χk+n,1
~
~
~~~

~~~~
~
~
 ~~~~~
S 1+1+k+n
~~~
~~~~
~
~
1
S 1 ∧σn
S ∧α
~~~~
~
~

~
~
~~~~
S 1 ∧ S 1 ∧ Xn ~~~~~~
~~
~~~~
~
~
S 1 ∧τ~~~~
 ~~~~/ S 1 ∧ Xn+1 .
S 1 ∧ Xn ∧ S 1
S 1+k+n+1α∧S 1

S 1 ∧σnThe upper left part of this diagram does not commute ! The two composites from S 1+k+n+1 to S 1 ∧Xn ∧S 1
differ by the automorphisms of S 1+k+n+1 and S 1 ∧ Xn ∧ S 1 which interchanges the outer two sphere
coordinates in each case. This coordinate change in the source induces multiplication by −1; the coordinatechange in the target is a map of degree −1, so after a single suspension it also induces multiplication by −1
on homotopy groups (see Lemma 2.3).
Example 2.18 (Shift). There is another construction for symmetric spectra which, like the suspension,
reindexes the homotopy groups. The shift of a symmetric spectrum X is given by(sh X)n = X1+nwith action of Σn via the monomorphism (1 × −) : Σn −→ Σ1+n which is explicitly given by (1 × γ)(1) = 1
and (1 × γ)(i) = γ(i − 1) + 1 for 2 ≤ i ≤ 1 + n. The structure maps of sh X are the reindexed structuremaps for X.
For any symmetric spectrum X, integer k and large enough n we haveπ(k+1)+n (sh X)n = πk+(1+n) X1+n ,and the maps in the colimit system for πk+1 (sh X) are the same as the maps in the colimit system for πk X.
Thus we get πk+1 (sh X) = πk X.
Warning: the suspension and shift construction both shift the homotopy groups, but, however, there
is in general no morphism between S 1 ∧ X and sh X which induces an isomorphism of homotopy groups.
This is closely related to the phenomenon of ‘semistability’, which we discuss in more detail in Section 4.5.
There is an important natural morphism λ : S 1 ∧ X −→ sh X for every symmetric spectrum X whose leveln component λn : S 1 ∧ Xn −→ X1+n is the composite(2.19)

∼
=

σ

χn,1

n
S 1 ∧ Xn −−−−→ Xn ∧ S 1 −−−→ Xn+1 −−−→ X1+n .

twist

One should note that using only the structure map σn without the twist isomorphism and permutation
χn,1 = (1, . . . , n, n + 1) does not yield a morphism of symmetric spectra ! The morphism λ is not in generala π∗ -isomorphism, but when it is, the symmetric spectrum X is called semistable (compare Theorem 4.44).
We can iterate the shift construction and get (shm X)n = Xm+n . In every level of the symmetric
spectrum shm X the symmetric group Σm acts via the ‘inclusion’ (− × 1) : Σm −→ Σm+n , and these actions
are compatible with the structure maps. So in this way shm X becomes a Σm -symmetric spectrum.
Example 2.20 (Twisted smash product). The twisted smash product starts from a number m ≥ 0, a
pointed Σm -space (or Σm -simplicial set) L and a symmetric spectrum X and produces a new symmetric
spectrum which we denote L.m X. This construction is a simultaneous generalization of semifree symmetric
spectra (Example 2.13) and the smash product of a space and a spectrum (Example 2.15) and also provides
a left adjoint to the shift construction (Example 2.18). Once the internal smash product of symmetric2. EXAMPLES

27

spectra is available, we will identify the twisted smash product L .m X with the smash product of Gm L
and X, see Proposition 3.5 below.
We define the twisted smash product L .m X as a point in levels smaller than m and in general by(L .m X)m+n = Σ+m+n ∧Σm ×Σn L ∧ Xn .
The structure map σm+n : (L .m X)m+n ∧ S 1 −→ (L .m X)m+n+1 is obtained from Id ∧σn : L ∧ Xn ∧ S 1 −→L ∧ Xn+1 by inducing up.
Here are some special cases. Taking X = S gives semifree and free symmetric spectra as
L .m S = Gm L respectively(Σ+m ∧ K) .m S = Fm K .

For m = 0 we get
K .0 X = K ∧ X ,the levelwise smash product of K and X. The twisted smash product has an associativity property in theform of a natural isomorphism0
L .m (L0 .n X) ∼= (Σ+m+n ∧Σm ×Σn L ∧ L ) .m+n X .

The twisted smash product is related by various adjunctions to other constructions. As we noted at theend of Example 2.18, the m-fold shift of a symmetric spectrum Z has an action of Σm through spectrum
automorphisms, i.e., shm Z is a Σm -symmetric spectrum. The levelwise smash product L ∧ X (in thesense of Example 2.15) of the underlying space of L and X also is a Σm -symmetric spectrum through theaction on L. Given a morphism f : L .m X −→ Z of symmetric spectra, we can restrict the component
in level m + n to the summand 1 ∧ L ∧ Xn in (L .m X)m+n and obtain a Σm × Σn -equivariant based map
f¯n = fm+n (1 ∧ −) : L ∧ Xn −→ Zm+n = (shm Z)n . The compatibility of the fm+n ’s with the structuremaps translates into the property that the maps f¯ = {f¯n }n≥0 form a morphism of Σm -symmetric spectrafrom L ∧ X to shm Z. Conversely, every Σm -equivariant morphism L ∧ X −→ shm Z arises in this way froma morphism f : L .m X −→ Z. In other words, the assignment f 7→ f¯ is a natural bijection of functors(2.21)SpΣ (L .m X, Z) ∼= Σm -SpΣ (L ∧ X, shm Z) .

The case m = 1 and L = S 0 gives a bijection,SpΣ (S 0 .1 X, Z) ∼= SpΣ (X, sh Z) ,natural in the symmetric spectra X and Z, which shows that X 7→ S 0 .1 X is left adjoint to shifting. [Samefor modules]
Example 2.22 (Function spectra). If X is a symmetric spectrum and K a pointed space, we define asymmetric function spectrum X K by
(X K )n = XnK = map(K, Xn )with Σn -action induced by the action on Xn . The structure map is the composite
K
XnK ∧ S 1 −→ (Xn ∧ S 1 )K −→ Xn+1

where the first map is adjoint to the evaluation map XnK ∧ S 1 ∧ K −→ Xn ∧ S 1 and the second is applicationof map(K, −) to the structure map of X. [Same for modules]
We note that if X is an Ω-spectrum, then so is X K , provided we also assume that
• K is a CW-complex (when in the context of topological spaces), or• X is levelwise a Kan complex (when in the context of simplicial sets).
Indeed, under either hypothesis, the map mapping space functor map(K, −) takes the weak equivalenceσ̃n : Xn −→ ΩXn+1 to a weak equivalencemap(K,σ̃n )K
XnK = map(K, Xn ) −−−−−−−→ map(K, ΩXn+1 ) ∼).
= Ω(Xn+1

28

I. BASIC DEFINITIONS AND EXAMPLES+

If R is a symmetric ring spectrum and L an unpointed space, then RL is again a symmetric ring+L+L+
spectrum. The multiplication maps RnL ∧ Rm−→ Rn+mare the composites
map(diagonal,µn,m )∧map(L+ , Rn ) ∧ map(L+ , Rm ) −−→ map(L+ ∧ L+ , Rn ∧ Rm ) −−−−−−−−−−−−−→ map(L+ , Rn+m )+using the diagonal map L+ −→ (L × L)+ ∼= L+ ∧ L+ . Associativity of the multiplication on RL comes+from associativity of R and coassociativity of the diagonal map. The unit map ιn : S n −→ RnL is the0+composite of the unit map of R with the map Rn ∼= RnS −→ RnL induced by the based map L+ −→ S 00which takes all of L to the non-basepoint of S . If the multiplication of R is commutative, then so is the+multiplication of RL , since the diagonal map is cocommutative.
In good cases (namely if X is semistable), the homotopy groups of X K are the values of the reduced
cohomology theory represented by X on the space K, i.e,πk (X K ) ∼= X̃ −k (K) .
A concrete example if X = HA, the Eilenberg-Mac Lane spectrum of an abelian group A. Then
πk (HAK ) ∼= H̃ −k (K; A) ,the reduced singular cohomology of K with coefficients in A. If A is a ring, then HA becomes a ring
spectrum and this isomorphism takes the product of homotopy groups to the cup product in singularcohomology.
Example 2.23 (Loops). The loop ΩX of a symmetric spectrum X is defined by applying the functorΩ = map(S 1 , −) levelwise, where the structure maps do not interact with the new loop coordinate. In otherwords, ΩX is the special case of a function spectrum as in the previous example. We claim that looping
shifts the homotopy groups.
We use the isomorphism α : πk+n Ω(Xn ) ∼= π1+k+n Xn defined by sending a representing continuous
map f : S k+n −→ Ω(Xn ) to the class of the adjoint fˆ : S 1+k+n −→ Xn given by fˆ(s ∧ t) = f (t)(s), wheres ∈ S 1 , t ∈ S k+n . As n varies, these particular isomorphisms are compatible with stabilization maps, sothey induce an isomorphism∼=

α : πk (ΩX) −−→ π1+k X
on colimits. The composite
α−1

S 1 ∧−π∗ X −−−−−→ π1+∗ (S 1 ∧ X) −−−→ π∗ (Ω(S 1 ∧ X))
is the map induced by the adjunction unit X −→ Ω(S 1 ∧ X) on homotopy, where S 1 ∧ − is the isomorphismof Example 2.16. Similarly, the composite(S 1 ∧−)−1απ1+∗ (S 1 ∧ (ΩX)) −−−−−−−→ π∗ (ΩX) −−→ π1+∗ X
is the map induced by the adjunction counit S 1 ∧ ΩX −→ X on homotopy.
What we said about the loop spectrum works as well for symmetric spectra of simplicial sets as longas they are levelwise Kan complexes.
Example 2.24 (Mapping spaces). There is a whole space, respectively simplicial set, of morphisms
between two symmetric spectra. For symmetric spectra X and Y of topological spaces, every morphismfrom f : X −→ Y consists of a family of based continuous maps {fn : Xn −→ Yn }n≥0 which satisfy
someconditions. So the set of morphisms from X to Y is a subset of the product of mapping spaces
Qn≥0 T (Xn , Yn ) and we give it the subspace topology of the (compactly generated) product topology. We
denote this mapping space by map(X, Y ).
Now suppose that X and Y are symmetric spectra of simplicial sets. Then the mapping space map(X, Y )is the simplicial set whose n-simplices are the spectrum morphisms from ∆[n]+ ∧ X to Y . For a monotone
map α : [n] −→ [m] in the simplicial category ∆, the map α∗ : map(X, Y )m −→ map(X, Y )n is given byprecomposition with α∗ ∧ IdX : ∆[n]+ ∧ X −→ ∆[m]+ ∧ X. The morphism space has a natural basepoint,2. EXAMPLES

29

namely the trivial map from ∆[0]+ ∧ X to Y . We can, and will, identify the vertices of map(X, Y ) withthe morphisms from X to Y using the natural isomorphism ∆[0]+ ∧ X ∼= X.
The topological and simplicial mapping spaces are related by various adjunctions. We list some ofthese. For a simplicial spectrum X and a topological spectrum Y there is an isomorphism of simplicial setsmap(X, S(Y )) ∼= S(map(|X|, Y ))which on vertices specializes to the adjunction between singular complex and geometric realization.
Furthermore, for a pointed space K and topological symmetric spectra X and Y we have adjunction
homeomorphisms
T (K, map(X, Y )) ∼= T (K ∧ X, Y ) ∼= map(X, Y K ) .
For free symmetric spectra we have homeomorphisms
map(Fm K, Y ) ∼= T (K, Ym ) .
For K = S 0 this specializes to a homeomorphism map(Fm S 0 , Y ) ∼= Ym . In the context of symmetric spectraof simplicial sets, the analogous isomorphisms of mapping simplicial sets hold as well.
We have associative and unital composition maps
map(Y, Z) ∧ map(X, Y ) −→ map(X, Z) .
Indeed, for symmetric spectra of topological spaces this is just the observation that composition of morphisms is continuous for the mapping space topology. For symmetric spectra of simplicial sets the composition maps are given on n-simplices bySpΣ (∆[n]+ ∧ Y, Z) ∧ SpΣ (∆[n]+ ∧ X, Y ) −→ SpΣ (∆[n]+ ∧ X, Z)
7−→ g ◦ (Id∆[n]+ ∧f ) ◦ (diag+ ∧ IdX )g∧f

Example 2.25 (Internal Hom spectra). Symmetric spectra have internal function objects: for symmetric spectra X and Y we define a symmetric spectrum Hom(X, Y ) in level n by
Hom(X, Y )n = map(X, shn Y )with Σn -action induced by the action on shn Y as described in Example 2.18. The structure map σn :Hom(X, Y )n ∧ S 1 −→ Hom(X, Y )n+1 is the composite
map(X,χ1,n )map(X,λshn Y )map(X, shn Y ) ∧ S 1 −→ map(X, S 1 ∧ shn Y ) −−−−−−−−−−→ map(X, sh1+n Y ) −−−−−−−−→ map(X, shn+1 Y )where λshn Y : S 1 ∧ shn Y −→ sh(shn Y ) is the natural morphism defined in (2.19).
The internal function spectrum Hom(−, −) is adjoint to the internal smash product of symmetric
spectra, to be discussed in Section 3. A natural isomorphism of symmetric spectra Hom(Fm S 0 , Y ) ∼= shm Yis given at level n by
χn,mHom(Fm S 0 , Y )n = map(Fm S 0 , shn Y ) ∼= (shn Y )m = Yn+m −−−→ Ym+n = (shm Y )n .
In the special case m = 0 we have F0 S 0 = S, which gives a natural isomorphism of symmetric spectra
Hom(S, Y ) ∼=Y.
Example 2.26 (Endomorphism ring spectra). For every symmetric spectrum X, the symmetric function
spectrum Hom(X, X) defined in Example 2.25 has the structure of a symmetric ring spectrum which we callthe endomorphism ring spectrum of X. The multiplication map µn,m : Hom(X, X)n ∧ Hom(X, X)m −→Hom(X, X)n+m is defined as the composite
shm ∧ Id

map(X, shn X) ∧ map(X, shm X) −−−−−→ map(shm X, shm+n X) ∧ map(X, shm X)◦

map(X,χm,n )−−−−→ map(X, shm+n X) −−−−−−−−−→ map(X, shn+m X)
where the second map is the composition pairing of Example 2.24. While this construction always works on
the pointset level, one can only expect Hom(X, X) to be homotopically meaningful under certain conditions
on X. The stable model structures which we discuss later will explain which conditions are sufficient.

30

I. BASIC DEFINITIONS AND EXAMPLES

In much the same way as above we can define associative and unital action maps Hom(X, Z)n ∧Hom(X, X)m −→ Hom(X, Z)n+m and Hom(X, X)n ∧ Hom(Z, X)m −→ Hom(Z, X)n+m for any othersymmetric spectrum Z. This makes Hom(X, Z) and Hom(Z, X) into right respectively left modules overthe endomorphism ring spectrum of X.
Example 2.27 (Telescope and diagonal of a sequence). We will sometimes be confronted with a sequenceof morphisms of symmetric spectra(2.28)f0

f1

f2

X 0 −−→ X 1 −−→ X 2 −−→ · · ·of which we want to take a kind of colimit in a homotopy invariant way, and such that the homotopy groupsof the ‘colimit’ are the colimits of the homotopy groups. We describe two constructions which do this job,
the mapping telescope and the diagonal.
The mapping telescope teli X i of the sequence (2.28) is a classical construction for spaces which weapply levelwise to symmetric spectra. It is defined as the coequalizer of two maps of symmetric spectra
W
+
i
i
// W
i≥0 [i, i + 1] ∧ X
i≥0 X
Here [i, i + 1] denotes a copy of the unit interval (when in the context of spaces) respectively the 1-simplex∆[1] (when in the context of simplicial sets). One of the morphisms takes X i to {i + 1}+ ∧ X i by theidentity, the other one takes X i to {i + 1}+ ∧ X i+1 by the morphism f i .
The diagonal diagi X i of the sequence (2.28) is the symmetric spectrum given by(diagi X i )n = Xnn ,i.e., we take the n-th level of the n-th symmetric spectrum with its given Σn -action. The structure map(diagi X i )n ∧ S 1 −→ (diagi X i )n+1 is the composite around either way in the commutative squareXnn ∧ S 1

n
σn

n
fn+1

n
fn
∧Id


Xnn+1 ∧ S 1n/ Xn+1n+1σn


/ X n+1
n+1

Lemma 2.29. For every sequence of symmetric spectra (2.28), the k-th homotopy group of the diagonal
symmetric spectrum is naturally isomorphic to the colimit of the k-th homotopy groups of the spectra X i ,along the maps πk (f i ),(2.30)πk (diagi X i ) ∼= colimi πk (X i ) .

For symmetric spectra of simplicial sets or symmetric spectra of non-degenerately based spaces there is achain of two natural π∗ -isomorphisms between the diagonal diagi X i and the mapping telescope teli X i ofthe sequence.
Proof. The right hand side of (2.30) is a sequential colimit of groups which are themselves sequential
colimits, and it is thus the colimit over the poset N × N of the functor (n, i) 7→ πk+n (Xni ). The left handside πk (diagi X i ) equals the colimit over the diagonal terms in this system. Since the diagonal embedding
N −→ N × N is cofinal, the colimit over the diagonal terms is isomorphic to the colimit over N × N, which
proves the isomorphism (2.30).
To prove the second statement we use the ‘partial telescopes’ tel[0,n] X i , the coequalizer of two maps ofsymmetric spectraWn
Wn−1 i//[i, i + 1]+ ∧ X i
X
i=0

i=0

i

defined as before. The spectrum tel[0,n] X includes into the next spectrum tel[0,n+1] X i with (categorical)
colimit the mapping telescope. The morphism cn : tel[0,n] X i −→ X n which projects each wedge summand

2. EXAMPLES

31[i, i + 1]+ ∧ X i onto X i and then applies the morphism f n−1 · · · f i : X i −→ X n is a homotopy equivalence.
The commutative diagram of symmetric spectra/ tel[0,2] X i/ tel[0,1] X itel[0,0] X i
c0

c1


X0


/ X1

f0

/ ···

c2

f1


/ X2

f2

/ ···

induces a morphism
diagn (tel[0,n] X i ) −→ diagn X n
on diagonals which is thus levelwise a homotopy equivalence, hence a π∗ -isomorphism. On the other hand
we have a morphism of symmetric spectra
diagn (tel[0,n] X i ) −→ teli X iwhich is levelwise given by the inclusion of a partial telescope in the full mapping telescope. This morphism
is a π∗ -isomorphism by the same kind of cofinality argument as in the first part. Here we use that an(unstable) homotopy group of the mapping telescope of a sequence of simplicial sets or non-degenerately
based spaces is the colimits of the sequence of homotopy groups.
Both mapping telescope and diagonal preserve module structures. Suppose that each symmetric spectrum X i in the sequence (2.28) has the structure of right module over a symmetric ring spectrum R and that
all morphisms f i are R-linear. Then the mapping telescope is naturally an R-module since all constructions
used to build it preserves the action by the ring spectrum. The diagonal is naturally an R-module as well,with action maps (diagi X i )n ∧ Rm −→ (diagi X i )n+m equal to the composite around either way in thecommutative squareXnn ∧ Rmαnn,m(f n+m−1 ···f n )n ∧Id
Xnn+m ∧ Rmn
/ Xn+m



αn+m
n,m

(f n+m−1 ···f n )n+m

/ X n+mn+m

Let me point out two advantages of the diagonal construction over the mapping telescope of a sequenceof symmetric spectra. One advantage is that the formula (2.30) is proved formally, and it holds without any
assumptions on the symmetric spectra involved. So when we work with symmetric spectra of topologicalspaces, no hypothesis about non-degenerate basepoints is needed. Another advantage is that the diagonal
construction has nicer formal and in particular multiplicative properties, as we shall see, for example, in
Examples 2.33 and 4.65.
Example 2.31 (I-spaces). Symmetric spectra are intimately related to the category I of finite sets andinjective maps. Here we denote by I the category with objects the sets n = {1, . . . , n} for n ≥ 0 (where 0 is
the empty set) and with morphisms all injective maps. In other words, I is the subcategory of the category
Fin of standard finite sets (compare Remark 1.12) with only injective maps as morphisms. We denote by
T I the category of I-spaces, i.e., covariant functors from I to the category of pointed spaces.
Given an I-space T : I −→ T and a symmetric spectrum X, we can form a new symmetric spectrum
T ∧ X by setting(T ∧ X)n = T (n) ∧ Xn
with diagonal action of Σn (which equals the monoid of endomorphism of the object n of I). The structure
map is given byT (ι)∧σn(T ∧ X)n ∧ S 1 = T (n) ∧ Xn ∧ S 1 −−−−−→ T (n + 1) ∧ Xn+1 = (T ∧ X)n+1
where ι : n −→ n + 1 is the inclusion. If K is a pointed space and T the constant functor with values K,then T ∧ X is equal to K ∧ X, i.e., this construction reduces to the pairing of Example 2.15.

32

I. BASIC DEFINITIONS AND EXAMPLES

Example 2.32 (Monoid ring spectra). If R is a symmetric ring spectrum and M a topological or
simplicial monoid (depending on the kind of symmetric spectra), we can define a symmetric ring spectrum
R[M ] by R[M ] = M + ∧ R, i.e., the levelwise smash product with M with a disjoint basepoint added. The
unit map is the composite of the unit map of R and the morphism R ∼= {1}+ ∧ R −→ M + ∧ R induced by
the unit of M . The multiplication map µn,m is given by the composite
mult.∧µn,m(M + ∧ Rn ) ∧ (M + ∧ Rm ) ∼= (M × M )+ ∧ (Rn ∧ Rm ) −−−−−−−−−→ M + ∧ Rn+m .

If both R and M are commutative, then so is R[M ]. A right module over the symmetric ring spectrum
R[M ] amounts to the same data as an R-module together with a continuous (or simplicial) right action of
the monoid M by R-linear endomorphisms.
The construction of the monoid ring over S is left adjoint to the functor which takes a symmetricring spectrum R to the simplicial monoid R0 . If R is semistable (see Theorem 4.44), then the homotopy
groups of R[M ] are the R-homology groups of the underlying space of M , with the Pontryagin product asmultiplication. In the special case of a discrete spherical monoid ring, the homotopy groups are the monoid
ring, in the ordinary sense, of the homotopy groups, i.e., there is a natural isomorphismπ∗ R[M ] ∼= (π∗ R)[M ] .
Example 2.33 (Ring spectra from multiplicative I-spaces). We can use the construction which pairs an
I-space with a symmetric spectrum (see Example 2.31) to produce symmetric ring spectra which model thesuspension spectra of certain infinite loop spaces such as BO, the classifying space of the infinite orthogonalgroup, even if these do not have a strictly associative multiplication. This works for infinite loop spaces
which can be represented as ‘monoids of I-spaces’, as we now explain.
The symmetric monoidal sum operation restricts from the category Fin of standard finite sets to thecategory I. Thus I has a symmetric monoidal product ‘+’ given by addition on objects and defined formorphisms f : n −→ n0 and g : m −→ m0 we define f + g : n + m −→ n0 + m0 by(f (i)if 1 ≤ i ≤ n, and(f + g)(i) =g(i − n) + n0 if n + 1 ≤ i ≤ n + m.
The product + is strictly associative and has the object 0 as a strict unit. The symmetry isomorphism isthe shuffle map χn,m : n + m −→ m + n.
Consider an I-space T : I −→ T with a pairing, i.e., an associative and unital natural transformationµn,m : T (n) ∧ T (m) −→ T (n + m). If R is a symmetric ring spectrum, then the smash product T ∧ R (seeExample 2.31) becomes a symmetric ring spectrum with respect to the multiplication map(T ∧ R)n ∧ (T ∧ R)m −→ (T ∧ R)n+m
defined as the compositeµn,m ∧µn,m

Id ∧twist∧Id

T (n) ∧ Rn ∧ T (m) ∧ Rm −−−−−−−−→ T (n) ∧ T (m) ∧ Rn ∧ Rm −−−−−−−→ T (n + m) ∧ Rn+m .
If the transformation µ is commutative in the sense that the square
T (n) ∧ T (m)

µn,m

twist


T (m) ∧ T (n)/ T (n + m)
µm,n

T (χn,m )/ T (m + n)

commutes for all n, m ≥ 0 and if the multiplication of R is commutative, then the product of T ∧ R is also
commutative. This construction generalizes monoid ring spectra (see Example 2.32): if M is a topological(respectively simplicial) monoid, then the constant I-functor with values M + inherits an associative andunital product from M which is commutative if M is. The smash product of a ring spectrum R with sucha constant multiplicative functor equals the monoid ring spectrum R[M ].

2. EXAMPLES

33

Later we shall also discuss an internal ‘tensor product’ of I-spaces and an internal smash product ofsymmetric spectra and see that these constructions are intimately related.
A more interesting instance of this construction is a commutative symmetric ring spectrum whichmodels the suspension spectrum of the space BO+ , the classifying space of the infinite orthogonal groupwith a disjoint basepoint. Here we start with the ‘I-topological group’ O, a functor from I to topologicalgroups whose value at n is O(n), the n-th orthogonal group. The behavior on morphisms is determined byrequiring that a permutation γ ∈ Σn acts as conjugation by the permutation matrix associated to γ andthe inclusion ι : n −→ n + 1 inducesA 0ι∗ : O(n) −→ O(n + 1) , A 7−→.
0 1
A general injective set map α : n −→ m then induces the group homomorphism α∗ : O(n) −→ O(m) givenbyAα−1 (i),α−1 (j) if i, j ∈Im(α),(α∗ A)i,j =1
if i = j and i 6∈Im(α),
0if i 6= j and i or j is not contained in Im(α).
Orthogonal sum of matrices gives a natural transformation of group valued functors

A 0O(n) × O(m) −→ O(n + m) , (A, B) 7−→.
0 B
This transformation is unital, associative and commutative, in a sense which by now is hopefully clear.
The classifying space functor B takes topological groups to pointed topological spaces and commutes withproducts up to unital, associative and commutative homeomorphism. So by taking classifying spaces
objectwise we obtain an I-space BO with values BO(n) = BO(n). This I-space inherits a unital, associativeand commutative product in the sense discussed above, but with respect to the cartesian product, as
opposed to the smash product, of spaces. So if we add disjoint basepoints and perform the construction
above, we obtain a symmetric spectrum BO+ ∧ S whose value in level n is the space BO(n)+ ∧ S n . By ourprevious discussion, this is a commutative symmetric ring spectrum. Since the connectivity of the maps
Bι∗ : BO(n) −→ BO(n + 1) tends to infinity with n, the underlying symmetric spectrum of BO ∧ S isπ∗ -isomorphic to the suspension spectrum Σ∞ BO+ [justify].
This construction can be adapted to yield commutative symmetric ring spectra which model the suspension spectra of BSO, BSpin, BU, BSU and BSp, which disjoint basepoints added. In each case, therespective family of classical groups fits into an ‘I-topological group’ with commutative product, and fromthere we proceed as for the orthogonal groups. More examples of the same kind are obtained from familiesof discrete groups which fit into ‘I-groups’ with commutative product, for example symmetric groups, alternating groups or general or special linear groups over some ring. [is commutativity of the ring needed?
check BSp]
Example 2.34 (Matrix ring spectra). If R is a symmetric ring spectrum and k ≥ 1 we define thesymmetric ring spectrum Mk (R) of k × k matrices over R by
Mk (R) = map(k + , k + ∧ R) .
Here k + = {0, 1, . . . , k} with basepoint 0, and so Mk (R) is a k-fold product of a k-fold coproduct (wedge)of copies of R. So ‘elements’ of Mk (R) are more like matrices which in each row have at most one nonzero
entry. The multiplicationµn,m : map(k + , k + ∧ Rn ) ∧ map(k + , k + ∧ Rm ) −→ map(k + , k + ∧ Rn+m )sends f ∧ g to the composite
g

f ∧Rm

µn,m

k + −−→ k + ∧ Rm −−−−−→ k + ∧ Rn ∧ Rm −−−→ Rn+m .

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I. BASIC DEFINITIONS AND EXAMPLES

Since homotopy groups take wedges and products to direct sums, we get a natural isomorphism of gradedabelian groupsπ∗ (Mk (R)) ∼= Mk (π∗ R) .
If R semistable, then so is Mk (R) and the above is an isomorphism of graded rings [prove this later].
Example 2.35 (Inverting m). We consider an integer m and define S[1/m], the sphere spectrum withm inverted by starting from the sphere spectrum and using a map ϕm : S 1 −→ S 1 of degree m as the newunit map ι1 . Since the multiplication on S is commutative, centrality is automatic. So S[1/m] has the same
spaces and symmetric group actions as S, but the n-th unit map ιn of S[1/m] is the n-fold smash power of
ϕm , which is a self map of S n of degree mn. The unit maps form a morphism S −→ S[1/m] of symmetricring spectra which on homotopy groups induces an isomorphismπ∗ S[1/m] ∼= π∗ S ⊗ Z[1/m] .
For m = 0, the homotopy groups are thus trivial and for m = 1 or m = −1 the unit morphism S −→ S[1/m]is a π∗ -isomorphism.
Example 2.36 (Inverting homotopy elements). Let R be a symmetric ring spectrum and let x : S 1 −→R1 be a central map of pointed spaces, i.e., the square
Id ∧x

Rn ∧ S 1/ Rn ∧ R1µn,1/ Rn+1χn,1τS 1 ∧ Rnx∧Id

/ R1 ∧ R n

/ R1+nµ1,ncommutes for all n ≥ 0. We define a new symmetric ring spectrum R[1/x] as follows. For n ≥ 0 we set
R[1/x]n = map(S n , R2n ) ,the n-fold loop space of R2n . The group Σn acts on S n by coordinate permutations, on R2n via restriction
along the diagonal embedding ∆ : Σn −→ Σ2n given by(
2 · γ((i + 1)/2) − 1 for i odd,∆(γ)(i) =2 · γ(i/2)
for i even,and by conjugation on the whole mapping space. The multiplication µn,m : R[1/x]n ∧ R[1/x]m −→R[1/x]n+m is the map
map(S n , R2n ) ∧ map(S m , R2m ) −→ map(S n+m , R2(n+m) )
f ∧g

7−→

µ2n,2m ◦ (f ∧ g) ,which is associative since smashing of maps and the product of R are. The multiplication map is Σn × Σm equivariant since the original multiplication maps are equivariant and since the diagonal embeddings ∆n :
Σn −→ Σ2n , ∆m : Σm −→ Σ2m and ∆n+m : Σn+m −→ Σ2(n+m) satisfy∆n (γ) × ∆m (τ ) = ∆n+m (γ × τ ) .
We have R[1/x]0 = R0 and the 0th unit map for R[1/x] is the same as for R.
Next we define pointed maps jn : Rn −→ map(S n , R2n ) as the adjoints of the maps
Id ∧x∧nµn,1,...,1

χ×n,2

Rn ∧ S n −−−−−→ Rn ∧ R1∧n −−−−−→ Rn+n −−−→ R2n
where χ×n,2 ∈ Σn2 is the multiplicative shuffle defined by(
2j − 1for 1 ≤ j ≤ n,×
χn,2 (j) =2(j − n) for n + 1 ≤ j ≤ n + n.
(See Remark 1.12 for more background on the multiplicative shuffle and why to expect it here.) Since themap µn,1,...,1 ◦ x(n) : Rn ∧ S n −→ Rn+n is Σn × Σn -equivariant and the diagonal embedding ∆ : Σn −→ Σ2n

2. EXAMPLES

35

×satisfies ∆(γ) ◦ χ×n,2 = χn,2 (γ × γ) in Σn+n ; together these imply that the adjoint jn is Σn -equivariant. The
maps jn are multiplicative in the sense of the relation µn,m (jn ∧ jm ) = jn+m µn,m ; this boils down to the××
relation χ×
n+m,2 ◦ (1 × χn,m × 1) = χn,2 × χm,2 in Σ(n+m)2 .
n
We define unit maps S −→ R[1/x]n as the composite of the unit map of R with jn ; this finishes thedefinition of R[1/x] which is again a symmetric ring spectrum and comes with a morphism of symmetricring spectra [centrality of the unit of R[1/x]]

j : R −→ R[1/x] .
Note that the map x does not enter in the definition of the spaces R[1/x]n , and it is not used in defining
the multiplication of R[1/x], but it enters in the definition of the unit map of the ring spectrum R[1/x].
Thus the colimit systems which define the homotopy groups of R and R[1/x] consist of the same groups, but
the effect of x is twisted into the morphisms in the sequence, and so the homotopy groups of R and R[1/x]are potentially different. We show in Proposition 4.67 below that if R is semistable, then so is R[1/x] andthe effect of the morphism j : R −→ R[1/x] on the graded rings of homotopy groups is precisely invertingthe class in π0 R represented by the map x.
Example 2.37 (Adjoining roots of unity). As an application of the localization construction of Example 2.36 we construct a commutative symmetric ring spectrum which models the ‘Gaussian integers over S’
with 2 inverted. To construct it, we start with the spherical group ring S[C4 ] of the cyclic group of order4, a commutative symmetric ring spectrum as in Example 2.32. We invert the element
1 − t2 ∈ Z[C4 ] = π0 S[C4 ]where t ∈ C4 is a generator, and defineS[1/2, i] = S[C4 ][1/(1 − t2 )] .
In more detail, the space S[C4 ]1 = C4+ ∧ S 1 is a wedge of 4 circles and the map from π1 S[C4 ]1 to the stablegroup π0 S[C4 ] is surjective. So 1 − t2 ∈ π0 S[C4 ] can be represented by a based map x : S 1 −→ S[C4 ]1 towhich we apply Example 2.36. The monoid ring spectrum S[C4 ] is commutative and semistable, and so
Corollary 4.69 below shows that the ring of homotopy groups of S[C4 ][1/(1 − t2 )] is obtained from the ringπ∗ S[C4 ] by inverting the class 1 − t2 in π0 .
Because (1 + t2 )(1 − t2 ) = 0 in the group ring Z[C4 ], inverting 1 − t2 forces 1 + t2 = 0, so t becomes asquare root of −1. Since (1 − t2 )2 = 2(1 − t2 ), inverting 1 − t2 also inverts 2, and in fact
π0 S[1/2, i] = Z[C4 ][1/(1 − t2 )] ∼= Z [1/2, i] .
The ring spectrum S[1/2, i] is π∗ -isomorphic as a symmetric spectrum to a wedge of 2 copies of S[1/2], andthus deserves to be called the ‘Gaussian integers over S’ with 2 inverted. Moreover, S[1/2, i] is a Moore
spectrum for the ring Z[1/2, i], i.e., its integral homology is concentrated in dimension zero.
If p is a prime number and n ≥ 1, we can similarly adjoin a primitive pn -th root of unity to the sphere
spectrum, provided we are also willing to invert p in the homotopy groups. We first form the monoid ring
spectrum S[Cpn ] of the cyclic group of order pn , let t ∈ Cpn denote a generator and invert the elementf = p − (tq(p−1) + tq(p−2) + · · · + tq + 1) in Z[Cpn ] = π0 S[Cpn ], where q = pn−1 . This defines
S[1/p, ζ] = S[Cpn ][1/f ] .
We have f 2 = pf , so inverting f also inverts the prime p and forces the expression p − f to become 0 in thelocalized ring. If we let ζ denote the image of t in the localized ring, then the latter says that ζ is a root of
the cyclotomic polynomial, i.e.,ζ q(p−1) + ζ q(p−2) + · · · + ζ q + 1 = 0
where again q = pn−1 . In fact we have Z[Cpn ][1/f ] = Z[1/p, ζ] where ζ is a primitive pn -th root of unity;
moreover the commutative symmetric ring spectrum S[1/p, ζ] is a Moore spectrum for the ring Z[1/p, ζ].
We can do the same constructions starting with any semistable commutative symmetric ring spectrum
R instead of the sphere spectrum, yielding a new commutative symmetric ring spectrum R[1/p, ζ]. If p is

36

I. BASIC DEFINITIONS AND EXAMPLES

already invertible and the cyclotomic polynomial above is irreducible in π0 R, then this adjoins a primitive
pn -th root of unity to the homotopy ring of R.
These examples are a special case of a much more general phenomenon: every number ring can be‘lifted’ to an extension of the sphere spectrum by a commutative symmetric ring spectrum, provided wealso invert the ramified primes. However, the only proofs of this general fact that I know use obstruction
theory, and so we cannot give a construction which is as explicit and simple as the one above for adjoining
roots of unity.
2.3. Restrictions from other kinds of spectra.
Example 2.38 (Continuous functors). By a continuous functor we mean a functor F : T −→ T fromthe category of pointed spaces to itself which is pointed in that it takes one-point spaces to one-point spaces
and continuous in the sense that for all pointed spaces K and L the map
F : T (K, L) −→ T (F (K), F (L))
is continuous with respect to the compact open topology on the mapping spaces. The (continuous !) map
l 7→(k7→k∧l)F

L −−−−−−−−→ T (K, K ∧ L) −−−−−→ T (F (K), F (K ∧ L)) .
then has an adjoint
F (K) ∧ L −→ F (K ∧ L)
which we call the assembly map. The assembly map is natural in K and L, it is unital in the sense that thecomposite
assembly
F (K) ∼= F (K) ∧ S 0 −−−−−→ F (K ∧ S 0 ) ∼= F (K)is the identity and it is associative in the sense that the diagram(F (K) ∧ L) ∧ M

ass.∧Id/ F (K ∧ L) ∧ M

ass.

/ F ((K ∧ L) ∧ M )
F (∼
=)

∼
=


F (K) ∧ (L ∧ M )
/ F (K ∧ (L ∧ M ))assembly

commutes for all K, L and M , where the vertical maps are associativity isomorphisms for the smash product.
As usual, there is also a simplicial version. A simplicial functor is an enriched, pointed functor F :sset* −→ sset* from the category of pointed simplicial sets to itself. So F assigns to each pointed simplicial
set K a pointed simplicial set F (K) and to each pair K, L of pointed simplicial sets a morphism of pointed
simplicial sets
F : map(K, L) −→ map(F (K), F (L))
which is associative and unital and such that F (∗) ∼= ∗. The restriction of F to vertices is then a functor
in the usual sense. The same kind of adjunctions as for continuous functors provides a simplicial functor
with an assembly map F (K) ∧ L −→ F (K ∧ L), again unital and associative.
To every continuous (respectively simplicial) functor F we can associate a symmetric spectrum of spaces(respectively of simplicial sets) F (S) by
F (S)n = F (S n )nwhere Σn permutes the coordinates of S . The structure map σn : F (S n ) ∧ S 1 −→ F (S n+1 ) is an instanceof the assembly map. More generally, we can evaluate a continuous (simplicial) functor F on a symmetric
spectrum X and get a new symmetric spectrum F (X) by defining F (X)n = F (Xn ) with structure map thecomposite
assembly

F (σn )F (Xn ) ∧ S 1 −−−−−→ F (Xn ∧ S 1 ) −−−−→ F (Xn+1 ) .
Some of the symmetric spectra which we described are restrictions of continuous functors to spheres,for example suspension spectra and Eilenberg-Mac Lane spectra. Free symmetric spectra Fm K or semifree
symmetric spectra Gm K do not arise this way (unless m = 0 or K = ∗) and cobordism spectra like M O
and M U don’t either.

2. EXAMPLES

37

A consequence of the formal properties of the assembly map is that the structure of a triple on acontinuous or simplicial functor T yields a multiplication on the symmetric spectrum T (S). Indeed, using
the assembly map twice and the triple structure map produces multiplication maps
T (K) ∧ T (L) −→ T (K ∧ T (L)) −→ T (T (K ∧ L)) −→ T (K ∧ L) ;here K and L are pointed spaces. If we apply this to spheres, we get Σp × Σq -equivariant maps
T (S p ) ∧ T (S q ) −→ T (S p+q )which provide the multiplication. The unit maps come from the natural transformation Id −→ T by
evaluating on spheres. Here are some examples.
• The identity triple gives the sphere spectrum as a symmetric ring spectrum.
• Let Gr be the reduced free group triple, i.e., it sends a pointed set K to the free group generated
by K modulo the normal subgroup generated by the basepoint. Since Gr(S n ) is weakly equivalent
to ΩS n+1 , which in the stable range is equivalent to S n , the unit maps form a π∗ -isomorphism
S −→ Gr(S). The same conclusion would hold with the free reduced monoid functor, also known
as the ‘James construction’ J, since J(S n ) is also weakly equivalence to ΩS n+1 as soon as n ≥ 1.
• Let M be a topological monoid and consider the pointed continuous functor K 7→ M + ∧ K. The
multiplication and unit of M make this into a triple whose algebras are pointed sets with left
M -action. The associated symmetric ring spectrum is the spherical monoid ring S[M ].
• Let A be a ring and consider the free reduced A-module triple Ã[K] = A[K]/A[∗]. Then Ã[S] =HA, the Eilenberg-Mac Lane ring spectrum. We shall see later [ref] that for every symmetric
spectrum of simplicial sets X the symmetric spectrum Ã[X] is π∗ -isomorphic to the smash product
HA ∧ X.
• Let B be a commutative ring and consider the triple X 7→ I(B̃(X)), the augmentation ideal of
the reduced polynomial algebra over B, generated by the pointed set X. The algebras over this
triple are non-unital commutative B-algebras, or augmented commutative B-algebras (which areequivalent categories). The ring spectrum associated to this triple is denoted DB, and it is closely
related to topological André-Quillen homology for commutative B-algebras. The ring spectrum
DB is rationally equivalent to the Eilenberg-Mac Lane ring spectrum HB, but DB has torsion inhigher homotopy groups.
More generally, if we evaluate a triple T on a symmetric ring spectrum R, then the resulting spectrum T (R)is naturally a ring spectrum with multiplication maps
T (µn,m )T (Rn ) ∧ T (Rm ) →− T (Rn ∧ Rm ) −−−−−→ T (Rn+m ) .
Example 2.39 (Γ-spaces). Many continuous or simplicial functors arise from so called Γ-spaces, andthen the associated symmetric spectra have special properties. The category Γ is a skeletal category of thecategory of finite pointed sets: there is one object n+ = {0, 1, . . . , n} for every non-negative integer n, andmorphisms are the maps of sets which send 0 to 0. (Γ is really equivalent to the opposite of Segal’s category
Γ, cf. [55]). A Γ-space is a covariant functor from Γ to the category of spaces or simplicial sets taking 0+
to a one point space (simplicial set). A morphism of Γ-spaces is a natural transformation of functors. We
follow the established terminology to speak of Γ-spaces even if the values are simplicial sets.
A Γ-space X can be extended to a continuous (respectively simplicial, depending on the context) functor
by a coend construction. If X is a Γ-space and K a pointed space or simplicial set, the value of the extended
functor on K is given by
Z n+ ∈ΓK n ∧ X(n+ ) ,where we use that K n = map(n+ , K) is contravariantly functorial in n+ . We will not distinguish notationally
between the original Γ-space and its extension. The extended functor is continuous respectively simplicial.
In the simplicial context, the extension of a Γ-space admits the following different (but naturally
isomorphic) description. First, X can be prolonged, by direct limit, to a functor from the category of

38

I. BASIC DEFINITIONS AND EXAMPLES

pointed sets, not necessarily finite, to pointed simplicial sets. Then if K is a pointed simplicial set we geta bisimplicial set [k] 7→ X(Kk ) by evaluating the (prolonged) Γ-space degreewise. The simplicial set X(K)defined by the coend above is naturally isomorphic to the diagonal of this bisimplicial set.
Symmetric spectra which arise from Γ-spaces have special properties. Here we restrict to Γ-spaces ofsimplicial sets, where things are easier to state. First, every simplicial functor which arises from a Γ-space
X preserves weak equivalences of simplicial sets, see [11, Prop. 4.9]. So if f : A −→ B is a level equivalenceof symmetric spectra of simplicial sets, then X(f ) : X(A) −→ X(B) is again a level equivalence. We shallsee later that X(−) also preserves π∗ -isomorphisms and stable equivalences [ref]. Another special property
is that symmetric spectra of the form X(S) for Γ-spaces of simplicial sets X are connective and the colimit
systems for the stable homotopy groups stabilize in a uniform way. This is because for every Γ-space X,
the simplicial set X(S n ) is always (n − 1)-connected [11] and the structure map X(S n ) ∧ S 1 −→ X(S n+1 )is 2n-connected [34, prop. 5.21]. Moreover, up to π∗ -isomorphisms, Γ-spaces model all connective spectra(see Theorem 5.8 of [11] [also reference to [55]?])A Γ-space X is called special if the map X((k + l)+ ) −→ X(k + ) × X(l+ ) induced by the projections
from (k + l)+ ∼= k + ∨ l+ to k + and l+ is a weak equivalence for all k and l. In this case, the weak map∼

X(∇)

X(1+ ) × X(1+ ) ←− X(2+ ) −−−→ X(1+ )induces an abelian monoid structure on π0 (X(1+ )). Here ∇ : 2+ −→ 1+ is defined by ∇(1) = 1 = ∇(2).
The Γ-space X is called very special if it is special and the monoid π0 (X(1+ )) is a group. By Segal’s theorem([55, Prop. 1.4] or [11, Thm. 4.2]), the spectrum X(S) associated to a special Γ-space X by evaluation on
spheres is a positive Ω-spectrum.
If X is very special, then X(S) is even an Ω-spectrum (i.e., from the 0th level on). In particular,
the homotopy groups of a very special Γ-space X are naturally isomorphic to the homotopy groups of thesimplicial set X(1+ ).
Example 2.40 (Orthogonal spectra). An orthogonal spectrum consists of the following data:• a sequence of pointed spaces Xn for n ≥ 0• a base-point preserving continuous left action of the orthogonal group O(n) on Xn for each n ≥ 0• based maps σn : Xn ∧ S 1 −→ Xn+1 for n ≥ 0.
This data is subject to the following condition: for all n, m ≥ 0, the iterated structure map
σ m : Xn ∧ S m −→ Xn+mis O(n) × O(m)-equivariant. The orthogonal group acts on S m since this is the one-point compactificationof Rn and O(n) × O(m) acts on the target by restriction, along orthogonal sum, of the O(n + m)-action.
A morphism f : X −→ Y of orthogonal spectra consists of O(n)-equivariant based maps fn : Xn −→ Yn
for n ≥ 0, which are compatible with the structure maps in the sense that fn+1 ◦ σn = σn ◦ (fn ∧ IdS 1 ) for
all n ≥ 0.
An orthogonal ring spectrum R consists of the following data:• a sequence of pointed spaces Rn for n ≥ 0• a base-point preserving continuous left action of the orthogonal group O(n) on Rn for each n ≥ 0• O(n) × O(m)-equivariant multiplication maps µn,m : Rn ∧ Rm −→ Rn+m for n, m ≥ 0, and• O(n)-equivariant unit maps ιn : S n −→ Rn for all n ≥ 0.
This data is subject to the same associativity and unit conditions as a symmetric ring spectrum (seeDefinition 1.3) and a centrality condition for every unit map ιn . In the unit condition, permutations suchas χn,m ∈ Σn+m have to be interpreted as permutation matrices in O(n + m). An orthogonal ring spectrum
R is commutative if for all n, m ≥ 0 the relation χn,m ◦µn,m = µm,n ◦twist holds as maps Rn ∧Rm −→ Rm+n .
A morphism f : R −→ S of orthogonal ring spectra consists of O(n)-equivariant based maps fn :Rn −→ Sn for n ≥ 0, which are compatible with the multiplication and unit maps (in the same sense as forsymmetric ring spectra).
Orthogonal spectra are ‘symmetric spectra with extra symmetry’ in the sense that every orthogonal
spectrum X has an underlying symmetric spectrum U X. Here (U X)n = Xn and the symmetric group acts

2. EXAMPLES

39

by restriction along the monomorphism Σn −→ O(n) given by permutation matrices. The structure mapsof U X are the structure maps of X. Many spectra that we have introduced above have this extra symmetry,
i.e., they are underlying orthogonal spectra. Examples are the sphere spectrum, suspension spectra or moregenerally any symmetric spectrum which is obtained from a continuous functor by evaluation on spheres.
The various Thom spectra such as M O and M U arise from orthogonal spectra by forgetting symmetry, butthey do not extend to continuous functors.
There is a more natural notion where we use vector spaces with inner product to index the spaces inan orthogonal spectrum. A coordinate free orthogonal spectrum X consists of the following data:• a pointed space X(V ) for each inner product space V , i.e., finite dimensional real vector spacewith a euclidian scalar product,• a base-point preserving continuous left action of the orthogonal group O(V ) of V on X(V ), foreach inner product space V ,• a structure map σV,W : X(V ) ∧ S W −→ X(V ⊕ W ) for each pair of inner product spaces V and
W which is O(V ) × O(W )-equivariant. Here S W is the one-point compactification of W on which
the group O(W ) acts by extension of the action on W , fixing the basepoint at infinity. The group
O(V ) × O(W ) acts on the target by restriction of the O(V ⊕ W )-action.
This data should satisfy two conditions: the composite
σV,0X(V ) ∼= X(V ) ∧ S 0 −−−→ X(V ⊕ 0) ∼= X(V )∼
should be the identity [isometries V = W should also act] and the square

X(V ) ∧ S W ∧ S Z

Id ∧µW,Z/ X(V ) ∧ S W ⊕Z
σV,W ⊕Z

σV,W ∧Id
X(V ⊕ W ) ∧ S Z

σV ⊕W,Z/ X(V ⊕ W ⊕ Z)

commutes for all inner product spaces V, W and Z.
A morphism f : X −→ Y of coordinate free orthogonal spectra consists of O(V )-equivariant pointed
maps f (V ) : X(V ) −→ Y (V ) for all V which are compatible with the structure maps in the sense thatf (V ⊕ W ) ◦ σV,W = σV,W ◦ (f (V ) ∧ Id) for all V and W .
A coordinate free orthogonal spectrum X gives rise to a coordinate free symmetric spectrum U X(see Remark 1.11) by forgetting symmetry. For a finite set A the space (U X)A is X(RA ), the value of
X at the inner product space RA which has A as orthonormal basis. [define the structure maps α∗ :(U X)A ∧ S B−α(A) −→ (U X)B ]Example 2.41 (Unitary spectra). Unitary spectra are the complex analogues of orthogonal spectra, andthey again come in a coordinatized and a coordinate free flavor. A coordinate free unitary spectrum assigns
a pointed spaces X(V ) for each complex inner product space V , i.e., finite dimensional complex vector spacewith a hermitian metric, together with a base-point preserving continuous left action of the unitary group
U (V ) of V on X(V ), and U (V ) × U (W )-equivariant structure maps σV,W : X(V ) ∧ S W −→ X(V ⊕ W )which are associative and unital.
Given a unitary spectrum X we can produce an orthogonal spectrum Φ(X) as follows. We set
Φ(X)(V ) = map(S iV , X(C ⊗R V )) ,where i ∈ C is a square root of −1 and S iV is the one-point compactification of the imaginary part ofthe complex inner product space C ⊗R V . The orthogonal group acts on S iV , on X(C ⊗R V ) via thecomplexification map O(V ) −→ U (C ⊗R V ) and on the mapping space by conjugation. The structure map
σV,W : Φ(X)(V ) ∧ S W −→ Φ(X)(V ⊕ W ) is adjoint to the map
map(S iV , X(C ⊗R V )) ∧ S W ∧ S i(V ⊕W ) ∼= map(S iV , X(C ⊗R V )) ∧ S iV ∧ S C⊗R W
eval∧Id

σC⊗V,C⊗W−−−−−→ X(C ⊗R V ) ∧ S C⊗R W −−−−−−−→ X(C ⊗R (V ⊕ W ))

40

I. BASIC DEFINITIONS AND EXAMPLES

where we have made identifications such as S W ∧ S iW ∼== S C⊗R W and C ⊗R V ⊕ C ⊗R W ∼= S W ⊕iW ∼C ⊗R (V ⊕ W ).
The functor Φ : SpU −→ SpO turns unitary ring spectra into orthogonal ring spectra. An exampleof this is the complex cobordism spectrum M U of Example 2.9 which arises naturally as a unitary ring
spectrum, made into an orthogonal spectrum via the functor Φ. More precisely, the symmetric sequencedenoted M U in Example 2.9 comes from a unitary spectrum with V th space
M U (V ) = EU (V )+ ∧U (V ) S V ,the Thom space of the vector bundle over BU (V ) with total space EU (V ) ×U (V ) V .
Example 2.42 (Periodic complex cobordism). We define the periodic complex cobordism spectrum
M U P , a unitary spectrum, as follows. For a complex inner product space V we consider the ‘full Grassmannian’ of V ⊕ V . A point in this ‘full Grassmannian’ is any complex subvector space of V ⊕ V , andthis space is topologized as the disjoint union of the Grassmannians of k-dimensional subspaces of V ⊕ V
for k = 0, . . . , 2 dim(V ). Over the ‘full Grassmannian’ sits a tautological hermitian vector bundle (of nonconstant rank!): the total space of this bundle consist of pairs (X, x) where X is a complex subvector spaceof V ⊕ V and x ∈ X. We define (M U P )(V ) as the Thom space of this tautological vector bundle, i.e., thequotient space of the unit disc bundle by the sphere bundle. The multiplication(M U P )(V ) ∧ (M U P )(W ) −→ (M U P )(V ⊕ W )sends (X, x) ∧ (Y, y) to (X + Y, (x, y)) where X + Y is the image of X ⊕ Y under the isometry Id ∧τ ∧ Id :(V ⊕ V ) ⊕ (W ⊕ W ) ∼= (V ⊕ W ) ⊕ (V ⊕ W ). The unit map S V −→ (M U P )(V ) sends x ∈ V to (∆(V ), (v, v))
where ∆(V ) is the diagonal copy of W
V in V ⊕ V . [explain that M U P is a wedgeof suspended copies ofW
M U , i.e., there is a π∗ -isomorphism k∈Z S 2k ∧ M U −→ M U P ; real version k∈Z S k ∧ M O ' M OP ]Example 2.43 (S-modules). We describe a functor Φ : MS −→ SpΣ from the category of S-modules
in the sense of Elmendorf, Kriz, Mandell and May [19] to the category of symmetric spectra which preserves0homotopy groups and multiplicative structures. For this we need the S-module S ∧L LS -1 = S ∧L LΣ∞
1 S
11defined in [19, II 1.7], which we abbreviate to Sc . What matters is not the precise form of Sc , but that it
is a cofibrant desuspension of the sphere S-module, i.e., it comes with a weak equivalence Sc-1 ∧ S 1 −→ S,
where S 1 denotes the circle. For n > 0 we define Sc-n to be the n-fold smash power of the S-module Sc-1 ,endowed with the permutation action of the symmetric group on n letters. We set Sc0 = S, the unit of thesmash product; here the notation is slightly misleading since Sc0 is not cofibrant. The functor Φ is thengiven by
Φ(X)n = MS (Sc-n , X)
where the right hand side is the topological mapping space in the category of S-modules. The symmetricgroup acts on the mapping space through the permutation action of the source. The desuspension map
Sc-1 ∧ S 1 −→ S induces a map
M (S -n , X) −→ M (S -(n+1) ∧ S 1 , X) ∼= T (S 1 , M (S -(n+1) , X))
S

c

S

S

c

c

whose adjoint
MS (Sc-n , X) ∧ S 1 −→ MS (Sc-(n+1) , X)
makes Φ(X) into a symmetric spectrum. For n ≥ 1, the S-module S -n is a cofibrant model of the (-n)-spherec

spectrum. So the functor Φ takes weak equivalences of S-modules to maps which are level equivalences
above level 0, and the i-th homotopy group of the space Φ(X)n is isomorphic to the (i − n)-th homotopy
group of the S-module X by [19, II 1.8]. In particular there is a natural isomorphism of stable homotopy
groups π∗ Φ(X) ∼= π∗ X.
If R is an S-algebra with multiplication µ : R ∧ R −→ R and unit i : S −→ R, then Φ(R) becomes asymmetric ring spectrum with multiplication mapsµ∧
MS (Sc-m , R) ∧ MS (Sc-n , R) −−→ MS (Sc-(m+n) , R ∧ R) −−→ MS (Sc-(m+n) , R) .

3. SMASH PRODUCT41

The unit maps i0 : S0 −→ Φ(S)0 = MS (S, R) is the unit i and the unit map S 1 −→ Φ(S)1 = MS (Sc-1 , R) isi
adjoint to the composite Sc-1 ∧ S 1 −→ S −→ R. If R is a commutative S-algebra, then Φ(R) is a commutative
symmetric ring spectrum.
Different kinds of (categories of) spectra yield symmetric spectra:
Γ-spaces/ unitary spectra/ continuous functors
TTTT
TTTT
TTTT
Φ
TTTT

*
/ orthogonal spectraMS/ symmetric spectra

The triangle only commute up to natural π∗ -isomorphism: for every continuous functor F there is a naturalπ∗ -isomorphism whose V th term is the map F (S V ) −→ map(S iV , F (S C⊗R V )) adjoint to the assembly map
F (S V ) ∧ S iV −→ F (S C⊗R V ). We did not say how to make an orthogonal spectrum from an S-module; thisconstruction can be found in [38].
3. Smash productOne of the main features which distinguishes symmetric spectra from the more classical spectra without
symmetric group actions is the internal smash product. The smash product of symmetric spectra is verymuch like the tensor product of modules over a commutative ring. To stress that analogy, we recall threedifferent ways to look at the classical tensor product and then give analogies involving the smash productof symmetric spectra.
In the following, R is a commutative ring and M, N and W are right R-modules.
(A) Tensor product via bilinear maps. A bilinear map from M and N to another right R-module
W is a map b : M × N −→ W such that for each m ∈ M the map b(m, −) : N −→ W is R-linear and foreach n ∈ N the map b(−, n) : M −→ W is R-linear. The tensor product M ⊗R N is the universal example
of a right R-module together with a bilinear map from M × N . In other words, there is a specified bilinear
map i : M × N −→ M ⊗R N such that for every R-module W the map
HomR (M ⊗R N, W ) −→ BilinR (M × N, W ) ,f 7→ f ◦ i

is bijective. As usual, the universal property characterizes the pair (M ⊗R N, i) uniquely up to preferred
isomorphism.
(B) Tensor product as an adjoint to internal Hom. The category of right R-modules has‘internal Hom-objects’: the set HomR (N, W ) of R-linear maps between two right R-modules N and W isnaturally an R-module by pointwise addition and scalar multiplication. For fixed right R-modules M andN , the functor HomR (M, HomR (N, −)) : mod-R −→ mod-R is representable and tensor product M ⊗R N
can be defined as a representing R-module. This point of view is closely related to the first approach since
the R-modules HomR (M, HomR (N, W )) and BilinR (M × N, W ) are naturally isomorphic.
(C) Tensor product as a construction. Often the tensor product M ⊗R N is introduced as a
specific construction, usually the following: M ⊗R N is the free R-module generated by symbols of the formm ⊗ n for all m ∈ M and n ∈ N subject to the following set of relations• (m + m0 ) ⊗ n = m ⊗ n + m0 ⊗ n , m ⊗ (n + n0 ) = m ⊗ n + m ⊗ n0
• (mr) ⊗ n = (m ⊗ n) · r = m ⊗ (nr)for all m, m0 ∈ M , n, n0 ∈ N and r ∈ R. Since this is a minimal set of relations which make the map
M × N −→ M ⊗R N given by (m, n) 7→ m ⊗ n into a bilinear map, the tensor product is constructed as tohave the universal property (A).
Now we introduce the smash product of symmetric spectra in three ways, analogous to the ones above.

42

I. BASIC DEFINITIONS AND EXAMPLES(A) Smash product via bilinear maps. We define a bimorphism b : (X, Y ) −→ Z from a pair ofsymmetric spectra (X, Y ) to a symmetric spectrum Z as a collection of Σp ×Σq -equivariant maps of pointed
spaces or simplicial sets, depending on the context,
bp,q : Xp ∧ Yq −→ Zp+q
for p, q ≥ 0, such that the diagram(3.1)

Xp ∧ Yq ∧ S 1
nn
Id ∧σq nnnn
nbp,q ∧Id
nn
nnn

vnnn
Xp ∧ Yq+1
Zp+q ∧ S 1
QQQ
QQQ
QQQ
σp+q
Q
bp,q+1 QQQQ
Q(

Zp+q+1 oId ∧twist/ Xp ∧ S 1 ∧ Yq
σp ∧Id

Xp+1 ∧ Yq
bp+1,q

1×χ1,qZp+1+qcommutes for all p, q ≥ 0. In Exercise 5.7 we give a justification for calling this notion ‘bimorphism’.
The smash product X ∧ Y can now we introduced as the universal example of a symmetric spectrumwith a bimorphism from X and Y . More precisely, we will show in (C) below that for every pair ofsymmetric spectra (X, Y ) the functor which assign to Z ∈ SpΣ the set of bimorphism from (X, Y ) to Z
is representable. A smash product of X and Y is then a representing object, i.e., a pair consisting of asymmetric spectrum X ∧ Y and a bimorphism ι : (X, Y ) −→ X ∧ Y such that for every symmetric spectrum
Z the map
(3.2)

SpΣ (X ∧ Y, Z) −→ Bimor((X, Y ), Z) ,f 7−→ f i = {fp+q ◦ ip,q }p,q

is bijective. Very often only the object X ∧ Y will be referred to as the smash product, but one should
keep in mind that it comes equipped with a specific, universal bimorphism. We will often refer to thebijection (3.2) as the universal property of the smash product of symmetric spectra.
(B) Smash product as an adjoint to internal Hom. In Example 2.25 we introduced ‘internal
Hom objects’ in the category of symmetric spectra. For every pair of symmetric spectra (X, Y ) we defined
another symmetric spectrum Hom(X, Y ) such that the morphism from X to Y are (in natural bijectionto) the vertices of the 0th level of Hom(X, Y ). We claim that for fixed symmetric spectra X and Y , thefunctor Hom(X, Hom(Y, −)) : SpΣ −→ SpΣ is representable. The smash product X ∧ Y can then be
defined as a representing symmetric spectrum. This point of view can be reduced to perspective (A) since
the sets SpΣ (X, Hom(Y, Z)) and Bimor((X, Y ), Z) are in natural bijection (see Exercise 5.7). In particular,
since the functor Bimor((X, Y ), −) is representable, so is the functor SpΣ (X, Hom(Y, −)). [extend this toan isomorphism of spectra Hom(X, Hom(Y, Z)) ∼= Bimor((X, Y ), Z)](C) Smash product as a construction. Now we construct a symmetric spectrum X ∧ Y from two
given symmetric spectra X and Y . We want X ∧ Y to be the universal recipient of a bimorphism from(X, Y ), and this pretty much tells us what we have to do. For n ≥ 0 we define the nth level (X ∧ Y )n as thecoequalizer, in the category of pointed Σn -spaces or pointed Σn -simplicial sets (depending on the context),of two maps__
1
αX , αY :Σ+Σ+n ∧Σp ×Σ1 ×Σq Xp ∧ S ∧ Yq −→n ∧Σp ×Σq Xp ∧ Yq .
p+1+q=np+q=nThe wedges run over all non-negative values of p and q which satisfy the indicated relations. The map αX
takes the wedge summand indexed by (p, 1, q) to the wedge summand indexed by (p + 1, q) using the map
σpX ∧ Id : Xp ∧ S 1 ∧ Yq −→ Xp+1 ∧ Yq

3. SMASH PRODUCT

43

and inducing up. The other map αY takes the wedge summand indexed by (p, 1, q) to the wedge summandindexed by (p, 1 + q) using the composite
Id ∧σqY

Id ∧twist

Id ∧χq,1

Xp ∧ S 1 ∧ Yq −−−−−→ Xp ∧ Yq ∧ S 1 −−−−→ Xp ∧ Yq+1 −−−−−→ Xp ∧ Y1+q
and inducing up.
The structure map (X ∧ Y )n ∧ S 1 −→ (X ∧ Y )n+1 is induced on coequalizers by the wedge of the maps
1
Σ+
−→ Σ+
n ∧Σp ×Σq Xp ∧ Yq ∧ S
n+1 ∧Σp ×Σq+1 Xp ∧ Yq+1

induced from Id ∧σqY : Xp ∧Yq ∧S 1 −→ Xp ∧Yq+1 . One should check that this indeed passes to a well-defined
map on coequalizers. Equivalently we could have defined the structure map by moving the circle past Yq ,using the structure map of X (instead of that of Y ) and then shuffling back with the permutation χ1,q ; thedefinition of (X ∧ Y )n+1 as a coequalizer precisely ensures that these two possible structure maps coincide,and that the collection of maps_x∧y7→1∧x∧yprojection
Xp ∧ Yq −−−−−−−−→
Σ+n ∧Σp ×Σq Xp ∧ Yq −−−−−−→ (X ∧ Y )p+q
p+q=n

form a bimorphism – and in fact a universal one.
The smash product X ∧ Y is a functor in both variables. This is fairly evident from the construction(C), but it can also be deduced from the universal property (A) or the adjunction (B) as follows. If we use
the universal property (A) the contravariant functoriality of the set Bimor((X, Y ), Z) in X and Y turns intofunctoriality of the representing objects. In more detail, if f : X −→ X 0 and g : Y −→ Y 0 are morphismsof symmetric spectra, then the collection of pointed maps


i0p,q
fp ∧gq00
00
Xp ∧ Yq −−−−→ Xp ∧ Yq −−→ (X ∧ Y )p+q
p,q≥0
0

0

forms a bimorphism (X, Y ) −→ X ∧ Y . So there is a unique morphism of symmetric spectra f ∧ g :X ∧ Y −→ X 0 ∧ Y 0 such that (f ∧ g)p+q ◦ ip,q = i0p,q ◦ (fp ∧ gq ) for all p, q ≥ 0. The uniqueness part of theuniversal property implies that this is compatible with identities and composition in both variables.
If we define the smash product as a representing object for the functor Hom(X, Hom(Y, −)), thenfunctoriality in X and Y follows from functoriality of the latter functor in X and Y .
Now that we have constructed a smash product functor we can investigate its formal and homotopical properties. The formal properties will be discussed in the rest of this section, but we postpone thehomotopical analysis until Section 5 of Chapter II.
The first thing to show is that the smash product is symmetric monoidal. Since ‘symmetric monoidal’is extra data, and not a property, we are obliged to construct associativity isomorphisms
αX,Y,Z : (X ∧ Y ) ∧ Z −→ X ∧ (Y ∧ Z) ,symmetry isomorphisms
τX,Y : X ∧ Y −→ Y ∧ X
and right unit isomorphisms
rX : X ∧ S −→ X
which satisfy a certain list of coherence conditions. We then define left unit isomorphisms lX : S ∧ X −→ X
as the composite of the symmetry isomorphism τS,X and the right unit rX .
First construction. We can obtain all the isomorphisms of the symmetric monoidal structure justfrom the universal property. So suppose that for each pair of symmetric spectra (X, Y ) a smash productX ∧ Y and a universal bimorphism i = {ip,q } : (X, Y ) −→ X ∧ Y have been chosen.

44

I. BASIC DEFINITIONS AND EXAMPLES

For construction the associativity isomorphism we notice that the family
ip,q ∧Id
ip+q,rXp ∧ Yq ∧ Zr −−−−→ (X ∧ Y )p+q ∧ Zr −−−−→ ((X ∧ Y ) ∧ Z)p+q+r
p,q,r≥0and the family
Id ∧iq,rip,q+rXp ∧ Yq ∧ Zr −−−−−→ Xp ∧ (Y ∧ Z)q+r −−−−→ (X ∧ (Y ∧ Z))p+q+r
p,q,r≥0both have the universal property of a tri morphism (whose definition is hopefully clear) out of X, Y and Z.
The uniqueness of representing objects gives a unique isomorphism of symmetric spectra
αX,Y,Z : (X ∧ Y ) ∧ Z ∼= X ∧ (Y ∧ Z)
such that (αX,Y,Z )p,q,r ◦ ip+q,r ◦ (ip+q ∧ Id) = ip,q+r ◦ (Id ∧iq,r ).
The symmetry isomorphism τX,Y : X ∧ Y −→ Y ∧ X corresponds to the bimorphism
n
o
ιq,p
χq,p
twist
(3.3)
Xp ∧ Yq −−−→ Yq ∧ Xp −−→ (Y ∧ X)q+p −−−→ (Y ∧ X)p+q.
p,q≥0

The block permutation χq,p is crucial here: without it the diagram (3.1) would not commute and wewould not have a bimorphism. If we restrict the composite τY,X ◦ τX,Y in level p + q along the map
ip,q : Xp ∧ Yq −→ (X ∧ Y )p+q we get ip,q again. Thus τY,X ◦ τX,Y = IdX∧Y and τY,X is inverse to τX,Y .
In much the same spirit, the universal properties can be used to provide a right unit isomorphism.
Because of the commuting left part of the diagram (3.1) a bimorphism b : (X, S) −→ Z is completely
determined by the components bp,0 : Xp ∧ S 0 −→ Zp , which constitute a morphism b•,0 : X −→ Z;
moreover, every morphism from X to Z arises in this way from a unique bimorphism out of (X, S). Hencethe morphism rX : X ∧ S −→ X corresponding to the bimorphism consisting of the iterated structure maps
σ m : Xn ∧ S m −→ Xn+m is an isomorphism of symmetric spectra.
Second construction. The coherence isomorphisms can also be obtained from the construction of
the smash product in (C) above, as opposed to the universal property. In level n the spectra (X ∧ Y ) ∧ Z
and X ∧ (Y ∧ Z) are quotients of the spaces_+Σ+n ∧Σp+q ×Σr Σp+q ∧Σp ×Σq Xp ∧ Yq ∧ Zr
p+q+r=nrespectively_+Σ+n ∧Σp ×Σq+r Xp ∧ Σq+r ∧Σq ×Σr Yq ∧ Zr

.

p+q+r=n

The wedges run over all non-negative values of p, q and r which sum up to n. We get a well-defined maps
between these two wedges by wedging over the maps


+
+
+
Σ+
n ∧Σp+q ×Σr Σp+q ∧Σp ×Σq Xp ∧ Yq ∧ Zr ←→ Σn ∧Σp ×Σq+r Xp ∧ Σq+r ∧Σq ×Σr Yq ∧ Zrσ ∧ ((τ ∧ x ∧ y) ∧ z)

7−→(σ(τ × 1)) ∧ (x ∧ (1 ∧ y ∧ z))σ(1 × γ) ∧ ((1 ∧ x ∧ y) ∧ z)←−σ ∧ (x ∧ (γ ∧ y ∧ z))

where σ ∈ Σn , τ ∈ Σp+q , γ ∈ Σq+r , x ∈ Xp , y ∈ Yq and z ∈ Zr .
The symmetry isomorphism τX,Y : X ∧ X −→ Y ∧ X is obtained by wedging over the maps+Σ+n ∧Σp ×Σq Xp ∧ Yq −→ Σn ∧Σq ×Σp Yq ∧ Xp

σ∧x∧y

7−→

(σχp,q ) ∧ y ∧ xwhere σ ∈ Σn , x ∈ Xp and y ∈ Yq and passing to quotient spaces. The shuffle permutation χp,q is needed
to make this map well-defined on quotients.

3. SMASH PRODUCT45Theorem 3.4. The associativity, symmetry and unit isomorphisms make the smash product of symmetric spectra into a symmetric monoidal product with unit object the sphere spectrum S. This product isclosed symmetric monoidal in the sense that the smash product is adjoint to the internal Hom spectrum,i.e., there is an adjunction isomorphism
Hom(X ∧ Y, Z) ∼= Hom(X, Hom(Y, Z)) .
Proof. We have to verify that several coherence diagrams commute. We start with the pentagon
condition for associativity. Given a fourth symmetric spectrum W we consider the pentagon((W ∧ X) ∧ Y ) ∧ Z
WWWWWgg
WWWαWWW∧X,Y,Z
g
WWWWW
g
g
g
g
g
WWWWW
g
+
sgggg
(W ∧ (X ∧ Y )) ∧ Z(W ∧ X) ∧ (Y ∧ Z)
NNN
NNN
ooo
N
ooo
o
o
αW,X∧Y,ZNNN
αN'
wooo W,X,Y ∧Z/
W ∧ (X ∧ (Y ∧ Z)))W ∧ ((X ∧ Y ) ∧ Z)
gg
αW,X,Y ∧Id
gggg

Id ∧αX,Y,Z

If we evaluate either composite at level o + p + q + r and precompose with
io,p ∧Id ∧ Id

Wo ∧ Xp ∧ Yq ∧ Zr −−−−−−−→ (W ∧ X)o+p ∧ Yq ∧ Zr
io+p,q ∧Id

io+p+q,r−−−−−−→ ((W ∧ X) ∧ Y )o+p+q ∧ Zr −−−−−→ (((W ∧ X) ∧ Y ) ∧ Z)o+p+q+r
then both ways around the pentagon yield the composite
Id ∧ Id ∧iq,r

Wo ∧ Xp ∧ Yq ∧ Zr −−−−−−−→ Wo ∧ Xp ∧ (Y ∧ Z)q+r
Id ∧ip,q+r

io,p+q+r−−−−−−→ Wo ∧ (X ∧ (Y ∧ Z))p+q+r −−−−−→ (W ∧ (X ∧ (Y ∧ Z)))o+p+q+r .
So the uniqueness part of the universal property shows that the pentagon commutes.
Coherence between associativity and symmetry isomorphisms means that the two composites from theupper left to the lower right corner of the diagram(X ∧ Y ) ∧ Z

αX,Y,Z/ X ∧ (Y ∧ Z)

τX,Y ∧Z/ (Y ∧ Z) ∧ X
αY,Z,X

τX,Y ∧Id(Y ∧ X) ∧ Z

αY,X,Z/ Y ∧ (X ∧ Z)

Id ∧τX,Z/ Y ∧ (Z ∧ X)

should be equal, and the same kind of argument as for the pentagon relation for associativity works.
It remains to check the coherence conditions relating associativity and symmetry isomorphisms to theunit morphisms. We define the left unit isomorphism lX : S ∧ X ∼= X as the composite lX = rX τS,X of theright unit and the symmetry isomorphism. Then the unit isomorphism are compatible with symmetry, and∼=furthermore we have lS = rS : S ∧ S −→ S since both arise from the bimorphism S n ∧ S m −→ S n+m madeup from the canonical isomorphisms. Finally, the triangle
αX,S,Y/ X ∧ (S ∧ Y )(X ∧ S) ∧ Y
NNN
pp
NNN
ppp
p
N
p
N
rX ∧IdNN&xppp Id ∧lY
X ∧Yshould commute, which is true since it holds after restriction with the maps
ip,q ∧Id

ip+q,r(Xp ∧ S q ) ∧ Yr −−−−→ (X ∧ S)p+q ∧ Yr −−−−→ ((X ∧ S) ∧ Y )p+q+r .
46

I. BASIC DEFINITIONS AND EXAMPLES

Now we identify the smash products of certain kinds of symmetric spectra and relate it by natural
maps to other constructions. We start by describing the smash product with a semifree spectrum. In
Example 2.20 we introduced the twisted smash product L .m X of a pointed Σm -space L and a symmetric
spectrum X.
Let X be a symmetric spectrum and L be a pointed Σm -space (or Σm -simplicial set) for some m ≥ 0.
The twisted smash product L .m X was defined in Example 2.20 and consists of a point in levels smaller
than m is given in general by(L .m X)m+n = Σ+m+n ∧Σm ×Σn L ∧ Xn .
In order to link L .m X to Gm L ∧ X we note that as n varies, the (m, n)-components
ιm,nL ∧ Xn = (Gm L)m ∧ Xn −−−→ (Gm L ∧ X)m+n = (shm (Gm L ∧ X))nof the universal bimorphism ι : (Gm L, X) −→ Gm L ∧ X in fact define a morphism of Σm -symmetric spectrab̄ : L ∧ X −→ shm (Gm L ∧ X). By the adjunction (2.21) this morphism corresponds to a morphism of
symmetric spectra b : L .m X −→ Gm L ∧ X.
Proposition 3.5. Let L be a pointed Σm -space (or Σm -simplicial set) for some m ≥ 0 and X asymmetric spectrum. Then the morphism of symmetric spectrab : L .m X −→ Gm L ∧ X
is a natural isomorphism.
Proof. In (2.21) we constructed a natural bijection∼ Σm -SpΣ (L ∧ X, shm Z) .
SpΣ (L .m X, Z) =The adjunctions between the smash product and mapping spaces, the definition of the homomorphism
spectrum and the fact that Gm is adjoint to evaluation at level m yield a natural bijection
Σm -SpΣ (L ∧ X, shm Z)) ∼= Σm - sset* (L, map(X, shm Z))= Σm - sset* (L, Hom(X, Z)m ) ∼= SpΣ (Gm L, Hom(X, Z)) .
Combining all these isomorphisms gives a representation of the functor SpΣ (Gm L, Hom(X, −)) by thesymmetric spectrum L .m X. Since the smash product Gm L ∧ X represents the same functor, we get apreferred isomorphism L .m X ∼= Gm L ∧ X, which in fact equals the morphism b.
We specialize the previous proposition in several steps. The special case m = 0 provides a natural
isomorphism
K ∧ X = K .0 X ∼= (Σ∞ K) ∧ X
for pointed spaces (or simplicial sets) K and symmetric spectra X. We can also consider a Σm -space L
and a Σn -space L0 . If we spell out all definitions we see that L .m (Gn L0 ) is isomorphic to the semifree
0
symmetric spectrum Gm+n (Σ+m+n ∧Σm ×Σn L ∧ L ). So Proposition 3.5 specializes to a natural isomorphism
(3.6)

0
0
∼
Gm+n (Σ+m+n ∧Σm ×Σn L ∧ L ) = Gm L ∧ Gn L .

The isomorphism is adjoint to the Σm+n -equivariant map
00Σ+m+n ∧Σm ×Σn L ∧ L −→ (Gm L ∧ Gn L )m+n

which in turn is adjoint to the Σm × Σn -equivariant map
im,nL ∧ L0 = (Gm L)m ∧ (Gn L0 )n −−−→ (Gm L ∧ Gn L0 )m+n
given by the universal bimorphism. So the isomorphism (3.6) rephrases the fact that a bimorphism from(Gm L, Gn L0 ) to Z is uniquely determined by its (m, n)-component, which can be any Σm × Σn -equivariant
map L ∧ L0 −→ Zm+n . The isomorphism (3.6), and the ones which follow below, are suitably associative,
commutative and unital.

3. SMASH PRODUCT47

As a special case we can consider smash products of free symmetric spectra. If K and K 0 are pointed
0+0spaces or simplicial set then we have Fm K = Gm (Σ+m ∧ K) and Fn K = Gn (Σn ∧ K ), so the isomorphism (3.6) specializes to an associative, commutative and unital isomorphism
Fm+n (K ∧ K 0 ) ∼= Fm K ∧ Fn K 0 .
As the even more special case for m = n = 0 we obtain a natural isomorphism of suspension spectra∼ Σ∞ (K ∧ L)
(Σ∞ K) ∧ (Σ∞ L) =for all pairs of pointed spaces (or pointed simplicial sets) K and L.
The tensor product of I-spaces is taken to the smash product by the smash product paring of Example 2.31. More precisely, for I-spaces T, T 0 and symmetric spectra X and X 0 there is a natural and coherent
isomorphism(T ∧ X) ∧ (T 0 ∧ X 0 ) ∼= (T ∧ T 0 ) ∧ (X ∧ X 0 ) .
[elaborate] Note that here the various smash product signs have three different meanings: three smashproducts are pairings between an I-space and a symmetric spectrum, two smash products are internal
smash products of symmetric spectra and one is the internal smash product of I-spaces.
The bimorphism
i1+p,q(sh X)p ∧ Yq = X1+p ∧ Yq −−−−→ (X ∧ Y )1+p+q = sh(X ∧ Y )p+q
corresponds to a natural homomorphism of symmetric spectra
ξX,Y : (sh X) ∧ Y −→ sh(X ∧ Y ) .
The homomorphism ξX,Y is compatible with the unit and associativity isomorphisms in the sense that thefollowing diagrams commute((sh X) ∧ Y ) ∧ Z

αsh X,Y,Z/ (sh X) ∧ (Y ∧ Z)

(sh X) ∧ S
OOO
OOrOsh X
OOO
OOO'

ξX,Y ∧Idsh(X ∧ Y ) ∧ Z
ξX∧Y,Zsh((X ∧ Y ) ∧ Z)

7 sh X
ooo
o
o
o
ooo

ooo sh(rX )sh(X ∧ S)
ξX,S

ξX,Y ∧Z

sh(αX,Y,Z )/ sh(X ∧ (Y ∧ Z))

Moreover, the map ξS,Y ‘is’ the morphism λY : S 1 ∧Y −→ sh Y is the following sense: we have sh S = Σ∞ S 1and so (sh S) ∧ Y ∼= S 1 ∧ Y . with this identification, the following square commutes(sh S) ∧ Y

ξS,Y

∼
=


S1 ∧ Y/ sh(S ∧ Y )

λYsh(lY )/ sh Y

There are natural composition morphisms◦ : Hom(Y, Z) ∧ Hom(X, Y ) −→ Hom(X, Z)
which are associative and unital with respect to a unit map S −→ Hom(X, X) adjoint to the identity of X
(which is a vertex in level 0 of the spectrum Hom(X, X)). The composition morphism is obtained, by theuniversal property of the smash product, from the bimorphism consisting of the maps
shm ∧ Id

map(Y, shn Z) ∧ map(X, shm Y ) −−−−−→ map(shm Y, shm+n Z) ∧ map(X, shm Y )◦map(X,χm,n )−−−−→ map(X, shm+n Z) −−−−−−−−−→ map(X, shn+m Z)

48

I. BASIC DEFINITIONS AND EXAMPLES

where the second map is the composition pairing of Example 2.24. If we specialize to X = Y = Z, we
recover the multiplication of the endomorphism ring spectrum as defined in Example 2.26.
If X and Y are symmetric spectra we can also define natural coherent morphisms
X K ∧ Y L −→ (X ∧ Y )K∧L
for pointed spaces (simplicial sets) K and L and morphisms
map(A, X) ∧ map(B, Y ) −→ map(A ∧ B, X ∧ Y )andHom(A, X) ∧ Hom(B, Y ) −→ Hom(A ∧ B, X ∧ Y )for symmetric spectra A and B.
Now we can make precise the idea that symmetric ring spectra are the same as monoid objects in thesymmetric monoidal category of symmetric spectra with respect to the smash product.
Construction 3.7. Let us define an implicit symmetric ring spectrum as a symmetric spectrum R
together with morphisms µ : R ∧ R −→ R and ι : S −→ R which are associative and unital in the sensethat the following diagrams commute(R ∧ R) ∧ R

αR,R,R/ R ∧ (R ∧ R)

Id ∧µ

µ

µ∧Id


R∧R

/ R∧R

/R

µ

ι∧Id
/ R ∧ R o Id ∧ιS ∧ RN
R∧S
NNN
ppp
NNN
p
p
p
µ
N
ppprR
lR NNNNN'  wpppp
R

We say that the implicit symmetric ring spectrum (R, µ, ι) is commutative if the multiplication is unchanged
when composed with the symmetric isomorphism, i.e., if the relation µ ◦ τR,R = µ holds.
Given an implicit symmetric ring spectrum (R, µ, ι) we can make the collection of Σn -spaces Rn into asymmetric ring spectrum in the sense of the original Definition 1.3 as follows. As unit maps we simply takethe components of ι : S −→ R in levels 0 and 1. We define the multiplication map µn,m : Rn ∧Rm −→ Rn+mas the composite
in,m

µn+m

Rn ∧ Rm −−−→ (R ∧ R)n+m −−−−→ Rn+m .
Then the associativity condition for µ above directly translates into the associativity condition of Definition 1.3 for the maps µn,m . Evaluating the two commuting unit triangles in level 0 gives the unit conditionof Definition 1.3. Spelling out the condition µ(ι ∧ Id) = lR = rR ◦ τS,R in level 1 + n and composing withthe map i1,n : S 1 ∧ Rn −→ (S ∧ R)1+n gives the centrality condition of Definition 1.3. Finally, the conditionµ(Id ∧ι) = rR in level n + 1 composed with ιn,1 : Rn ∧ S 1 −→ (R ∧ S)n+1 shows that µn,1 ◦ (Id ∧ι1 ) equals
the structure map σn : Rn ∧ S 1 −→ Rn+1 of the underlying symmetric spectrum of R. So the conceivably
different meaning of ‘underlying symmetric spectrum’ in the sense of Remark 1.6 (iii) in fact coincides withthe underlying spectrum R.
Theorem 3.8. The construction 3.7 which turns an implicit symmetric ring spectrum into a symmetricring spectrum in sense of the original Definition 1.3 is an isomorphism between the category of implicit
symmetric ring spectra and the category of symmetric ring spectra. The functor restricts to an isomorphismfrom the category of commutative implicit symmetric ring spectra to the category of commutative symmetricring spectra.
Proof.



Now that we have carefully stated and proved Theorem 3.8 we will start to systematically blur the distinction between implicit and explicit symmetric ring spectra. Whenever convenient we use the isomorphismof categories to go back and forth between the two notions without further mentioning.

3. SMASH PRODUCT49

Example 3.9 (Smash product of ring spectra). Here is a construction of a new symmetric ring spectrum
from old ones for which the possibility to define ring spectra ‘implicitly’ is crucial. If R and S are symmetricring spectra, then the smash product R ∧ S has a natural structure as symmetric ring spectrum as follows.
The unit map is defined from the unit maps of R and S as the composite
−1
−1
rS,S
=lS,S

ι∧ι

S ←−−−−−−− S ∧ S −−−−−→ R ∧ S .
The multiplication map of R ∧ S is defined from the multiplications of R and S as the composite
Id ∧τS,R ∧Idµ∧µ(R ∧ S) ∧ (R ∧ S) −−−−−−−−→ (R ∧ R) ∧ (S ∧ S) −−−→ R ∧ S ,where we have suppressed some associativity isomorphisms. It is a good exercise to insert these associativity
isomorphisms and observe how the hexagon condition for associativity and symmetry isomorphisms enters
the verification that the product of R ∧ S is in fact associative.
Example 3.10. Another class of examples which can only be given as implicit symmetric ring spectra
are symmetric ring spectra ‘freely generated’ by a symmetric spectrum. These come in two flavors, anassociative and a commutative (and associative) version.
Given a symmetric spectrum X we define the tensor algebra as the symmetric spectrum_X· · ∧ X}TX =| ∧ ·{zn≥0nwith the convention that a 0-fold smash product is the unit object S. The unit morphism ι : S −→ T X
is the inclusion of the wedge summand for n = 0. The multiplication is given by ‘concatenation’, i.e., therestriction of µ : T X ∧ T X −→ T X to the (n, m) wedge summand is the canonical isomorphism∼=

X ∧n ∧ X ∧m −→ X ∧(n+m)
followed by the inclusion of the wedge summand indexed by n+m. In order to be completely honest here weshould throw in several associativity isomorphisms; strictly speaking already the definition of T X requires
choices of how to associate expressions such as X ∧ X ∧ X and higher smash powers. However, all of this istaken care of by the coherence conditions of the associativity (and later the symmetry) isomorphisms, andwe will not belabor this point any further.
Given any symmetric ring spectrum R and a morphism of symmetric spectra f : X −→ R we can define
a new morphism fˆ : T X −→ R which on the nth wedge summand is the composite
f ∧n

µn

X ∧n −−→ R∧n −−→ R .
Here µn is the iterated multiplication map, which for n = 0 has to be interpreted as the unit morphismι : S −→ R. This extension fˆ : T X −→ R is in fact a homomorphism of (implicit) symmetric ring spectra.
Moreover, if g : T X −→ R is any homomorphism of symmetric ring spectra then g = ĝ1 for g1 : X −→ R
the restriction of g to the wedge summand indexed by 1. Another way to say this is that
Homring spectra (T X, R) −→ SpΣ (X, R) ,g 7→ g1

is a natural bijection. In fact, this bijection makes the tensor algebra functor into a left adjoint of theforgetful functor from symmetric ring spectra to symmetric spectra.
The construction has a commutative variant. We define the symmetric algebra generated by a symmetric
spectrum X as_
PX =(X ∧n )/Σn .
n≥0Here Σn permutes the smash factors [elaborate] of X ∧n using the symmetry isomorphisms, and we takethe quotient symmetric spectrum. This symmetric spectrum has unique unit and multiplication maps suchthat the quotient morphism T X −→ P X becomes a homomorphism of symmetric ring spectra. So the unit

50

I. BASIC DEFINITIONS AND EXAMPLES

morphism ι : S −→ P X is again the inclusion of the wedge summand for n = 0 and the multiplication is
the wedge of the morphisms(X ∧n )/Σn ∧ (X ∧m )/Σm −→ (X ∧(n+m) )/Σn+m
induced on quotients by X ∧n ∧ X ∧m ∼= X ∧(n+m) .
Example 3.11. For two abelian groups A and B, a natural morphism of symmetric spectra
HA ∧ HB −→ H(A ⊗ B)
is obtained, by the universal property (3.2), from the bilinear morphism(HA)n ∧ (HB)m = (A ⊗ Z[S n ]) ∧ (B ⊗ Z[S m ])−→ (A ⊗ B) ⊗ Z[S n+m ] = (H(A ⊗ B))n+m
given by!
Xiai · xi


∧


X
j

bj · yj  7−→X(ai ⊗ bj ) · (xi ∧ yj ) .

i,jA unit map S −→ HZ is given by the inclusion of generators. With respect to these maps, H becomes alax symmetric monoidal functor from the category of abelian groups to the category of symmetric spectra.
As a formal consequence, H turns a ring A into a symmetric ring spectrum with multiplication map
HA ∧ HA −→ H(A ⊗ A) −→ HA .
This is the ‘implicit’ construction of an Eilenberg-Mac Lane ring spectrum whose explicit variant appearedin Example 2.7. Similarly, an A-module structure on B gives rise to an HA-module structure on HB.
The definition of the symmetric spectrum HA makes just as much sense when A is a simplicial abeliangroup; thus the Eilenberg-Mac Lane functor makes simplicial rings into symmetric ring spectra, respecting
possible commutativity of the multiplications.
4. Homotopy groups, M-modules and semistability
As we shall explain in Section 4 of Chapter II, formally inverting the π∗ -isomorphisms between symmetric spectra leaves ‘too many homotopy types’. Instead, we will later introduce a strictly larger classof stable equivalences, defined as the morphisms which induce isomorphisms on all cohomology theories.
In order to understand the relationship between π∗ -isomorphisms and stable equivalences, it is useful to
exploit extra algebraic structure on the homotopy groups of a symmetric spectrum. This extra structure isan action of the injection monoid M, the monoid of injective self-maps of the set of natural numbers undercomposition. The M-modules that come up, however, have a special property which we call tameness, see
Definition 4.11. Tameness has strong algebraic consequences and severely restricts the kinds of M-modules
which can arise as homotopy groups of symmetric spectra.
An important class of symmetric spectra is formed by the semistable symmetric spectra. Within thisclass, stable equivalences coincide with π∗ -isomorphisms, so it is very useful to recognize a given symmetric
spectrum as semistable. In Theorem 4.44, we characterize the semistable symmetric spectra as those forwhich the M-action on homotopy groups is trivial.
4.1. Exact sequences of stable homotopy groups. In this section we construct long exact sequences of homotopy groups from a morphism of symmetric spectra. The long exact homotopy sequence
involving the homotopy fibre also exists unstably. However, a fundamental property of stable homotopy
groups of spectra which is not satisfied by (unstable) homotopy groups of spaces is that also the mapping
cone fits into a long exact homotopy sequence.
The mapping cone C(f ) of a morphism of symmetric spectra f : X −→ Y is defined as(4.1)

C(f ) = ([0, 1] ∧ X) ∪f Y ,4. HOMOTOPY GROUPS, M-MODULES AND SEMISTABILITY

51

where [0, 1] is pointed by 0 ∈ [0, 1], so that [0, 1] ∧ X is the cone of X. At level n, C(f )n is just the pointed
mapping cone of fn : Xn −→ Yn . The mapping cone comes with an inclusion Y −→ C(f ) whose quotientC(f )/Y is the spectrum S 1 ∧ X. [how do we identify [0, 1]/{0, 1} with S 1 ?]We define a connecting homomorphism δ : π1+k C(f ) −→ πk X as the composite
π1+k (proj)
π1+k C(f ) −−−−−−−−→ π1+k (S 1 ∧ X) ∼= πk X ,(4.2)

where the first map is the effect of the projection C(f ) −→ S 1 ∧ X on homotopy groups, and the second
map is the inverse of the isomorphism S 1 ∧ − : πk X −→ π1+k (S 1 ∧ X) introduced in Example 2.16.
There is construction ‘dual’ to the mapping cone, namely the homotopy fibre. Let f : X −→ Y be amorphism between symmetric spectra. The homotopy fibre F (f ) is the symmetric spectrum
F (f ) = ∗ ×Y Y [0,1] ×Y X
i.e., the pullback in the cartesian square
(4.3)

/X

p

F (f )(∗,f )

Y [0,1](ev0 ,ev1 )/ Y ×Y

Here evi : Y [0,1] −→ Y for i = 0, 1 is the ith evaluation map which takes a path ω ∈ Y [0,1] to ω(i), i.e.,the start or endpoint. So levelwise a point in F (f ) is a pair (ω, x) where ω is a path in Y starting at thebasepoint and x is a lift of the endpoint, i.e., f (x) = ω(1).
There are morphisms
i

p

f

ΩY −
→ F (f ) −→X−→Y
the first two being given byi(ω) = (ω, ∗)respectivelyp(ω, x) = x .

The composite pi is the trivial map and the composite f p comes with a preferred null-homotopy H :[0, 1]+ ∧ F (f ) −→ Y [specify], i.e., H starts with the constant map at the basepoint and ends with f p. Thus
H factors over a well-defined morphism(4.4)

h : S 1 ∧ F (f ) −→ Y /f (X) .

We define a connecting homomorphism δ : π1+k Y −→ πk F (f ) as the composite
(4.5)

α−1

πk (i)

π1+k Y −−→ πk (ΩY ) −−−−→ πk F (f ),where α : πk (ΩY ) −→ π1+k Y is the isomorphism defined in Example 2.23.
The ‘duality’ between the two constructions manifests itself when we take morphisms into a thirdsymmetric spectrum, in the form of a natural isomorphism
Hom(C(f ), Z) ∼= F (Hom(f, Z) : Hom(Y, Z) −→ Hom(X, Z)) ,along with analogous isomorphisms for homotopy classes and mapping spaces.
The constructions of mapping cone and homotopy fibre make perfect sense for a morphism between
symmetric spectra of simplicial sets, provided the interval [0, 1] is replaced by the simplicial 1-simplex ∆[1]
throughout. The simplicial face operators d0 , d1 : ∆[0] −→ ∆[1] then take the roles of the inclusions of thetwo endpoints of the interval.
The following elementary lemma will be needed below to establish a long exact sequence of homotopy
groups. Here, CA = [0, 1] ∧ A is the cone of a based space A.
Lemma 4.6. Let f : A −→ B be a based continuous maps between based spaces and let pA : CA −→S 1 ∧A and pB : CB −→ S 1 ∧B denote the projections. [specify an identification between S 1 and [0, 1]/0 ∼ 1]

52

I. BASIC DEFINITIONS AND EXAMPLES(i) The triangle
CA ∪f CB
MMM
q
MM∗∪p
MMMB
q
q
q
MM&
q
q
xq
/ S1 ∧ B
S1 ∧ ApA ∪∗ qqq

τ ∧fcommutes up to homotopy, where τ : S 1 −→ S 1 is the map of degree -1 given by τ (t) = 1 − t.
(ii) Let β ∈ πm B be a homotopy class in the kernel of πm (i) : πm B −→ πm C(f ). Then there exists a
homotopy class α ∈ π1+m (S 1 ∧ A) such that (S 1 ∧ f )∗ (α) = S 1 ∧ β in π1+m (S 1 ∧ B).
Proof. (i) We define a based homotopy H : CA × [0, 1] −→ S 1 ∧ B by the formulaH([s, a], t) = [2 − s − t, f (a)]which has to be interpreted as the basepoint if 2 − s − t ≥ 1. Another based homotopy H 0 : CB × [0, 1] −→S 1 ∧ B is given by the formula
H 0 ([s, b], t) = [s − t, b]
which has to be interpreted as the basepoint if s ≤ t. The two homotopies are compatible in the sense that
H([1, a], t) = [1 − t, f (a)] = H 0 ([1, f (a)], t)
for all t ∈ [0, 1] and a ∈ A. So H and H 0 glue together and yield a homotopy
0H∪H(CA ∪f CB) × [0, 1] ∼= (CA × [0, 1]) ∪f ×Id (CB × [0, 1]) −−−−→ S 1 ∧ B .

For t = 0 this homotopy starts with the map ∗ ∪ pB , and it ends for t = 1 with the map (τ ∧ f ) ◦ (pA ∪ ∗).
(ii) Let b : S m −→ B be a representative of β and let H : C(S m ) −→ C(f ) be a based nullhomotopy ofthe composite of b with i : B −→ C(f ), i.e., such that H[1, x] = i(b(x)) for all x ∈ S m . We collaps 1 × S m
in C(S m ) and the image of B in C(f ) and get a map H̄ : S 1 ∧ S m −→ S 1 ∧ A induced by H on the quotient
spaces. We claim that the homotopy class α = [H] has the required property.
To prove the claim we need the homotopy equivalence p ∪ ∗ : C(S m ) ∪1×S m C(S m ) −→ S 1 ∧ S m whichcollapses the second cone. We obtain a sequence of equalities and homotopies(S 1 ∧ f ) ◦ H̄ ◦ (p ∪ ∗) = (S 1 ∧ f ) ◦ (pA ∪ ∗) ◦ (H ∪ C(b))' (τ ∧ B) ◦ (∗ ∪ pB ) ◦ (H ∪ C(b))= (τ ∧ B) ◦ (S 1 ∧ b) ◦ (∗ ∪ p)
= (S 1 ∧ b) ◦ (τ ∧ S m ) ◦ (∗ ∪ p) ' (S 1 ∧ b) ◦ (p ∪ ∗)Here H ∪ C(b) : C(S m ) ∪1×S m C(S m ) −→ C(f ) ∪B CB ∼= CA ∪f CB. The two homotopies result frompart (i) applied to f respectively the identity of S m , and we used the naturality of various constructions.
Since the map p ∪ ∗ is a homotopy equivalence, this proves that the map (S 1 ∧ f ) ◦ H̄ which represents(S 1 ∧ f )∗ (α) is homotopic to S 1 ∧ b.
Proposition 4.7. Let f : X −→ Y be a morphism of symmetric spectra.
(i) The long sequence of abelian groups
πk f

πk (incl)

δ· · · −→ πk X −−−−→ πk Y −−−−−→ πk C(f ) −−→ πk−1 X −→ · · ·is exact.
(ii) In the simplicial context, suppose also that X and Y are levelwise Kan complexes. Then the longsequence of abelian groups
π1+k fδ

πk p· · · −→ π1+k X −−−−−→ π1+k Y −→ πk F (f ) −−−−→ πk X −→ · · ·is exact.

4. HOMOTOPY GROUPS, M-MODULES AND SEMISTABILITY

53

Proof. (i) In the simplicial context, homotopy groups are defined after geometric realization, which
commutes with mapping cones. So it suffices to treat the case of symmetric spectra of spaces. We showthat the sequence
πk f

πk (incl)

πk (S 1 ∧f )πk (proj)

πk X −−−−→ πk Y −−−−−→ πk C(f ) −−−−−→ πk (S 1 ∧ X) −−−−−−→ πk (S 1 ∧ Y )is exact; when we substitute definition (4.2) of the boundary map δ, this becomes the exact sequence ofpart (i).
Exactness at πk Y : the composite of f : X −→ Y and the inclusion Y −→ C(f ) is nullhomotopic, so it
induces the trivial map on πk . So it remains to show that every element in the kernel of πk (incl) : πk Y −→πk C(f ) is in the image of πk f .
Let β ∈ πk+n Yn represent an element in the kernel. By increasing n, if necessary, we can assume thatincl∗ (β) is trivial in πk+n C(fn ). By Lemma 4.6 (ii) there is a homotopy class α ∈ π1+k+n (S 1 ∧ Xn ) suchthat (S 1 ∧ fn )∗ (α) = S 1 ∧ β. The homotopy class α̃ = (−1)k+n · (τS 1 ,Xn )∗ (α) ∈ πk+n+1 (Xn ∧ S 1 ) thensatisfies (fn ∧ S 1 )∗ (α̃) = β ∧ S 1 , and thus (σn )∗ (α̃) ∈ πk+n+1 Xn+1 hits ι∗ (β) ∈ πk+n+1 Yn+1 . So the classrepresented by β in the colimit πk Y is in the image of πk f : πk X −→ πk Y .
Exactness at πk C(f ): If we apply the previous paragraph to the inclusion i : Y −→ C(f ) instead of f ,we see that the sequence
πk (i)πk (incli )πk Y −−−→ πk C(f ) −−−−−→ πk C(i)is exact. We claim that the collaps map∗ ∪ p : C(i) ∼= CY ∪f CX −→ S 1 ∧ X
is a homotopy equivalence, and thus induces an isomorphism of homotopy groups. Since the compositeof the homotopy equivalence ∗ ∪ p : C(i) −→ S 1 ∧ X with the inclusion of C(f ) equals the projectionC(f ) −→ S 1 ∧ X, we can replace the group πk C(i) by the isomorphic group πk (S 1 ∧ X) and still obtain anexact sequence.
To prove the claim we define a homotopy inverser : S 1 ∧ X −→ CY ∪f CX
by the formula(r([s, x]) =[2s, x] ∈ CX
[2 − 2s, f (x)] ∈ CY

for 0 ≤ s ≤ 1/2, andfor 1/2 ≤ s ≤ 1,

which is to be interpreted levelwise. [specify the homotopies r(∗ ∪ p) ' Id and (∗ ∪ p)r ' Id]
Exactness at πk (S 1 ∧ Y ): If we apply the previous paragraph to the inclusion i : Y −→ C(f ) insteadof f , we see that the sequence
πk (inclf )πk (incli )πk Y −−−−−−→ πk C(f ) −−−−−→ πk (C(incl))
is exact. We claim that the collaps map
C(proj) ∼= C(CX ∪f Y ) ∪proj (S 1 ∧ X) −→ S 1 ∧ Y[define; give details] is a homotopy equivalence, so induces an isomorphism of homotopy groups. Moreover,the compositeincl

S 1 ∧ X −−→ C(proj) −→ S1 ∧ Y
is homotopic to the morphism τ ∧ f : S 1 ∧ X −→ S 1 ∧ Y , whose effect on homotopy groups is the negative ofπk (S 1 ∧ f ). Since the sign has no effect on the kernel, we can replace the group πk C(proj) by the isomorphic
group πk (S 1 ∧ Y ) and still obtain an exact sequence.
(ii) Pass to the colimit from the exact sequences of unstable homotopy groups.
54

I. BASIC DEFINITIONS AND EXAMPLES

We draw some consequences of Proposition 4.7. For every morphism A −→ B of symmetric spectra
which is levelwise an h-cofibration [define] (in the topological context) respectively levelwise injective (inthe simplicial context), the quotient spectrum B/A is level equivalent to the mapping cone. Dually, if
f : X −→ Y is a morphism of symmetric spectra which is levelwise a Serre fibration of spaces respectively
Kan fibration of simplicial sets, the strict fibre F is level equivalent to the homotopy fibre. This gives:
Corollary 4.8. (i) Suppose f : A −→ B is a h-cofibration of symmetric spectra of topological spaces
or an injective morphism of symmetric spectra of simplicial sets. Denote by p : B −→ B/A the quotient
map. Then there is a natural long exact sequence of homotopy groups
πk (f )πk (p)

δ· · · −→ πk A −−−→ πk B −−−→ πk (B/A) −−→ πk−1 A −→ · · ·where the connecting map δ is the composite of the inverse of the isomorphism πk C(f ) −→ πk (B/A) inducedby the level equivalence C(f ) −→ B/A which collapses the cone of A and the connecting homomorphism
πk C(f ) −→ πk−1 A defined in (4.2).
(ii) Suppose f : X −→ Y is a morphism of symmetric spectra which is levelwise a Serre fibration of
spaces respectively Kan fibration of simplicial sets. Denote by i : F −→ X the inclusion of the fibre overthe basepoint. Then there is a natural long exact sequence of homotopy groups
πk (i)πk (f )

δ

· · · −→ πk F −−−→ πk X −−−→ πk Y −−→ πk−1 F −→ · · ·where the connecting map δ is the composite of the connecting homomorphism πk Y −→ πk−1 F (f ) definedin (4.5) and the inverse of the isomorphism πk−1 F (f ) −→ πk−1 F induced by the level equivalence F −→F (f ) which sends x ∈ F to (const∗ , x).
(iii) Suppose that f : X −→ Y is an h-cofibration (when in the topological context) respectively levelwise
injective and X and Y are levelwise Kan complexes (when in the simplicial context). Then the morphism
h : S 1 ∧ F (f ) −→ Y /X (4.4) from the suspension of the homotopy fibre to the quotient of f is a π∗ isomorphism.
Proof. (iii) Compare the two long exact sequences and use the five lemma.

iCorollary 4.9. (i) For every family of symmetric spectra {X }i∈I and every integer k the canonical
map
!
M
_
i
i
πk X −→ πk
X
i∈I

i∈I

is an isomorphism of abelian groups.
(ii) For every finite indexing set I, every family {X i }i∈I of symmetric spectra and every integer k the
canonical map
!
Y
Y
i
πk
X
−→
πk X ii∈Ii∈I

is an isomorphism of abelian groups.
(iii) For every finite family of symmetric spectra the natural morphism from the wedge to the productis a π∗ -isomorphism.
The restriction to finite indexing sets in part (ii) of the previous corollary is essential, compare Example 4.39.
Proof. (i) We first show the special case of two summands. If A and B are two symmetric spectra,
then the wedge inclusion iA : A −→ A ∨ B has a retration. So the associated long exact homotopy groupsequence of Proposition 4.7 (i) splits into short exact sequences
πk (iA )incl

0 −→ πk A −−−−→ πk (A ∨ B) −−→ πk (C(iA )) −→ 0 .
The mapping cone C(iA ) is isomorphic to (CA) ∨ B and thus homotopy equivalent to B. So we can replace
πk (C(iA )) by πk B and conclude that πk (A ∨ B) splits as the sum of πk A and πk B, via the canonical map.

4. HOMOTOPY GROUPS, M-MODULES AND SEMISTABILITY

55

The case of a finite indexing set I now follows by induction, and the general case follows since homotopy
groups of symmetric spectra commute with filtered colimits [since the image of every compact space in an
infinite wedge lands in a finite wedge].
(ii) Unstable homotopy groups commute with products, which for finite indexing sets are also sums,
which commute with filtered colimits.
(iii) This is a direct consequence of (i) and (ii). More precisely, for finite indexing set I and every
integer k the composite map
M_YY
πk X i −→ πk ( X i ) −→ πk ( X i ) −→πk X i
i∈I

i∈I

i∈I

i∈I

is an isomorphism, where the first and last maps are the canonical ones. These canonical maps are isomorphisms by parts (i) respectively (ii), hence so is the middle map.
Remark 4.10. A wedge of π∗ -isomorphisms is a π∗ -isomorphism, and [justify] a wedge of stable equivalences is a stable equivalence. This is a stable phenomenon; unstably, one needs a non-degeneracy conditions.
[Give an example]
4.2. M-action on homotopy groups. The definition of homotopy groups does not take the symmetric group actions into account; using these actions we will now see that the homotopy groups of asymmetric spectrum have more structure.
Definition 4.11. The injection monoid M is the monoid, under composition, of injective self-maps ofthe set ω = {1, 2, 3, . . . } of natural numbers. An M-module is a left module over the monoid ring Z[M].
We call an M-module W tame if for every element x ∈ W there exists a number n ≥ 0 with the following
property: for every element f ∈ M which fixes the set n = {1, . . . , n} elementwise we have f x = x.
As we shall soon see, the homotopy groups of a symmetric spectrum have a natural tame M-action.
An example of an M-module which is not tame is the free module of rank 1. Tameness has many algebraic
consequences which we discuss in the next section. We will see in Remark 4.41 that the M-action gives
all natural operations on the homotopy groups of a symmetric spectrum; more precisely, we show that thering of natural operations on π0 X is a completion of the monoid ring of M, so that an arbitrary operation
is a sum, possibly infinite, of operations by elements from M.
Construction 4.12. We define an action of the injection monoid M on the homotopy groups of asymmetric spectrum X. We break the construction up into two steps and pass through the intermediate
category of I-functors. The category I has an object n = {1, . . . , n} for every non-negative integer n,
including 0 = ∅. Morphisms in I all injective maps. An I-functor is a covariant functor from the category
I to the category of abelian groups.
Step 1: from symmetric spectra to I-functors. For every integer k we assign an I-functor π k X to thesymmetric spectrum X. On objects, this I-functor is given by(π k X)(n) = πk+n Xn
if k + n ≥ 2 and (π k X)(n) = 0 for k + n < 2. If α : n −→ m is an injective map and k + n ≥ 2, thenα∗ : (π k X)(n) −→ (π k X)(m) is given as follows. We choose a permutation γ ∈ Σm such that γ(i) = α(i)for all i = 1, . . . , n and setα∗ (x) = sgn(γ) · γ∗ (ιm−n(x))∗
where ι∗ : πk+n Xn −→ πk+n+1 Xn+1 is the stabilization map (1.5).
We have to justify that this definition is independent of the choice of permutation γ. Suppose γ 0 ∈ Σmis another permutation which agrees with α on n. Then γ −1 γ 0 is a permutation of m which fixed thenumbers 1, . . . , n, so it is of the form γ −1 γ 0 = 1 × τ for some τ ∈ Σm−n , where 1 is the unit of Σn . It
suffices to show that for such permutations the induced action on πk+m Xm via the action on Xm satisfies
the relation(4.13)(1 × τ )∗ (ιm−n(x)) = sgn(τ ) · (ιm−n
(x))
∗
∗

56

I. BASIC DEFINITIONS AND EXAMPLES

for all x ∈ πk+n Xn . To justify this we let f : S k+n −→ Xn represent x. Since the iterated structure map
σ m−n : Xn ∧ S m−n −→ Xm is Σn × Σm−n -equivariant, we have a commutative diagram
S k+m
Id ∧τ

f ∧Id

σ m−n

f ∧Id


/ Xn ∧ S m−n

/ Xm
1×τ

Id ∧τ



S k+m

/ Xn ∧ S m−n

σ m−n


/ Xm

The composite through the upper right corner represents (1 × τ )∗ (ιm−n(x)). Since the effect on homo∗
topy groups of precomposing with a coordinate permutation of the sphere is multiplication by the sign ofthe permutation, the composite through the lower left corner represents sgn(τ ) · (ι∗m−n (x)). This proves
formula (4.13) and completes the definition of α∗ : (π k X)(n) −→ (π k X)(m).
The inclusion n −→ n + 1 induces the map ι∗ over which the colimit πk X is formed, so if we denotethe inclusion by ι, then two meanings of ι∗ are consistent. We let N denote the subcategory of I whichcontains all objects but only the inclusions as morphisms, and then we have
πk X = colimN π k X .
Step 2: from I-functors to tame M-modules. The next observation is that for any I-functor F thecolimit of F , formed over the subcategory N of inclusions, has a natural left action by the injection monoid
M. Applied to the I-functor π k X coming from a symmetric spectrum X, this yields the M-action on thestable homotopy group πk X.
We let Iω denote the category with objects the sets n for n ≥ 0 and the set ω and with all injective maps
as morphisms. So Iω contains I as a full subcategory and contains one more object ω whose endomorphism
monoid is M. We will now extend an I-functor F to a functor from the category Iω in such a way thatthe value of the extension at the object ω is the colimit of F , formed over the subcategory N of inclusions.
It will thus be convenient, and suggestive, to denote the colimit of F , formed over the subcategory N ofinclusions, by F (ω) and not introduce new notation for the extended functor. The M-action on the colimitof F is then the action of the endomorphisms of ω in Iω on F (ω).
So we set F (ω) = colimN F and first define β∗ : F (n) −→ F (ω) for every injection β : n −→ ω asfollows. We set m = max{β(n)}, denote by β|n : n −→ m the restriction of β and take β∗ (x) to be theclass in the colimit represented by the image of x under(β|n )∗ : F (n) −→ F (m) .
It is straightforward to check that this is a functorial extension of F , i.e., for every morphism α : k −→ n
in I we have (βα)∗ (x) = β∗ (α∗ (x)).
Now we let f : ω −→ ω be an injective self-map of ω, and we want to define f∗ : F (ω) −→ F (ω). If[x] ∈ F (ω) is an element in the colimit represented by a class x ∈ F (n), then we set f∗ [x] = [(f |n )∗ (x)]where f |n : n −→ ω is the restriction of f and f∗ : F (n) −→ F (ω) was defined in the previous paragraph.
Again it is straightforward to check that this definition does not depend on the representative x of the class[x] in the colimit and that the extension is functorial, i.e., we have (f α)∗ (x) = f∗ (α∗ (x)) for injections
α : n −→ ω as well as (f g)∗ [x] = f∗ (g∗ [x]) when g is another injective self-map of ω. As an example, if we
also write ι : n −→ ω for the inclusion, then we have ι∗ (x) = [x] for x ∈ F (n).
The definition just given is in fact the universal way to extend an I-functor F to a functor on the
category Iω , i.e., we have just constructed a left Kan extension of F : I −→ Ab along the inclusion
I −→ Iω . However, we do not need this fact, so we omit the proof.
A trivial but important observation straight from the definition is that the action of the injection
monoid M on the colimit of any I-functor F is tame in the sense of Definition 4.11: every element in thecolimit F (ω) is represented by a class x ∈ F (n) for some n ≥ 0; then for every element f ∈ M which fixes
the numbers 1, . . . , n, we have f∗ [x] = [x].

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57

Example 4.14. To illustrate the action of the injection monoid M on the homotopy groups of asymmetric spectrum X we make it explicit for the injection d : ω −→ ω given by d(i) = i + 1, which will
also play an important role later. For every n ≥ 1, the map d and the cycle (1, 2, . . . , n, n + 1) = χn,1 ofΣn+1 agree on n, so d acts on πk X as the colimit of the system
πk X 0ι∗ι∗
πk+1 X1/ πk+1 X1

ι∗−(1,2)∗ ◦ι∗ι∗/ πk+2 X2ι∗/ πk+2 X2

ι∗

/ ···

ι∗

/

ι∗

(−1)n (1,2,...,n,n+1)∗ ◦ι∗(1,2,3)∗ ◦ι∗/ πk+3 X3/ πk+n Xn

ι∗

/ ···


/ πk+n+1 Xn+1

ι∗

ι∗

/

(at least for k ≥ 0; for negative values of k only a later portion of the system makes sense).
Remark 4.15. The stable homotopy group πk X of a symmetric spectrum X can also be calculated
from the system of stable as opposed to unstable homotopy groups of the individual spaces Xn . For us, thesmth stable homotopy group πm
K of a pointed space K is the colimit of the sequence of abelian groups(4.16)

S1∧

S1∧

S1∧

πm K −−−→ π1+m (S 1 ∧ K) −−−→ π2+m (S 2 ∧ K) −−−→ · · ·[not consistent with earlier definition as πk (Σ∞ K)] where we stabilize from the left. Smashing with thessidentity of S 1 from the right provides a stabilization map (even an isomorphism) πm
K −→ πm+1(K ∧ S 1 ).
sss
For a symmetric spectrum we can then define an I-functor π k X by setting (π k X)(n) = πk+n Xn on objects(with no restriction on k + n) and defining the action of a morphism n −→ m in the same way as for the
I-functor π k X of unstable homotopy groups.
The map from the initial term to the colimit of the sequence (4.16) provides a natural transformations
πm K −→ πm
K which is compatible with stabilization, so it defines a morphism of I-functors π k X −→ π sk X
for every symmetric spectrum X. The induced map on colimits∼=

colimN π k X −−→ colimN π sk X
is bijective and thus an isomorphism of M-modules.
Example 4.17. Here is an alternative perspective on the I-functor π k X associated to a symmetric
spectrum X. In Example 2.31 we associated to every I-space T : I −→ T a symmetric spectrum T ∧ S.
This construction has a right adjoint Ω• : SpΣ −→ T I defined as follows. If X is a symmetric spectrum,
we set(Ω• X)(n) = map(S n , Xn )on objects, where the symmetric group Σn acts by conjugation, i.e., (γ∗ f )(x) = γf (γ −1 x) for f : S n −→ Xn
and γ ∈ Σn .
If α : n −→ m is an injective map then α∗ : map(S n , Xn ) −→ map(S m , Xm ) is given as follows. We
choose a permutation γ ∈ Σm such that γ(i) = α(i) for all i = 1, . . . , n and setα∗ (f ) = γ∗ (σ m−n (f ∧ S m−n )) ,i.e., we let γ acts as just defined on the composite
f ∧S m−n

σ m−n

Sm ∼
= S n ∧ S m−n −−−−−−→ Xn ∧ S m−n −−−−→ Xm .
The proof of the relation (4.13) above in fact shows that this definition is independent of the choice of
permutation γ. Functoriality of the assignment α 7→ α∗ is then straightforward.
The adjunctionSpΣ (T ∧ S, X) ∼= T I (T, Ω• X)
takes a morphism ϕ : T ∧ S −→ X to the natural transformation ϕ̂ : T −→ Ω• X whose value at the object
n is the adjoint T (n) −→ map(S n , Xn ) of ϕn : T (n) ∧ S n −→ Xn .
The isomorphism πk map(S n , Xn ) ∼= πk+n Xn (adjoint the loop coordinates to the right) gives an isomorphism of abelian groups between πk (Ω• X)(n) and (π k X)(n). The Σn -action on the source sphere inmap(S n , Xn ) induces the sign action on homotopy groups, so the above isomorphism is Σn -equivariant.

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I. BASIC DEFINITIONS AND EXAMPLES

Since the stabilization map ι∗ : πk+n Xn −→ πk+n+1 Xn+1 corresponds precisely to the effect of ι∗ :map(S n , Xn ) −→ map(S n+1 , Xn+1 ) on πk , we in fact have an isomorphism of I-functors πk (Ω• X) ∼= (π k X).
We now give a construction which associates to an I-functor with Σm -action F (i.e., a covariant functor
F : I −→ Z[Σm ]-mod) a new I-functor .m F and give a formula for the M-module (.m F )(ω). This will be
relevant later when we identify the M-action on the homotopy groups of semifree symmetric spectra.
Given F : I −→ Z[Σm ]-mod we define a new I-functor .m F by (.m F )(k) = 0 for k < m and(4.18)(.m F )(m + n) = Z[Σm+n ] ⊗Σm ×Σn F (n) .

We define .m F on morphisms α : m + n −→ m + k in I as follows. We choose a permutation γ ∈ Σm+k
which agrees with α on m + n and defineα∗ : (.m F )(m + n) = Z[Σm+n ] ⊗Σm ×Σn F (n) −→ Z[Σm+k ] ⊗Σm ×Σk F (k) = (.m F )(m + k)by α∗ (τ ⊗ x) = γ(τ × 1k−n ) ⊗ ι∗ (x) where ι : n −→ k is the inclusion. [functorial...]We define a homomorphism of monoids × : Σm × M −→ M by(γ(i)
for 1 ≤ i ≤ m, and(4.19)(γ × f )(i) =f (i − m) + m for m + 1 ≤ i.
We denote by Z[M]+m the monoid ring of M with its usual left multiplication action, but with action by
the monoid Σm × M via restriction along the homomorphism × : Σm × M −→ M. Since F takes values inΣm -modules, the colimit F (ω) not only has an action of M, but also a compatible left action by the groupΣm . So we can form
Z[M]+m ⊗Σm ×M F (ω)
which is a left M-module via the left multiplication action of Z[M] on itself. The point of this constructionis that it describes the colimit of the functor .m F as an M-module:
Lemma 4.20. For every I-functor with Σm -action F the natural map
Z[M]+m ⊗Σm ×M F (ω) −→ (.m F )(ω)
f ⊗ [x]7−→ f · [1 ⊗ x]

is an isomorphism of M-modules.
Proof. We define a morphism in the other direction by(.m F )(ω) −→ Z[M]+m ⊗Σm ×M F (ω) ,[γ ⊗ x] 7−→ γ ⊗ [x]where x ∈ F (n) and γ ∈ Σm+n . The main point is to check that the formulas for both maps are actuallywell-defined, i.e., they indeed factor over the tensor product over Σm × M respectively over Σm × Σn andare independent of the representative x respectively [γ ⊗ x] in the colimit F (ω) respectively (.m F )(ω). We
omit these routine verifications, and after that it is clear that both maps are M-linear and inverse to eachother.

4.3. Algebraic properties of tame M-modules. In this section we discuss some algebraic properties of tame M-modules. It turns out that tameness is a rather restrictive condition. We start with aresult which controls a lot of the homological of the monoid ring of M.
Lemma 4.21. The classifying space BM of the injection monoid M is contractible.
Proof. The classifying space BM is the geometric realization of the nerve of the category BM with
one object whose monoid of endomorphisms is M. Let t ∈ M be given by t(i) = 2i. We define an injective
endomorphism ct : M −→ M as follows. For f ∈ M and i ∈ ω we set(i
if i is odd, andct (f )(i) =2 · f (i/2) if i is even.

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59

Even though t is not bijective, the endomorphism ct behaves like conjugation by t in the sense that theformula ct (f ) · t = t · f holds. Thus t provides a natural transformation from the identity functor of BM
to B(ct ). On the other hand, if s ∈ M is given by s(i) = 2i − 1, then ct (f ) · s = s for all f ∈ M, so s
provides a natural transformation from the constant functor of BM with values 1 ∈ M to B(ct ). Thus
via the homotopies induced by t and s, the identity of BM is homotopic to a constant map, so BM iscontractible.

We introduce some useful notation and terminology. For an injective map f : ω −→ ω we write |f |for the smallest number i ≥ 0 such that f (i + 1) 6= i + 1. So in particular, f restricts to the identity on{1, . . . , |f |}. An element x of an M-module W has filtration n if for every f ∈ M with |f | ≥ n we havef x = x. We denote by W (n) the subgroup of W of elements of filtration n; for example, W (0) is the set ofelements fixed by all f ∈ M. We say that x has filtration exactly n if it lies in W (n) but not in W (n−1) .
By definition, an M-module W is tame if and only if every element has a finite filtration, i.e., if the groups
W (n) exhaust W .
The following lemmas collect some elementary observations, first for arbitrary M-modules and then fortame M-modules.
Lemma 4.22. Let W be any M-module.
(i) If two elements f and g of M coincide on n = {1, . . . , n}, then f x = gx for all x ∈ W of
filtration n.
(ii) For n ≥ 0 and f ∈ M set m = max{f (n)}. Then f · W (n) ⊆ W (m) .
(iii) We denote by d ∈ M the map given by d(i) = i + 1. If x ∈ W has filtration exactly n with n ≥ 1,then dx has filtration exactly n + 1.
(iv) Let V ⊆ W be an M-submodule such that the action of M on V and W/V is trivial. Then theaction of M on W is also trivial.
Proof. (i) We can choose a bijection γ ∈ M which agrees with f and g on n, and then γ −1 f andγ g fix n elementwise. So for x of filtration n we have (γ −1 f )x = x = (γ −1 g)x. Multiplying by γ gives
f x = gx.
(ii) If g ∈ M satisfies |g| ≥ m, then gf and f agree on n. So for all x ∈ W (n) we have gf x = f x by (i),which proves that f x ∈ W (m) .
(iii) We have d · W (n) ⊆ W (n+1) by part (ii). To prove that d increases the exact filtration we consider
x ∈ W (n) with n ≥ 1 and show that dx ∈ W (n) implies x ∈ W (n−1) .
For f ∈ M with |f | = n − 1 we define g ∈ M by g(1) = 1 and g(i) = f (i − 1) + 1 for i ≥ 2.
Then we have gd = df and |g| = n. We let h be the cycle h = (f (n) + 1, f (n), . . . , 2, 1) so that we have|hd| = f (n) = max{f (n)}. Then f x ∈ W (f (n)) by part (ii) and so−1

f x = (hd)(f x) = h(g(dx)) = (hd)x = x .
Altogether this proves that x ∈ W (n−1) .
(iv) Since the M-action is trivial on V and W/V , every f ∈ M determines an additive map δf :W/V −→ V such that x − f x = δf (x + V ) for all x ∈ W . These maps satisfy δf g (x) = δf (x) + δg (x) andso δ is a homomorphism from the monoid M to the abelian group of additive maps from W/V to V . By
Lemma 4.21 the classifying space BM of the monoid M is contractible, so H 1 (BM, A) = Hom(M, A) is
trivial for every abelian group A. Thus δf = 0 for all f ∈ M, i.e., M acts trivially on W .
Corollary 4.23. The assignment n 7→ W (n) extends to an I-functor W (•) in such a way that W 7→W
is right adjoint to the functor which assigns to an I-functor F the M-module F (ω). The counit of
the adjunction (W (•) )(ω) −→ W is injective with image the subgroup of elements
S of finite filtration, whichis also the largest tame submodule of W . The assignment W 7→ (W (•) )(ω) = n W (n) is right adjoint to
the inclusion of tame M-modules into all M-modules.
(•)

Proof. To define the I-functor W (•) on morphisms α : n −→ m in the category I we choose any
extension α̃ : ω −→ ω of α and define α∗ : W (n) −→ W (m) as the restriction of α̃· : W −→ W . This really

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I. BASIC DEFINITIONS AND EXAMPLES

has image in W (m) by part (ii) of Lemma 4.22 and is independent of the extension by (i) of that lemma.
The rest is immediate.
Lemma 4.24. Let W be a tame M-module.
(i) Every element of M acts injectively on W .
(ii) If the filtration of elements of W is bounded, then W is a trivial M-module.
(iii) If the map d given by d(i) = i + 1 acts surjectively on W , then W is a trivial M-module.
(iv) If W is finitely generated as an abelian group, then W is a trivial M-module.
Proof. (i) Consider f ∈ M and x ∈ W (n) with f x = 0. Since f is injective, we can choose h ∈ Mwith |hf | ≥ n. Then x = (hf )x = h(f x) = 0, so f acts injectively.
(ii) Lemma 4.22 (iii) implies that if W = W (n) for some n ≥ 0, then n = 0, so the M-action is trivial.
(iii) Suppose M does not act trivially, so that W (0) 6= W . Let n be the smallest positive integer suchthat W (0) 6= W (n) . Then by part (iii) of Lemma 4.22, any x ∈ W (n) − W (0) is not in the image of d, so ddoes not act surjectively.
(iv) The union of the nested sequence of subgroups W (0) ⊆ W (1) ⊆ W (2) ⊆ · · · is W . Since finitely
generated abelian groups are Noetherian, we have W (n) = W for all large enough n. By part (ii), themonoid M must act trivially.
Parts (i), (iii) and (iv) of Lemma 4.24 can fail for non-tame M-modules: we can let f ∈ M act on theabelian group Z as the identity if the image of f : ω −→ ω has finite complement, and we let f acts as 0 if
its image has infinite complement.
Example 4.25. We introduce some important tame M-modules Pn for n ≥ 0. The module Pn is thefree abelian group with basis the set of ordered n-tuples of pairwise distinct elements of ω (or equivalently
the set of injective maps from n to ω). The monoid M acts from the left on this basis by componentwise
evaluation, i.e., f (x1 , . . . , xn ) = (f (x1 ), . . . , f (xn )), and it acts on Pn by additive extension. For n = 0,
there is only one basis element, the empty tuple, and so P0 is isomorphic to Z with trivial M-action. Forn ≥ 1, the basis is countably infinite and the M-action is non-trivial. The module Pn is tame: the filtration(m)of a basis element (x1 , . . . , xn ) is the maximum of the components. So the filtration subgroup Pn isgenerated by the n-tuples all of whose components are less than or equal to m. An equivalent way of saying(m)this is that Pn = Z[I(n, m)], the free abelian group generated by all injections from n to m; in particular,(m)
Pn is trivial for m < n.
The module Pn represents the functor of taking elements of filtration n: for every M-module W , themap
HomM-mod (Pn , W ) −→ W (n) , ϕ 7→ ϕ(1, . . . , n)is bijective.
We end this section with some homological algebra of tame M-modules. This will not be needed until
Section 4.2 of Chapter II where we construct a spectral sequence which converges to the ‘true’ homotopy
groups of a symmetric spectrum. The E 2 -term of that spectral sequence consists of Tor groups over themonoid ring Z[M].
Lemma 4.26. (i) Let Z[M]+ denote the monoid ring of M with its usual left action, but with rightaction through the monomorphism (1 × −) : M −→ M given by (1 × f )(1) = 1 and (1 × f )(i) = f (i − 1) + 1
for i ≥ 2. Then for every n ≥ 0 the map
κ : P1+n −→ Z[M]+ ⊗M Pn
which sends the generator (1, . . . , n + 1) to the element 1 ⊗ (1, . . . , n) of filtration n + 1 in Z[M]+ ⊗M Pn
is an isomorphism of M-modules.
(ii) For every n ≥ 0 and every abelian group A, the groups TorpZ[M] (Z, Pn ⊗ A) vanish in positive
dimensions.

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61(iii) For every n ≥ 0 and every Σn -module B we have a natural isomorphism
TorZ[M](Z, Pn ⊗Σn B) ∼= H∗ (Σn ; B) .
∗Proof. (i) For any n-tuple (x1 , . . . , xn ) of pairwise distinct natural numbers we can choose g ∈ Mwith g(i) = xi for 1 ≤ i ≤ n. Because off ⊗ (x1 , . . . , xn ) = f ⊗ g(1, . . . , n) = f (1 × g) · (1 ⊗ (1, . . . , n))the element 1 ⊗ (1, . . . , n) generates Z[M]+ ⊗M Pn , so the map κ is surjective. The map Z[M]+ ⊗M Pn −→P1+n which sends f ⊗ (x1 , . . . , xn ) to (f (1), f (x1 + 1), . . . , f (xn + 1)) is right inverse to κ since the composite
sends the generator (1, . . . , n + 1) to itself. So κ is also injective.
(ii) The groups TorZ[M](Z, A) are isomorphic to the singular homology groups with coefficients in A ofpthe classifying space BM of the monoid M. This classifying space is contractible by Lemma 4.21, so thegroups TorZ[M](Z, A) vanish for p ≥ 1, which proves the case n = 0.
p
For n ≥ 1, the M-modules P1+n ⊗ A and Z[M]+ ⊗M Pn ⊗ A are isomorphic by part (i). Since theM-bimodule Z[M]+ is free as a left and right module separately, the balancing property of Tor groups
yields
TorZ[M](Z, P1+n ⊗ A) ∼= Tor∗Z[M] (Z, Z[M]+ ⊗M Pn ⊗ A)∗∼
(Z ⊗M Z[M]+ , Pn ⊗ A) ∼(Z, Pn ⊗ A)
= TorZ[M]= TorZ[M]∗∗since Z ⊗M Z[M]+ is again the trivial right M-module Z. So induction on n shows that the groups(Z, Pn ⊗ A) vanish in positive dimensions.
TorZ[M]p(iii) Since Pn is free as a right Σn -module, the functor Pn ⊗Σn − is exact. The functor takes the freeΣn -module of rank 1 to Pn , so by part (ii) it takes projective Σn -modules to tame M-modules which areacyclic for the functor Z ⊗M −.
Thus if P• −→ B is a projective resolution of B by Σn -modules, then Pn ⊗Σn P• is a resolution ofPn ⊗Σn B which can be used to calculate the desired Tor groups. Thus we have isomorphisms(Z, Pn ⊗Σn B) = H∗ (Z ⊗M Pn ⊗Σn P• ) ∼TorZ[M]= H∗ (Z ⊗Σn P• ) = H∗ (Σn ; B) .
∗
4.4. Examples. We discuss several classes of symmetric spectra with a view towards the M-action
on the stable homotopy groups.
Example 4.27 (Eilenberg-Mac Lane spectra). Every tame M-module W can be realized as the homotopy group of a symmetric spectrum. For this purpose we modify the construction of the symmetric
Eilenberg-Mac Lane spectrum of an abelian group. We define a symmetric spectrum HW of simplicial setsby
(HW )n = W (n) ⊗ Z[S n ] ,where W (n) is the filtration n subgroup of W and Z[S n ] refers to the simplicial abelian group freely generated
by the simplicial set S n = S 1 ∧. . .∧S 1 , divided by the subgroup generated by the basepoint. The symmetricgroup Σn takes W (n) to itself and we let it act diagonally on (HW )n , i.e., on S n by permuting the smash
factors. If M acts trivially on W , then this is just the ordinary Eilenberg-Mac Lane spectrum introducedin Example 2.7. Note that HW is an Ω-spectrum if and only if the M-action on W is trivial.
Since (HW )n is an Eilenberg-Mac Lane space of type (W (n) , n) the homotopy
groups of the symmetricS
spectrum HW are concentrated in dimension zero where we have π0 HW ∼= n≥0 W (n) = W as M-modules.
As an aside we note that instead of the system n 7→ W (n) we could use any I-functor in the definition above;
this shows that every I-functor arises as the I-functor π 0 of a symmetric spectrum.
Example 4.28 (Twisted smash products). We describe the homotopy groups of a twisted smash productL .m X (see Example 2.20) as a functor of the homotopy groups of L ∧ X, using all available structure on
those. Since L .m X is isomorphic to Gm L ∧ X this gives a description of the homotopy groups of smash

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I. BASIC DEFINITIONS AND EXAMPLES

products with semifree spectra. Since free and semifree symmetric spectra are special cases of twisted smashproducts, this will specialize to formulas for the homotopy groups of free and semifree symmetric spectra.
So we consider a symmetric spectrum X and a non-degenerately based Σm -space (or pointed Σm simplicial set) L for some m ≥ 0. We construct a natural isomorphism of M-modules(4.29)

πk (L .m X) ∼= Z[M]+m ⊗Σm ×M (sgnm ⊗πk+m (L ∧ X)) .

Here we use the left Σm -action on πk+m (L ∧ X) through the action on L, twisted by the sign representation
sgnm of Σm , and Z[M]+m is the monoid ring of M with its natural left multiplication action and rightaction of Σm × M through the homomorphism × : Σm × M −→ M, compare (4.19).
We remark that as a right Σm × M-module, Z[M]+m is free of countably infinite rank. One possible
basis is given by the ‘(m, ∞)-shuffles’, i.e., by those bijections f ∈ M which satisfy f (i) < f (i + 1) for
all i 6= m. In other words, all bijective f which keep the sets m = {1, . . . , m} and {m + 1, m + 2, . . . } intheir natural order. So the isomorphism (4.29) in particular implies that the underlying abelian group ofπk (L .m X) is a countably infinite sum of copies of the underlying abelian group of πk+m (L ∧ X).
To establish the isomorphism (4.29) we calculate the I-functor π sk (L .m X) consisting of the stablehomotopy groups of the spaces in the spectrum L .m X and exploit that for any symmetric spectrum Y
the M-module πk Y can also be calculated as the colimit of π sk Y instead of the I-functor π k Y of unstablehomotopy groups, see Remark 4.15. We denote by sgnm the sign representation of the symmetric groupsΣm and recall that for every spectrum Y the action of Σm = I(m, m) on (π sk Y )(m) = πk+m
Ym is inducedby the action on Ym twisted by sgnm .
Since stable homotopy groups takes wedges to sums we have an isomorphism of Σm+n -modulessπ sk (L .m X)(m + n) = sgnm+n ⊗πk+(m+n)Σ+(4.30)
m+n ∧Σm ×Σn (L ∧ Xn )s∼(L ∧ Xn )= Z[Σm+n ] ⊗Z[Σm ×Σn ] sgnm ⊗ sgnn ⊗π(k+m)+n= Z[Σm+n ] ⊗Z[Σm ×Σn ] sgnm ⊗π sk+m (L ∧ X)
The isomorphism (4.30) is also compatible with stabilization maps, so it is an isomorphism of I-functors
π sk (L .m X) ∼= .m (sgnm ⊗π sk+m (L ∧ X))
where .m is the construction for I-functors with Σm -action defined in (4.18). Lemma 4.20 provides anisomorphism of M-modules(.m (sgnm ⊗π sk+m (L ∧ X)))(ω) ∼= Z[M]+m ⊗Σm ×Z[M] (sgnm ⊗πk+m (L ∧ X)) .
Combining these two isomorphisms gives (4.29).
Example 4.31 (Free and semifree symmetric spectra). We saw in Example 2.12 that the zeroth stablehomotopy group of the free symmetric spectrum F1 S 1 is free abelian of countably infinite rank. We now
refine this calculation to an isomorphism of M-modules π0 (Fm S m ) ∼= Pm , see (4.33) below; here Pm is the
M-module which represents taking filtration m elements, see Example 4.25. So while the groups π0 (Fm S m )are all additively isomorphic for different positive m, the M-action distinguishes them. In particular, there
cannot be a chain of π∗ -isomorphisms between Fm S m and Fn S n for n 6= m.
The calculation of the M-action on free and semifree symmetric spectra is a special case of the very
general formula (4.29) for the homotopy groups of a twisted smash product. Let L be a pointed space (orsimplicial set) with a left action by the symmetric group Σm , for some m ≥ 0. Recall that Gm L denotes
the semifree symmetric spectrum generated by L in level m, defined in Example 2.13, which is also equal
to the twisted smash product L .m S of L with the sphere spectrum. The functor Gm is left adjoint toevaluating a symmetric spectrum at level m, viewed as a functor with values in pointed Σm -spaces. When
we apply formula (4.29) to Gm L = L .m S we obtain an isomorphism of M-modules
πk (Gm L) = πk (L .m S) ∼= Z[M]+m ⊗Σm ×M (sgnm ⊗πk+m (L ∧ S)) .

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63

The homotopy groups of the spectrum L ∧ S = Σ∞ L are the stable homotopy groups of L. Since theM-action on πk+m (L ∧ S) is trivial [justify] we gets
Z[M]+m ⊗Σm ×M (sgnm ⊗πk+m (L ∧ S)) ∼L) .
= (Z[M]+m ⊗1×M Z) ⊗Σm (sgnm ⊗πk+m

The tame M-module Pm has a compatible right Σm -action which is given on the basis by permuting thecomponents of an m-tuple, i.e., (x1 , . . . , xm )γ = (xγ(1) , . . . , xγ(m) ). The map
Z[M]+m ⊗1×M Z −→ Pm ,f ⊗ 1 7−→ (f (1), . . . , f (m))

is an isomorphism of M-Σm -bimodules; so combining all these isomorphisms we finally get a natural
isomorphism of M-modules(4.32)sπk (Gm L) ∼L)(sgn) .
= Pm ⊗Σm (πk+m

On the right of the tensor symbol, the group Σm acts by what is induced on stable homotopy groups by
the action on L, twisted by sign.
Free symmetric spectra are special cases of semifree symmetric spectra. For a pointed space K (withouts+s∼
any group action) we have Fm K ∼= Gm (Σ+m ∧ K) and πk+m (Σm ∧ K) = Z[Σm ] ⊗ πk+m K as Σm -modules.
So (4.32) specializes to a natural isomorphism of M-modules(4.33)sπk (Fm K) ∼K .
= Pm ⊗ πk+msHere πk+m
K is the (k + m)th stable homotopy group of K; the monoid M acts only on Pm .

Example 4.34 (Loop and suspension). The loop ΩX and suspension S 1 ∧X of a symmetric spectrum X
are defined by applying the functors Ω respectively S 1 ∧− levelwise, where the structure maps do not interact
with the new loop or suspension coordinates, compare Examples 2.16 and 2.23. We already saw that loop
and suspension simply shift the homotopy groups, and we shall now prove that the M-action is unchangedin this process.
For every symmetric spectrum X the map S 1 ∧− : πk+n Xn −→ π1+k+n (S 1 ∧Xn ) is Σn -equivariant and anatural transformations of I-functors as n varies. So the induced morphism S 1 ∧− : πk X −→ π1+k (S 1 ∧X)
on colimits is M-linear, and hence, by Example 2.16, an isomorphism of M-modules.
The isomorphism α : πk+n Ω(Xn ) ∼= π1+k+n Xn which we used in Example 2.23 sends a representing
continuous map f : S k+n −→ Ω(Xn ) to the class of the adjoint fˆ : S 1+k+n −→ Xn given by fˆ(s ∧ t) =f (t)(s), where s ∈ S 1 , t ∈ S k+n . As n varies, these particular isomorphisms are compatible with thesymmetric group actions and stabilization maps, so they form an isomorphism of I-functors α : π k (ΩX) ∼=π 1+k X. Hence the induced isomorphism on colimits α : πk (ΩX) ∼= π1+k X is M-linear.
Example 4.35 (Shift). The shift is another construction for symmetric spectra which reindexes thehomotopy groups, but unlike the suspension, this construction changes the M-action in a systematic way.
The shift of a symmetric spectrum X was defined in Example 2.18 by (sh X)n = X1+n with action of Σn
via the monomorphism (1 × −) : Σn −→ Σ1+n . The structure maps of sh X are the reindexed structuremaps for X.
If we view Σn as the subgroup of M of maps which fix all numbers bigger than n, then the homomorphism (1 × −) : Σn −→ Σ1+n has a natural extension to a monomorphism (1 × −) : M −→ M given by(1 × f )(1) = 1 and (1 × f )(i) = f (i − 1) + 1 for i ≥ 2. The image of the monomorphism 1 × − is thesubmonoid of those g ∈ M with g(1) = 1. If W is an M-module, we denote by W (1) the M-module withthe same underlying abelian group, but with M-action through the endomorphism 1 × −. We call W (1)the shift of W . Since |1 × f | = 1 + |f |, shifting an M-module shifts the filtration subgroups, i.e., we have
W (1)(n) = W (1+n) for all n ≥ 0. Thus the M-module W (1) is tame if and only if W is.
For any symmetric spectrum X, integer k and large enough n we haveπ(k+1)+n (sh X)n = πk+(1+n) X1+n ,and the maps in the colimit system for πk+1 (sh X) are the same as the maps in the colimit system for πk X.
Thus we get πk+1 (sh X) = πk X as abelian groups. However, the action of a permutation on πk+1+n (sh X)n64

I. BASIC DEFINITIONS AND EXAMPLES

is shifted by the homomorphism 1 × −, so we have
(4.36)π∗+1 (sh X) = (π∗ X)(1)

as M-modules.
Example 4.37 (Shift adjoint). The shift functor has a left adjoint S 0 .1 given by (S 0 .1 X)0 = ∗ and(S 0 .1 X)1+n = Σ+
1+n ∧Σn Xn
for n ≥ 0. Here Σn acts from the right on Σ1+n via the monomorphism (1 × −) : Σn −→ Σ1+n . The+
1structure map (Σ+1+n ∧Σn Xn ) ∧ S −→ Σ1+n+1 ∧Σn+1 Xn+1 is induced by (− × 1) : Σ1+n −→ Σ1+n+1 (the‘inclusion’) and the structure map of X.
The effect on homotopy groups of the functor S 0 .0 is given as a special case of the very generalformula (4.29) for the homotopy groups of a twisted smash product. Indeed, that formula specializes to anatural isomorphism of M-modules(4.38)
πk (S 0 .1 X) ∼= Z[M]+ ⊗M πk+1 X .
Here Z[M]+ denotes the monoid ring of M with its usual left action, but with right action through themonomorphism (1 × −) : M −→ M given by (1 × f )(1) = 1 and (1 × f )(i) = f (i − 1) + 1 for i ≥ 2. As aright M-module, Z[M]+ is free of countably infinite rank (one possible basis is given by the transpositions(1, n) for n ≥ 1). So the isomorphism (4.38) in particular implies that the underlying abelian group ofπk (S 0 .1 X) is a countably infinite sum of copies of the underlying abelian group of πk+1 X.
The functor Z[M]+ ⊗M − is left adjoint to HomM (Z[M]+ , −), which is a fancy way of writing the algebraic shift functor W 7→ W (1). Under the isomorphism (4.38) and the identification (4.36), the adjunction
between shift and S 0 .1 as functors of symmetric spectra corresponds exactly to the adjunction between
W 7→ W (1) and Z[M]+ ⊗Z[M] − as functors of tame M-modules.
Example 4.39 (Infinite products). Finite products of symmetric spectra are π∗ -isomorphic to finite
wedges, so stable homotopy groups commute with finite products. But homotopy groups do not in generalcommute with infinite products. This should not be surprising because stable homotopy groups involves asequential colimit, and these generally do not preserve infinite products.
There are even two different ways in which commutation with products can fail. First we note that an
infinite product of a family {Wi }i∈I of tame M-modules is only tame if almost all the modules Wi havetrivial M-action.
Q Indeed, if there are infinitely many Wi with non-trivial M-action, then by Lemma 4.24 (ii)the product i∈I Wi contains tuples of elements whose filtrations are not bounded. We define the tameproduct of the family {Wi }i∈I by!
tameY[ Y (n)
Wi =
Wi
,
i∈I

n≥0

i∈I

which is the largest tame submodule of the product and thus the categorical product in the category oftame M-modules.
Now we consider a family {Xi }i∈I of symmetric spectra. Since the monoid M acts tamely on thehomotopy groups of any symmetric spectrum, the natural map from the homotopy groups of the productspectrum to the product of the homotopy groups always lands in the tame product. But in general, thisnatural map
!
tame
Y
Y
(4.40)πkXi −→πk X ii∈I

i∈I

need not be an isomorphism. As an example we consider the symmetric spectra (F1 S 1 )≤i obtained bytruncating the free symmetric spectrum F1 S 1 above level i, i.e.,((F1 S 1 )n for n ≤ i,
1 ≤i((F1 S ) )n =∗for n ≥ i + 1

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65

1
1 ≤i
with structure maps as a quotient spectrum
Q of F1 S . Then (F1 S ) has trivial homotopy groups for all i.
The 0th homotopy group of the product i≥1 (F1 S 1 )≤i is the colimit of the sequence of maps
Y (n)
Y
(n+1)
P1 −→
P1
i≥n

i≥n+1
(n)

which first projects away from the factor indexed by i = n and then takes a product of inclusions P1 −→
Qtame
L
(n+1)
P1
. The colimit is the quotient of the tame product i≥1 P1 by the sumi≥1 P1 ; so π0 of the
product is non-zero and even has a non-trivial M-action.
Remark 4.41. The injection monoid M gives essentially all natural operations on the homotopy groupsof symmetric spectra. More precisely, we now identify the ring of natural operations π0 X −→ π0 X witha completion of the monoid ring Z[M]. Moreover, tame M-modules can equivalently be described as thediscrete modules over the ring of operations. We will not need this information later, so we will be brief.
We define the ring Z[[M]] as the endomorphism ring of the functor π0 : SpΣ −→ Ab. So an elementof Z[[M]] is a natural self-transformation of the functor π0 , and composition of transformations gives the
product. The following calculation of this ring depends on the fact that the homotopy group functor π0
is pro-represented, in the level homotopy category of symmetric spectra, by the inverse system of freesymmetric spectra Fn S n , and that we know π0 (Fn S n ) by Example 4.31.
In more detail: for every n ≥ 0 we let jn ∈ πn (Fn S n )n be the wedge summand inclusion S n −→+
Σn ∧ S n = (Fn S n )n indexed by the unit element of Σn . Then evaluation at jn is a bijection[Fn S n , X] −→ πn Xn ,[f ] 7→ f∗ (jn )where the left hand side means homotopy classes of morphisms of symmetric spectra. We write λ :n1
Fn+1 S n+1 −→ Fn S n for the morphism adjoint the wedge summand inclusion S n+1 −→ Σ+
n+1 ∧ (S ∧ S ) =n(Fn S )n+1 indexed by the unit element of Σn+1 . Then we have
λ∗ (jn+1 ) = ι∗ (jn )in the group πn+1 (Fn S n )n+1 which implies that the squares[Fn S n , X]
[λ,X]

∼
=

ι∗



[Fn+1 S n+1 , X]

/ πn X n∼=/ πn+1 Xn+1

commute. Passage to colimits give a natural isomorphism
colimn [Fn S n , X] −→ π0 X .
From here the Yoneda lemma shows that we get an isomorphism of abelian groups(4.42)

β : Z[[M]] −→ lim π0 (Fn S n ) ,n(where the limit is taken over the maps π0 λ) by sending a natural transformation τ : π0 −→ π0 to the tuple{τFn S n [jn ]}n .
It remains to exhibit the ring Z[[M]] as a completion of the monoid ring Z[M]. The natural action of
M on the 0th homotopy group of a symmetric spectrum provides a ring homomorphism Z[M] −→ Z[[M]].
We define a left ideal In of Z[M] as the subgroup generated by all differences of the form f − g for all
f, g ∈ M such that f and g agree on n. If W is a tame M-module and if x ∈ W (n) has filtration n, thenIn · x = 0. So the action of the monoid ring Z[M] on any tame module automatically extends to an additivemap(lim Z[M]/In ) ⊗ W −→ W .
n

66

I. BASIC DEFINITIONS AND EXAMPLES(Warning: In is not a right ideal for n ≥ 1, so the completion does not a priori have a ring structure). Since
the homotopy groups of every symmetric spectrum form tame M-modules, this gives a map of abeliangroupsα : lim Z[M]/In −→ Z[[M]]nwhich extends the map from the monoid ring Z[M].
To prove that α is a bijection we show that the composite βα : limn Z[M]/In −→ limn π0 (Fn S n )with the isomorphism (4.42) is bijective. But this holds because the composite arise from compatible
isomorphisms
Z[M]/In −→ π0 (Fn S n ) , f + In 7−→ f · [jn ] ,∼ π0 (Fn S n ) from Example 4.31.
which in turn uses the isomorphisms Pn =We end this remark by claiming without proof that the extended action of Z[[M]] on a tame M-module
W makes it a discrete module in the sense that the action map
Z[[M]] × W −→ W
is continuous with respect to the discrete topology on W and the filtration topology on Z[[M]]. Conversely,
if W is discrete module over Z[[M]], then its underlying M-module is tame. This establishes an isomorphism
between the category of tame M-modules and the category of discrete Z[[M]]-modules.
4.5. Semistable symmetric spectra. The semistable spectra form an important class of symmetric
spectra since for these, the naively defined homotopy groups of (1.5) coincide with the ‘true’ homotopy
groups, i.e., morphisms in the stable homotopy category from the sphere spectra (a more precise definitionof the true homotopy groups will be given in Section 4.2 of Chapter II). As a slogan, for semistable
spectra the homotopy groups are ‘correct’, and they are pathological otherwise. The rigorous meaning of
this statement will only become clear later when we have introduced stable equivalences and the stablehomotopy category. Many symmetric spectra which arise naturally are semistable, compare Example 4.48.
In Theorem 4.44 below we define semistable symmetric spectra via several equivalent conditions. To
state these, we need a few definitions. As we have seen in Examples 2.16 and 2.18, the suspension andshift construction both shift the homotopy groups. However, there is in general no morphism of symmetric
spectra which realizes an isomorphism on homotopy groups. The two constructions are related by thenatural morphism λX : S 1 ∧ X −→ sh X which was defined in (2.19). We set RX = Ω(sh X) and obtain amorphism λ∗X : X −→ RX as the adjoint of λX . The n-th level of λ∗ differs from the adjoint structure map
σ̃n : Xn −→ ΩXn+1 by the isomorphism Ω(χn,1 ) : Ω(Xn+1 ) −→ Ω(X1+n ). So X (levelwise Kan when in
the simplicial context) is an Ω-spectrum if and only if the morphism λ∗X : X −→ RX is a level equivalence.
We iterate this construction and let R∞ X be the colimit of the sequence(4.43)λ∗R(λ∗ )R2 (λ∗ )X −−→ RX −−−−−→ R2 X −−−−−−→ · · · .

∞This construction comes with a canonical natural morphism λ∞
X : X −→ R X.

Theorem 4.44. For every symmetric spectrum X the following conditions are equivalent.
(i) The action of the monoid M is trivial on all homotopy groups of X.
(ii) The morphism λX : S 1 ∧ X −→ sh X is a π∗ -isomorphism.
(iii) The morphism λ∗X : X −→ Ω(sh X) is a π∗ -isomorphism.
∞(iv) The morphism λ∞
X : X −→ R X is a π∗ -isomorphism.
∞(v) The symmetric spectrum R X is an Ω-spectrum.
(vi) There exists a π∗ -isomorphism from X to an Ω-spectrum.
If conditions (i)-(vi) hold, then X is called semistable. [For symmetric spectra of simplicial sets, X shouldbe levelwise Kan in (iii), (iv) and (v).]Proof. For symmetric spectra of simplicial sets all relevant notions are defined by geometrically realizing to a topological spectrum. So it suffices to prove the theorem for symmetric spectra of topologicalspaces.

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67

To see that condition (i) implies condition (iii) we compare the effect of λ∗X : X −→ Ω(sh X) on
homotopy with the action of the special monoid element d given by d(i) = i + 1. The level n component
λ∗n : Xn −→ Ω(X1+n ) is adjoint to the composite∼=(1,...,n,n+1)σnS 1 ∧ Xn −−−−→ Xn ∧ S 1 −−−−−→ Xn+1 −−−−−−−−→ X1+n

twist

So the square/ πk+n+1 Xn+1ι∗

πk+n Xn
πk+n (λ∗n)

πk+n Ω(X1+n )(−1)n (1,...,n,n+1)∗∼=/ πk+1+n X1+n(−1)k αcommutes, where ι∗ is the stabilization map, and the isomorphism α is as in Example 2.23. The signs arise
as the effect of moving a sphere coordinate past k respectively n other coordinates. As n increases, themaps (−1)n (1, . . . , n, n + 1)∗ ◦ ι∗ : πk+n Xn −→ πk+1+n X1+n stabilize to the left multiplication of d ∈ M
on πk X, see Example 4.14. So the square(4.45)/ (πk X)(1)

d·

πk X
πk (λ∗X)πk Ω(sh X)∼=(−1)k α/ πk+1 (sh X)

commutes, and if M, hence in particular d, acts trivially, then λ∗X is a π∗ -isomorphism.
Conditions (ii) and (iii) are equivalent since we have a commutative square
πk X

πk (λ∗X)=S 1 ∧− ∼
π1+k (S 1 ∧ X)

/ πk Ω(sh X)∼= απ1+k (λX )/ π1+k (sh X)

in which both vertical maps are isomorphisms.
(iii) ⇒ (iv): The commutative square (4.45) implies a natural isomorphism of M-modules(4.46)
πk (R∞ X) ∼= (πk X)(∞) .
Here for an M-module V we denote by V (∞) the colimit of the sequence
d·

d·

d·

V −−→ V (1) −−→ V (2) −−→ · · ·(note that (1 × f )d = df for all f ∈ M, which means that d· : V −→ V (1) is indeed M-linear, and so thecolimit V (∞) is naturally an M-module). If condition (iii) holds for X then it also holds for Rn X for all
X and the morphism X −→ R∞ X is a π∗ -isomorphism.
(iv) ⇒ (v): As we saw in the proof of the previous implication, the effect of λ∞X on homotopy groups
coincides with the natural injection πk X −→ (πk X)(∞) obtained by iterated left multiplication by d ∈ M.
If that injection is a bijection the d acts bijectively on the homotopy groups of X. Hence every monoid
element acts identically on πk X. The commutative square
πk X(1n ×d)·∼=
πk+n (R∞ X)n/ πk X∼=ι∗/ πk+n+1 (R∞ X)n+1[define the vertical iso; check] thus shows that R∞ X is an Ω-spectrum.

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I. BASIC DEFINITIONS AND EXAMPLES

To see that condition (vi) implies condition (i) we show that the M-action on the homotopy groupsof an Ω-spectrum is always trivial. If X is an Ω-spectrum, then for every integer k the canonical map
πk X0 −→ πk X (for k ≥ 0) or the map π0 X−k −→ Xk (for k ≤ 0) is bijective. So every element of πk X has
filtration max(0, −k). But tame M-modules with bounded filtration necessarily have trivial M-action by
Lemma 4.24 (ii).
Proposition 4.47. Let X be a symmetric spectrum which satisfies one of the following conditions.
(i) For every k ∈ Z there is an n ≥ 0 such that the canonical map πk+n Xn −→ πk X is surjective.
(ii) Every even permutation γ ∈ Σn acts as the identity on the homotopy groups of Xn .
(iii) X is underlying an orthogonal spectrum.
(iv) The homotopy groups of X are dimensionwise finitely generated as abelian groups.
Then the M-action on all homotopy groups of X is trivial and so X is semistable.
Proof. (i) Under the assumption every element of πk X has filtration n. But tame M-modules withbounded filtration necessarily have trivial M-action by Lemma 4.24 (ii).
(ii) We consider more generally any I-functor F which takes all even permutations to identity maps.
Given f ∈ M and an element [x] ∈ F (ω) in the colimit represented by x ∈ F (n), then we can findm ≥ max{f (n)} and an even permutation γ ∈ Σm such that γ agrees with f on n. Since γ is even, wethen have f∗ [x] = [(f |n )∗ (x)] = [(γ|n )∗ (x)] = [γ∗ (ιm−n(x))] = [ιm−n(x)] = [x]. Thus the monoid M acts∗∗
trivially on the colimit F (ω).
(iii) The inclusion Σn −→ O(n) as permutation matrices sends all even permutations to the pathcomponent of the unit in O(n). So if the Σn -action on a pointed space Xn extends to an O(n)-action, thenall even permutations act as the identity on the homotopy groups of Xn . So part (ii) applies.
(iv) If πk X is finitely generated as an abelian group, then tameness forces the M-action to be trivial
on πk X (Lemma 4.24 (iv)).

Example 4.48. An important special case where condition (i) in Proposition 4.47 above holds is when
the homotopy groups of a symmetric spectrum X stabilize, i.e., for each k ∈ Z there exists an n ≥ 0 suchthat from the group πk+n Xn on, all maps in the sequence (1.5) defining πk X are isomorphisms.
Examples of symmetric spectra with stabilizing homotopy groups include all suspension spectra (Example 2.16), Ω-spectra, or Ω-spectra from some point Xn on. So it includes Eilenberg-Mac Lane spectra HA
associated to an abelian group (see Example 2.7) as well as spectra of topological K-theory (Example 2.10)
and algebraic K-theory (Example 2.11). The symmetric spectrum obtained from a Γ-space A (see Example 2.39) by evaluation on spheres is another example since the structure map A(S n ) ∧ S 1 −→ A(S n+1 ) is
(2n + 1)-connected [34, Prop. 5.21]. So all these kinds of symmetric spectra are semistable.
The various Thom spectra M O, M SO and M Spin of Example 2.8 or M U , M SU and M Sp of Example 2.9 are underlying orthogonal spectra, so they are all semistable.
Example 4.49. We collect some examples of symmetric spectra which are not semistable. Example 4.31
identifies the homotopy groups of free and semifree symmetric spectra assπk (Fm K) ∼K
= Pm ⊗ πk+m

respectivelysπk (Gm L) ∼L)(sgn) .
= Pm ⊗Σm (πk+mSince Pm is free of countably infinite rank as a right Σm -module, the free or semifree symmetric spectra
generated in positive level m are never semistable unless K respectively L has trivial stable homotopy
groups.
If W is a tame M-module with non-trivial M-action, then π0 HW ∼= W as M-modules and so thegeneralized Eilenberg-Mac Lane spectrum HW as defined in Example 4.27 is not semistable.
Example 4.39 shows that an infinite product of symmetric spectra with trivial homotopy groups canhave homotopy groups with non-trivial M-action. In particular, infinite products of semistable symmetric
spectra need not be semistable.
If X has at least one non-trivial homotopy group, then S 0 .1 X ∼= F1 S 0 ∧ X is not semistable by
Example 4.37.

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69

The ‘trivial M-action’ criterion is often handy for showing that semistability is preserved by certain
constructions. We give a few examples of this in the following proposition.
Example 4.50. If f : X −→ Y is any morphism of symmetric spectra, then the homotopy groups ofthe spectra X, Y and the mapping cone C(f ) = [0, 1]+ ∧ X ∪f Y are related by a long exact sequence oftame M-modules (we use that the M-action does not change under loop and suspension). Trivial tame
M-modules are closed under taking submodules, quotient modules and extensions (Lemma 4.22 (iv)); so if
two out of three graded M-modules π∗ X, π∗ Y and π∗ C(f ) have trivial M-action, then so does the third.
Thus the mapping cone of any morphism between semistable symmetric spectra is semistable.
If f : X −→ Y is an h-cofibration [define] of symmetric spectra, or simply an injective morphism whenin the simplicial context, then the mapping cone C(f ) is π∗ -isomorphic to the quotient Y /X. Thus if twoof the spectra X, Y and Y /X are semistable, then so is the third.
Example 4.51. Semistability is preserved under suspension, loop, wedges, shift and sequential colimits
along h-cofibrations (or injective morphisms when in the simplicial context) since these operations preserve
the property of M acting trivially on homotopy groups.
We shall see later (see Theorem II.5.17) that the smash product of two semistable symmetric spectra
is semistable if at least one of the factors is flat. Moreover, if X is a semistable symmetric spectrum and A
is a Γ-space, then A(X) is semistable (see Proposition II.5.18 (iii)) [also true for simplicial functors?]
Example 4.52. For a symmetric spectrum X and a pointed space K we let K ∧ X be the symmetric
spectrum obtained by smashing K levelwise with X (compare Example 2.15). For example, when K = S 1 isthe circle, this specializes to the suspension of X. We claim that if X is semistable and K is a CW-complex,
then the symmetric spectrum K ∧ X is again semistable.
We first prove the claim for finite dimensional CW-complexes by induction over the dimension. If K is
0-dimensional, then K ∧ X is a wedge of copies of X, thus semistable. If K has positive dimension n andK(n−1) is its (n − 1)-skeleton, then K/K(n−1) is a wedge of n-spheres and so the quotient of K ∧ X by thesubspectrum K(n−1) ∧ X is a wedge of n-fold suspension of X. By induction the subspectrum K(n−1) ∧ X issemistable; since the inclusion is an h-cofibration and the quotient spectrum is also semistable, so is K ∧ X.
For a general CW-complex K the symmetric spectrum K ∧ X is the sequential colimit, over h-cofibrations,of the smash product of X with the skeleta of K. So K ∧ X is semistable.
The geometric realization of any simplicial set is a CW-complex, so in the simplicial context we conclude
that for any pointed simplicial set K and any semistable symmetric spectrum X the symmetric spectrum
K ∧ X is again semistable.
Example 4.53. Let F : J −→ SpΣ be a functor from a small category J to the category of symmetric
spectra. If F (j) is semistable for each object j of J, then the homotopy colimit of F over J is semistable.
Indeed, the homotopy colimit is the geometric realization of the simplicial replacement q∗ F in thesense of Bousfield and Kan [10, Ch. XII, 5.1], a simplicial object of symmetric spectra. The spectrum of
n-simplices of q∗ F is a wedge, indexed over the n-simplices of the nerve of J, of spectra which occur asvalues of F . The geometric realization |q∗ F | is the sequential colimit, over h-cofibrations, of the realizationsof the skeleta skn q∗ F in the simplicial direction, so it suffices to show that each of these is semistable.
The skeleton inclusion realizes to an h-cofibration | skn−1 q∗ F | −→ | skn q∗ F | whose quotient symmetric
spectrum is a wedge, indexed over the non-degenerate n-simplices of the nerve of J, of n-fold suspensionsof spectra which occur as values of F . So the quotient spectra are semistable, and so by induction thesymmetric spectra | skn q∗ F | are semistable.
4.6. Homotopy groups of ring spectra. In Example 2.4 we constructed a product on the stablestems and an action of this graded ring on the homotopy groups of a symmetric spectrum. We now
extend this type of multiplication to symmetric ring spectra; however, the situation is more subtle, and ingeneral only those stable homotopy classes which are fixed by the M-action support a multiplication. Moreprecisely, we show:
Theorem 4.54. Let R be a symmetric ring spectrum. There is a natural structure of a graded ring on
the graded subgroup (π∗ R)(0) of M-fixed elements in the homotopy groups of R. If R is commutative, then

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I. BASIC DEFINITIONS AND EXAMPLES

the product on (π∗ R)(0) is graded-commutative. The homotopy groups of every right R-module naturally
form a graded right module over the graded ring (π∗ R)(0) .
Thus if R is semistable then the homotopy groups π∗ R form a graded ring which acts naturally on thehomotopy groups of every right R-module.
To prove Theorem 4.54 we consider a right R-module M and try to define a biadditive pairing fromπk M ×πl R to πk+l M in the same way as for R = S in Example 2.4 (an S-module is the same as a symmetric
spectrum). As we shall see, the case of general ring spectra is more subtle, and the condition of being fixedby M comes up naturally when checking that an unstable product is well-defined on stable homotopy
classes. Given two homotopy classes f ∈ πk+n Mn and g ∈ πl+m Rm we denote by f · g the homotopy classin πk+l+n+m Mn+m given by the composite(4.55)

Id ∧χl,n ∧Id

µn,m

f ∧g

S k+l+n+m −−−−−−−→ S k+n+l+m −−−−→ Mn ∧ Rm −−−−→ Mn+m .

This dot operation is associative, i.e., if h ∈ πj+q Rq is another homotopy class, then we have (f · g) · h =f · (g · h) in πk+l+j+n+m+q Rn+m+q . After spelling out the definitions, this associativity ultimately boils
down to the equality(k × χl,n × m × j × q) ◦ (k × l × χj,n+m × q) = (k × n × l × χj,m × q) ◦ (k × χl+j,n × m × q)
in the symmetric group Σk+l+j+q+n+m and the associativity of the action of R on M .
Using the dot product we can rewrite the stabilization map ι∗ : πk+n Mn −→ πk+n+1 Mn+1 as ι∗ (f ) =f · ι1 , where ι1 ∈ π1 R1 is the class of the unit map of R. In the special case M = R we can also multiply
with the unit map ι1 from the left, and then ι1 · g is the composite of the top row in the diagram
χl,1 ×m

S 1 ∧g/ S 1+l+m
S l+1+mP
PPP
PPP
χ1,l+mPP
l×χ1,m PPP' 
S l+m+1
g∧S 1/ S 1 ∧ Rm

ι1 ∧Rm/ R1 ∧ R m

µ1,m

χm,1

twist


/ Rm ∧ S 1/ R1+mO

Rm ∧ι1/ Rm ∧ R1µm,1/ Rm+1The diagram commutes, using centrality of ι1 , so that we have the relation
ι1 · g = (−1)m (χm,1 )∗ (g · ι1 )in πl+1+m R1+m . The image of the right hand side in the stable group πl R is precisely d · [g], where d· is theaction of the special monoid element d ∈ M, see Example 4.14. So we deduce the relation [ι1 · g] = d · [g]in πl R.
Now it is easy to investigate to what extent the dot product passes to an operation stable homotopy
groups. If we replace the right factor g ∈ πl+m Rm by the next representative ι∗ (g) then associativity gives
f · ι∗ (g) = f · (g · ι1 ) = (f · g) · ι1 = ι∗ (f · g) ,so the class [f · g] in the stable group πk+l M only depends on the class [g] in πl R, and not on the unstable
representative.
However, the dot product interacts differently with stabilization in the left factor, for we have
(4.56)ι∗ (f ) · g = (f · ι1 ) · g = f · (ι1 · g)

which in general will not have the same image in πk+l M as f · g. So in general we cannot expect that thedot product passes to a well-defined operation on πk M × πl R.
Let us now make the additional assumption that the stable class [g] ∈ πl R is fixed by the special monoid
element d ∈ M. Because of the relation [ι1 · g] = d · [g] = [g] in πl R and equation (4.56) we then get thatι∗ (f ) · g and f · g do have the same image in πk+l M . Note that if d[g] = [g], then [g] in fact has filtration
zero (by Lemma 4.22 (iii)) which means that it is fixed by all monoid elements. So by restricting to classes
in πl R which are M-fixed we get a well-defined paring· : πk M × (πl R)(0) −→ πk+l M .

4. HOMOTOPY GROUPS, M-MODULES AND SEMISTABILITY

71

In particular, if R is semistable, then we end up with a well-defined map · : πk M × πl R −→ πk+l M .
The proof that the dot operation is biadditive is the same as in the special case R = S, compare
Example 2.4. We claim that the product if M-linear in the left variable, i.e., we have (α · x) · y = α · (x · y)for α ∈ M, x ∈ πk M and y ∈ (πl R)(0) .[justify]
If we specialize to M = R then the product provides a biadditive graded pairing · : πk R × (πl R)(0) −→πk+l R on the homotopy groups of the symmetric ring spectrum R. By M-linearity in the left variable, thisrestricts to a biadditive pairing · : (πk R)(0) × (πl R)(0) −→ (πk+l R)(0) . Associativity for the unstable dot
product immediately implies associativity for its stabilized version, i.e., for every right R-module M the
diagram
πk M × (πl R)(0) × (πj R)(0)·×Id/ πk+l M × (πj R)(0)
·

Id ×·


πk M × (πl+j R)(0)/ πk+l+j M·

commutes. This finishes the proof of Theorem 4.54, except for the verification that the product on (π∗ R)(0)is graded-commutative if R is commutative. This will come out of the study of the product on the opposite
ring spectrum, which we discuss now.
Example 4.57 (Opposite ring spectrum). For every symmetric ring spectrum R we can define theopposite ring spectrum Rop by keeping the same spaces (or simplicial sets), symmetric group actions andopunit maps, but with new multiplication µop
given by the composite
n,m on R
µm,n

twist

χm,n

op
op
.
Rnop ∧ Rm= Rn ∧ Rm −−−→ Rm ∧ Rn −−−→ Rm+n −−−→ Rn+m = Rn+m

As a consequence of centrality of ι1 , the higher unit maps for Rop agree with the higher unit maps for R.
By definition, a symmetric ring spectrum R is commutative if and only if Rop = R.
For example, we have (HA)op = H(Aop ) for the Eilenberg-Mac Lane ring spectra (Example 2.7) of an
ordinary ring A and its opposite, we have (T ∧ R)op = T op ∧ Rop for the smash product of an I-space withmultiplication and a ring spectrum (Example 2.31), and R[M ]op = (Rop )[M op ] for the monoid ring spectra(Example 2.32) of a simplicial or topological monoid M and its opposite.
We claim that for every ring spectrum R and integer k the homotopy groups πk R and πk (Rop ) areequal (not just isomorphic) as M-modules and(π∗ (Rop ))(0) = ((π∗ R)(0) )op(4.58)(again equality) as graded rings, where the right hand side is the graded-opposite ring, i.e., the gradedabelian group π∗ R with new product x ·op y = (−1)kl y · x for x ∈ πk R and y ∈ πl R. In particular, if R issemistable, then we have π∗ (Rop ) = (π∗ R)op as graded rings.
Let us first check that πk R and πk (Rop ) are the same M-modules. This follows from the stronger claimthat the I-functors π k R and π k (Rop ) are equal. It is immediate from the definition of Rop that these two
I-functors agree on objects and bijective morphisms in the category I. There is something to check though
to see that π k R and π k (Rop ) agree on the inclusions n −→ n + 1 (and thus on all injections), i.e., thatthey have the same stabilization maps. The stabilization map ιop∗ for the opposite ring spectrum takes thehomotopy class of a map f : S k+n −→ Rn to the class of the composite(4.59)

S k+n+1

f ∧Id/ Rn ∧ S 1 Id ∧ι1 / Rn ∧ R1 twist / R1 ∧ Rn

µ1,n

/ R1+nχ1,n/2 Rn+1 .

µop
n,1Because the unit map ι1 is central, the composite of the last four maps in (4.59) is actually equal toµn,1 (Id ∧ι1 ). Thus the total composite in (4.59) equals the composite
f ∧Id

Id ∧ι

µn,1

1
S k+n+1 −−−→ Rn ∧ S 1 −−−−→
Rn ∧ R1 −−−→ Rn+1 ,72

I. BASIC DEFINITIONS AND EXAMPLES

which is precisely the effect of the stabilization map for R. So the I-functors π k R and π k (Rop ) are equaland hence their colimits πk R and πk (Rop ) are equal (or at least as equal as a colimit is canonical) as
M-modules.
Now consider two homotopy classes f ∈ πk+n Rn and g ∈ πl+m Rm which represents M-fixed classes in
πk R respectively πl R. Spelling out the definitions leads to the equation
f ·op g = (χm,n )∗ ◦ (g ·R f ) ◦ (χk,l × χn,m ) = (−1)kl+nm (χm,n )∗ (g · f )as elements of πk+l+n+m Rn+m . We already know that the element represented by this class in πk+l R is
M-fixed, and hence[(−1)nm (χm,n )∗ (g · f )] = d · [g · f ] = [g · f ]in πk+l R. Thus [f ·op g] = (−1)kl [g · f ], which proves the equation (4.58).
As a special case of (4.58) we obtain the claim about commutative ring spectrum made in Theorem 4.54.
If R is commutative, then R = Rop and so then (π∗ R)(0) agrees with its graded opposite ring, which meansthat the multiplication on (π∗ R)(0) is commutative in the graded sense.
The product on the homotopy groups of a symmetric ring spectrum can also be explained from adifferent angle using the smash product. For symmetric spectra X and Y smashing of representatives andcomposing with the universal bimorphism in,m : Xn ∧ Ym −→ (X ∧ Y )n+m passes to an M-linear biadditive
pairing· : πk X × (πl Y )(0) −→ πk+l (X ∧ Y ) .
The verification that this well-defined and the reason for restricting to M-fixed classes in the second factor
are the same as above. It is straightforward to check that for a symmetric ring spectrum R and a right
R-module M the multiplication as defined in Theorem 4.54 agrees with the composite·

απk M × (πl R)(0) −→ πk+l (M ∧ R) −−→ πk+l R .
where α : M ∧ R −→ M is the ‘external’ form of the action of R on M .
Again we have associativity, i.e., for every triple of symmetric spectra X, Y and Z the diagram
πk X × (πl Y )(0) × (πj Z)(0)
Id ×·

·×Id

/ πk+l (X ∧ Y ) × (πj Z)(0)·πk X × (πl+j (Y ∧ Z))(0)/ πk+l+j (X ∧ Y ∧ Z)·

commutes. The graded commutativity of the multiplication in the homotopy of a commutative ring spectrum also has a precursor: for all symmetric spectra X and Y the square(πk X)(0) × (πl Y )(0)·/ πk+l (X ∧ Y )(0)

twist


(πl Y )(0) × (πk X)(0)·πk+l (τX,Y )/ πl+k (Y ∧ X)(0)commutes up to the sign (−1)kl .
Example 4.60 (Killing a homotopy class). We describe a construction that can be used to ‘kill’ theaction of a homotopy class in a ring spectrum on a given module. We consider a symmetric ring spectrum
R and a map x : S l+m −→ Rm . For every right R-module M we define a morphism of symmetric spectraρx : S l+m ∧ M −→ shm M
in level n as the composite
twist

Id ∧x

αn,m

χn,m

S l+m ∧ Mn −−−→ Mn ∧ S l+m −−−→ Mn ∧ Rm −−−→ Mn+m −−−→ Mm+n = (shm M )n .
The name ρx stands for ‘right multiplication by x’. For example, if x = ι1 : S 1 −→ R1 is the unit map thenρι1 : S 1 ∧ M −→ sh M equals the map λM defined in (2.19). [relate the composite of ρy and ρx to ρx·y ]4. HOMOTOPY GROUPS, M-MODULES AND SEMISTABILITY

73

While the construction of the morphism ρx : S l+m ∧ M −→ shm M makes sense for any map x and
R-module M , we can only analyze it homotopically under additional assumptions.
Proposition 4.61. Let R be a symmetric ring spectrum, x : S l+m −→ Rm a based map and M a right
R-module.
(i) Suppose that the map x is central, i.e., the squareRn ∧ S l+m
τ

Id ∧x

/ Rn ∧ Rmµn,m/ Rn+mχn,m

S l+m ∧ Rnx∧Id

/ Rm ∧ Rn

µm,n/ Rm+ncommutes for all n ≥ 0. Then the morphism of symmetric spectra ρx : S l+m ∧ M −→ shm M is ahomomorphism of R-modules.
(ii) If the class [x] ∈ πl R in the l-th stable homotopy group is M-fixed and M is semistable, thenthe morphism ρx : S l+m ∧ M −→ shm M realizes right multiplication by [x] in homotopy. Moreprecisely, the diagram
πk M

·[x]

/ πk+l M=
S l+m ∧− ∼


πl+m+k (S l+m ∧ M )πl+m+k (ρx )/ πl+m+k (shm M )commutes up to the sign (−1)k(l+m) .
As usual, the sign in part (ii) above can be predicted by remembering that the group πk+l M is ‘naturally’equal to πk+l+m (shm M ) (compare Example 2.18), whereas the ‘natural’ target of the lower vertical map
is πl+m+k (shm M ); so secretly, k sphere coordinates move past l + m other coordinates, hence the sign.
Proof of Proposition 4.61. Part (i) is straightforward from the definitions. For (ii) we consider anelement f ∈ πk+n Mn which represents a class in πk M and chase it around both sides of the square. We
then have(4.62)(ρx )∗ (S l+m ∧ f ) = (χn,m )∗ (f · x) ◦ γ∗

in the group π(l+m)+(k+n) Mm+n , where γ∗ : S (l+m)+(k+n) −→ S (k+l)+(n+m) is the coordinate permutationinduced by the element γ ∈ Σl+m+k+n given by
i+kfor 1 ≤ i ≤ l,i + k + n for l + 1 ≤ i ≤ l + m,
γ(i) =
i − l − m for l + m + 1 ≤ i ≤ l + m + k,



i−m
for l + m + k + 1 ≤ i.
The permutation γ has sign (−1)k(l+m)+nm and since M is semistable the effect of the shuffle χn,m inthe stable homotopy group is the sign (−1)nm . So when we pass from the unstable homotopy groupπ(l+m)+(k+n) Mm+n to the stable group πl+m+k (shm M ), the equation (4.62) becomes πl+m+k (ρx )(S l+m ∧[f ]) = (−1)k(l+m) [f ] · [x], as claimed.
Now suppose that the hypothesis of both (i) and (ii) hold in Proposition 4.61, i.e., the map x is central,
its class [x] in πl R is M-fixed and M is semistable. We let M/x denote the mapping cone of the morphismρ̃x : S l ∧ M −→ Ωm (shm M ) which is adjoint to ρx . Then by Proposition 4.61 the morphism ρ̃x also realizes

74

I. BASIC DEFINITIONS AND EXAMPLES

multiplication by [x] on homotopy [levelwise Kan] in the sense that the square
·[x]

πk M

/ πk+l M∼= λ∗=S l ∧− ∼

πl+k (S l ∧ M )πl+k (ρ̃x )
/ πl+k (Ωm (shm M ))commutes up to the sign (−1)k(l+m) . The long exact homotopy sequence of a mapping cone (Proposition 4.7 (i)) breaks up into a short exact sequence of (π∗ R)(0) -modules [the connecting morphism is(π∗ R)(0) -linear]
0 −→ π∗ M/(π∗−l M ) · [x] −→ π∗ (M/x) −→ {π∗+l−1 M }[x] −→ 0
where the first map is induced by the composite of λ∗ : M −→ Ωm (shm M ) and the mapping cone inclusionΩm (shm M ) −→ M/x and {−}[x] denotes the submodule of homotopy classes annihilated by [x]. So weconclude that if x acts injectively on the homotopy groups of M , then the morphism M −→ M/x realizes
the quotient map π∗ M −→ π∗ M/(π∗−l M ) · [x] on homotopy groups.
In contrast, if the class [x] annihilates nonzero classes in the homotopy of M then we may not be able
realize the module π∗ M/(π∗−l M ) · [x] as the homotopy of an R-module. In fact, Toda brackets give thefirst obstructions to such a realization. If there are homogeneous elements m ∈ π∗ M and x, y ∈ (π∗ R)(0)such that mx = 0 = xy and the Toda bracket hm, x, yi does not contain 0, then the projection π∗ M −→π∗ M/(π∗−l M ) · [x] is not realizable as the effect on homotopy of any R-module homomorphism out of M .
[explain]
Example 4.63 (Killing a regular sequence). We can iteratively kill homotopy classes as in the previous
example and thereby kill the action of certain ideals in the homotopy groups of a symmetric ring spectrum.
We just saw that we can only control the homotopy groups of M/x if the homotopy class [x] which is killed
is not a zero divisor on π∗ M . So iterating the construction naturally leads us to consider regular sequences.
Recall that a sequence, finite or countably infinite, of homogeneous elements yi in a graded commutative
ring R∗ is a regular sequence for a graded R∗ -module M∗ if y1 acts injectively on M∗ and for all i ≥ 2 theelement yi acts injectively on M∗ /M∗ · (y1 , . . . , yi−1 ). A homogeneous ideal I of R∗ is a regular ideal for M∗
if it can be generated by a regular sequence, finite or countably infinite, for M∗ .
To simplify the exposition we now assume that the ring spectrum R we work over is semistable andcommutative. As in Example 4.60 this can be relaxed to the assumption that all relevant homotopy classes
are M-fixed and can be represented by central maps.
Proposition 4.64. Let R be a commutative semistable symmetric ring spectrum, M a semistable right
R-module and I a homogeneous ideal of π∗ R. If I is a regular ideal for the module π∗ M then there exists
a semistable R-module M/I and a homomorphism q : M −→ M/I of R-modules such that the induced
homomorphism of homotopy groupπ∗ (q) : π∗ M −→ π∗ (M/I)
is surjective and has kernel equal to (π∗ M )I.
Proof. We choose a sequence y1 , y2 , . . . of homogeneous elements of π∗ R which generate the ideal I
and form a regular sequence for π∗ M . We construct inductively a sequence of semistable R-modules M iand homomorphisms
q1
q2
q3
M = M 0 −→ M 1 −→ M 2 −→ · · ·such that the composite morphism M −→ M i is surjective on homotopy groups and has kernel equal to(π∗ M ) · (y1 , . . . , yi ).
The induction starts with i = 0, where there is nothing to show. In the ith step we let l be the dimensionof the homotopy class yi and choose a based map x : S l+m −→ Rm which represents yi ∈ πl R. By induction
the homotopy groups of M i−1 realize the π∗ R-module π∗ M/(π∗ M ) · (y1 , . . . , yi−1 ). Since we have a regular4. HOMOTOPY GROUPS, M-MODULES AND SEMISTABILITY

75

sequence for π∗ M the class yi = [x] acts injectively on the homotopy of M i−1 , so the morphism qi :M i−1 −→ M i−1 /x constructed in Example 4.60 realizes the projection π∗ M i−1 −→ π∗ M i−1 /(π∗ M i−1 ) · yi .
We can thus take M i = M i−1 /x; then the composite morphism M −→ M i is again surjective on homotopy
groups and its kernel is(π∗ M ) · (y1 , . . . , yi−1 ) + (π∗ M ) · yi = (π∗ M ) · (y1 , . . . , yi ) .
This finishes the argument if I is generated by a finite regular sequence.
If the generating sequence is countably infinite we define M/I as the diagonal (see Example 2.27) of
the above sequence of R-modules M i . Then the natural map
colimi π∗ Mi −→ π∗ (M/I)
is an isomorphism (see (2.30)), and the left hand side is isomorphic tocolimi (π∗ M/(π∗ M ) · (x1 , . . . , xi )) ∼= π∗ M/(π∗ M ) · (x1 , x2 , . . . ) = π∗ M/(π∗ M ) · I .
Example 4.65 (Inverting a homotopy class). In Example 2.36 we defined a new symmetric ring spectrum R[1/x] from a given symmetric ring spectrum R and a central map x : S 1 −→ R1 . We now generalize
this construction to central maps x : S l+m −→ Rm and also analyze it homotopically.
First we extend the localization construction to right R-modules M . We define a right R[1/x]-module
M [1/x] by
M [1/x]p = map(S (l+m)p , M(1+m)p ) .
The symmetric group Σp acts on S (l+m)p and M(1+m)p by permuting the p blocks of l + m respectively
1 + m coordinates, i.e., by restriction along the diagonal embeddings∆ : Σp −→ Σ(l+m)prespectively ∆ : Σp −→ Σ(1+m)p .

More precisely, the diagonal embedding ∆ : Σp −→ Σnp is defined by ∆(γ) = Idn ·γ (using the notation of
Remark 1.12) which unravels to∆(γ)(j + (i − 1)n) = j + (γ(i) − 1)for i ≤ j ≤ n and 1 ≤ i ≤ p. The action of Σp on the whole mapping space M [1/x]p is then by conjugation.
A special case of this is M = R and we now define Σp × Σq -equivariant action maps
αp,q : M [1/x]p ∧ R[1/x]q −→ M [1/x]p+q
as the composite
map(S (l+m)p , M(1+m)p ) ∧ map(S (l+m)q , R(1+m)q ) −→ map(S (l+m)(p+q) , M(1+m)(p+q) )
f ∧g

7−→

µ(1+m)p,(1+m)q ◦ (f ∧ g) .

The action maps are associative because smashing and the original action maps are. The verification themaps αp,q are Σp × Σq -equivariant ultimately boils down to the equivariant for the original action maps
and the fact that the diagonal embeddings ∆p : Σp −→ Σnp , ∆q : Σq −→ Σnq and ∆p+q : Σp+q −→ Σn(p+q)
satisfy∆p (γ) × ∆q (τ ) = ∆p+q (γ × τ ) .
The next piece of data we define are maps jp : Mp −→ M [1/x]p which we will later recognize as ahomomorphism of R-modules. We denote by xp : S (l+m)p −→ Rmp the composite
x(p)µm,...,m(p)
S (l+m)p −−→ Rm−−−−−→ Rmp ;because x is central the map xp is Σp -equivariant if we let Σp act on source and target through the diagonalembeddings. We define
jp : Mp −→ map(S (l+m)p , M(1+m)p ) = M [1/x]p

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I. BASIC DEFINITIONS AND EXAMPLES

as the adjoint toId ∧xp

αp,mpξ∗Mp ∧ S (l+m)p −−−−→ Mp ∧ Rmp −−−−→ Mp+mp −→ M(1+m)pwhere ξ ∈ Σ(1+m)p is given by(
1 + (i − 1)(1 + m)
ξ(i) =(j + 1) + (k − 1)(1 + m)

if 1 ≤ i ≤ p
if i = p + mk + j for 1 ≤ k ≤ p, 1 ≤ j ≤ m.

The map jp is Σp -equivariant. In terms of the adjoint of jp this means that for every permutationγ ∈ Σp the outer composites in the diagram
Mp ∧ S (l+m)p

Id ∧xpγ∧∆(γ)


Mp ∧ S (l+m)p/ Mp ∧ Rmp

αp,mp/ Mp+mpγ∧∆(γ)

Id ∧xp/ Mp ∧ Rmp

ξ/ M(1+m)pγ×∆(γ)∆(γ)

/ Mp+mpαp,mpξ/ M(1+m)pagree. The left square commutes by equivariance of xp , the middle square by the equivariance of the actions
maps of M . The right square commutes because the relationξ ◦ (γ × ∆(γ)) = ∆(γ) ◦ ξholds in the symmetric group Σ(1+m)p .
The collection of maps jp is multiplicative in the sense that the squares
Mp ∧ R q

αp,q/ Mp+qjp ∧jq
M [1/x]p ∧ R[1/x]qjp+q/ M [1/x]p+q

αp,qcommute [elaborate].
For M = R we define the unit maps ιn : S n −→ R[1/x]n as the composite of the unit map ιn : S n −→ Rn
for R with jn : Rn −→ R[1/x]n . [central] Then j : R −→ R[1/x] is a homomorphism of symmetric ring
spectra and j : M −→ M [1/x] is a morphism of R-modules if we view M [1/x] as an R-module by restrictionof scalars along j : R −→ R[1/x]. In other words, we have constructed a functor
mod- R −→ mod- R[1/x] ,M 7−→ M [1/x]∗and a natural transformation of R-modules j : M −→ j (M [1/x]).
Remark 4.66. The permutation ξ can be predicted as follows by the general rules which we introduced
in Remark 1.12. The natural ‘coordinate free’ target of the map αp,m,...,m is indexed by the set
p + m + ··· + m = 1 · p + m · p .
{z
}
|
p

This set is equal to the set (1 + m) · p, but the way the parenthesis arise naturally reminds us that weshould use the right distributivity isomorphism∼=p+m·p=1·p+m·p −→ (1 + m) · p
to identify the two sets. The right distributivity isomorphism is defined using the multiplicative shuffles
×
×
as χ×
p,1+m ◦ (χ1,p × χm,p ), which is precisely the permutation ξ. Note that in contrast, the definition of
the actions map αp,q does not need any permutations. Indeed, the natural coordinate free target of αp,q isindexed by the set (1 + m) · p + (1 + m) · q, which is equal to the set (1 + m) · (p + q). Here, however, theparenthesis suggest using the left distributivity isomorphism, which is the identity permutation.

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77

The relation ξ ◦ (γ × ∆(γ)) = ∆(γ) ◦ ξ which came up in the proof of the equivariance on the map jp
just expresses the fact that the multiplicative shuffle χ×n,m : n · m −→ m · n is a natural isomorphismξ

1·p+m·p×χ×1,p +χm,pγ+∆(γ) Id1 ·γ+Idm ·γ
1·p+m·p×χ×1,p +χm,p/ p · 1 + p · m = p · (1 + m)

χ×
p,1+m

γ·Id1 +γ·Idm γ·Id1+m/ p · 1 + p · m = p · (1 + m)+
/ (1 + m) · p
γ·Id1 ∆(γ)

χ×
p,1+m


/ (1 + m) · p
3ξNow we analyze the construction homotopically, under the additional assumption that the class of x in
πl R is M-fixed. The centrality condition on the map x implies that then the stable homotopy class [x] is
central in the graded ring (π∗ R)(0) . [when is R[1/x] equivalent to just changing the unit map ?]Proposition 4.67. Let R be a symmetric ring spectrum and x : S l+m −→ Rm a central map whoseclass [x] in πl R is M-fixed. Then for every semistable right R-module M [levelwise Kan] the R[1/x]-module
M [1/x] is again semistable [and levelwise Kan] and the morphism j : M −→ M [1/x] of R-modules induces
a natural isomorphism
(4.68)(π∗ M )[1/[x]] ∼= π∗ M [1/x]of graded modules over (π∗ R)(0) . In the special case M = R the morphism of symmetric ring spectra
j : R −→ R[1/x] induces an isomorphism of graded rings
(0)(π∗ R)(0) [1/[x]] ∼.
= π∗ R[1/x]Proof. As an R-module M [1/x] is equal to the diagonal [No: not correct Σn - action] (see Example 2.27) of the sequence of R-module homomorphisms
ρ̃xΩl+m shm (ρ̃x )M −−−−−→ Ωl+m (shm M ) −−−−−−−−−→ Ω(l+m)2 (shm2 M ) · · ·Ω(l+m)p shmp (ρ̃x )· · · Ω(l+m)p (shmp M ) −−−−−−−−−−−→ · · ·where ρ̃x is adjoint to the ‘right multiplication’ homomorphism defined in Example 4.60. The map j :M −→ M [1/x] is the canonical morphism from the initial term of a sequence to the diagonal spectrum.
By Proposition 4.61 (ii) the effect of ρ̃x on homotopy groups is right multiplication by the class [x] ∈πl R. So since the homotopy groups of the diagonal are isomorphic to the colimit of the homotopy groups(see (2.30)), we deduce that j induces the isomorphismπ∗ (M [1/x]) = π∗ diagp Ω(l+m)p (shmp M ) ∼= colimp π∗ Ω(l+m)p (shmp M ) ∼= colimp π∗+lp M
with the last colimit being taken over iterated multiplication by [x]. Since the right hand side is the resultof inverting [x] in π∗ M , this proves the first claim.
In the special case M = R we now know that the morphism of symmetric ring spectra j : R −→ R[1/x]induces an isomorphism of graded (π∗ R)(0) -modules
(0)(π∗ R)(0) [1/[x]] ∼.
= π∗ R[1/x]This is necessarily a multiplicative isomorphism.

An important special case of the above is when the symmetric ring spectrum R is commutative (whichmakes centrality of the map x is automatic) and semistable (so that all of π∗ R is M-fixed). For easierreference we spell out Proposition 4.67 in this special case.

78

I. BASIC DEFINITIONS AND EXAMPLES

Corollary 4.69. Let R be a commutative semistable symmetric ring spectrum [levelwise Kan] andx : S l+m −→ Rm a based map. Then R[1/x] is a commutative symmetric ring spectrum, the homomorphismof symmetric ring spectra j : R −→ R[1/x] sends the class [x] ∈ πl R to a unit in the l-th homotopy groupof R[1/x] and the induced morphism of graded commutative rings(π∗ R)[1/[x]] ∼= π∗ R[1/x]is an isomorphism.
Example 4.70 (Brown-Peterson, Johnson-Wilson spectra and Morava K-theory). If we apply themethod of ‘killing a regular sequence’ to the Thom spectrum M U we can construct a whole collection ofimportant spectra. In Example 2.9 we constructed M U as a commutative symmetric ring spectrum, andM U is semistable because it underlies an orthogonal spectrum (compare Proposition 4.47). As input for thefollowing construction we need the knowledge of the homotopy ring of M U . The standard way to perform
this calculation is in the following sequence of steps:• calculate, for each prime p, the mod-p cohomology of the spaces BU (n) and BU ,• use the Thom isomorphism to calculate the mod-p cohomology of the Thom spectrum M U as amodule over the mod-p Steenrod algebra,• use the Adams spectral sequence, which for M U collapses at the E2 -term, to calculate the pcompletion of the homotopy groups of M U ,• and finally assemble the p-local calculations into the integral answer.
When the dust settles, the result is that π∗ M U is a polynomial algebra generated by infinitely manyhomogeneous elements xi of dimension 2i for i ≥ 1. The details of this calculation can be found in [61]and [50] [check this; other sources?]. A very different geometric approach to this calculation was describedby Quillen [47], who determines the ring of cobordism classes of stably almost complex manifolds, which
by Thom’s theorem is isomorphic to π∗ M U . (Quillen’s argument, however, needs as an input the a priori
knowledge that the homotopy groups of M U are finitely generated in each dimension.)
Now fix a prime number p. Using the close connection between the ring spectrum M U and the theoryof formal groups laws one can make a particular choices for the (pn − 1)-th generator xpn −1 , the so-called
Hazewinkel generator, which is then denoted vn . Killing all polynomial generators except those of the form
xpn −1 produces a semistable M U -module BP with homotopy groups π∗ (BP ) = Z[v1 , v2 , v3 , . . . ] where thedegree of vn is 2pn − 2. Localizing at p produces a semistable M U -module BP , called the Brown-Peterson
spectrum, with homotopy groupsπ∗ BP ∼= Z(p) [v1 , v2 , v3 , . . . ] .
The original construction of this spectrum by Brown and Peterson was quite different, and we say more
about the history of BP in the ‘History and credits’ section at the end of this chapter.
Now we can keep going and kill more of the polynomial generators vi in the homotopy of BP , and possibly also invert another generator. In this way we can produce various M U -modules BP/I and (BP/I)[vn−1 ]together with M U -homomorphisms from BP whose underlying stable homotopy types play important roles
in stable homotopy theory. Some examples of spectra which we can obtained in this way are given in thefollowing table, along with their homotopy groups:
BP hni Z(p) [v1 , v2 , . . . , vn ]P (n)Fp [vn , vn+1 , . . . ]k(n)Fp [vn ]E(n) Z(p) [v1 , v2 , . . . , vn , vn−1 ]B(n) Fp [vn−1 , vn , vn+1 , . . . ]K(n) Fp [vn , vn−1 ][discuss uniqueness] The spectrum E(n) is referred to as the Johnson-Wilson spectrum and k(n) respectively
K(n) are the connective and periodic Morava K-theory spectra.
We have so far only constructed the spectra above as M U -modules. The way we have presented thehomotopy groups of the various spectra above does not only give graded modules over the homotopy ringof M U , but in fact graded commutative algebras. This already hints that the spectra have more structure.
In fact, all the spectra above can be constructed as M U -algebra spectra, so in particular as symmetric ring
spectra. We may or may not get back to this later.

EXERCISES

79

Construction 4.71 (Toda brackets). The homotopy groups of a symmetric ring spectrum have evenmore structure than that of a graded ring, they also have ‘secondary’ (and higher. . . ) forms of multiplications, called Toda brackets. We will restrict ourselves to the simplest kind of such brackets, namely triple
brackets (as opposed to four-fold, five-fold,. . . ) with single entries (as opposed to ‘matric’ Toda brackets).
In order to simplify the exposition we will only consider semistable ring spectra; the discussion works moregenerally if the operations and relations are suitably restricted to M-fixed classes.
So we let R be a semistable symmetric ring spectrum and M a right R-module. The Toda bracket
hx, y, zi is defined for every triple of homogeneous elements x ∈ π∗ M and y, z ∈ π∗ R which satisfies therelations xy = 0 = yz. If the dimensions of x, y and z are k, l and j respectively, then the bracket is not asingle homotopy class, but an entire coset in πk+l+j+1 M for the subgroup x · πl+j+1 R + πk+l+1 M · z. This
subgroup is called the indeterminacy of the bracket. [construct the bracket]
Here are some examples of non-trivial Toda bracket. In the stable stems, i.e., the homotopy groups ofthe sphere spectrum (compare the table in Example 2.4) we have
η 2 ∈ h2, η, 2i mod (0)ν 2 ∈ hη, ν, ηi mod (0)ησ +  ∈ hν, η, νi mod (0)6ν ∈ hη, 2, ηimod (12ν)
40σ ∈ hν, 24, νi mod (0) ∈ hη, ν, 2νi mod (ησ)

Toda brackets satisfy a number of relations. The first are a kind of ‘higher form of associativity’ andare often referred to as ‘juggling formulas’. For the juggling formula we need another homogeneous classw ∈ π∗ R satisfying zw = 0. Then we havex · hy, z, wi = ±hx, y, zi · wsmodule the common indeterminacy x · π|yz|+1· w. [fix the sign] An example of the juggling formula isη · h2, η, 2i = hη, 2, ηi · 2
which holds in π3s without indeterminacy. By the table above, the first bracket contains η 3 while the second
3bracket contains 12ν. So we get the multiplicative relation η = 12ν as a consequence of the Toda brackets
involving 2 and η.
The first brackets η 2 ∈ h2, η, 2i in the table above is a special case of Toda’s relation
ηx ∈ h2, x, 2i

which holds for all 2-torsion classes x in the homotopy of every commutative [?] symmetric ring spectrum.
This in turn is a special case of relations which hold between Toda brackets and power operations in thehomotopy ring of commutative symmetric ring spectra. We plan to get back to this later. Another example
of a non-trivial Toda bracket is u ∈ h1, η, 2i (modulo 2u) in π2 KU . Here the complex topological K-theory
spectrum KU is viewed just as a symmetric spectrum (and not as a ring spectrum). So the Bott class u
and 1 are homotopy classes of KU , while η and 2 have to be viewed as elements in the stable stems.
It is straight forward to check that homomorphisms of symmetric ring spectra preserve Toda brackets.
More precisely, for every homomorphism f : S −→ R we havef∗ (hx, y, zi) ⊆ hf∗ (x), f∗ (y), f∗ (z)i
whenever the bracket on the left is defined. The indeterminacy of the right hand side may be larger than
the image of the indeterminacy of the bracket hx, y, zi, which is why in general we only have containment,
not necessarily equality, as subsets on πk+l+j+1 R. For example, the relation η 2 ∈ h2, η, 2i holds in thehomotopy of every ring spectrum (with possibly bigger indeterminacy, and possibly with η 2 = 0); since η 2
is non-zero, in the homotopy of real topological K-theory KO (compare the table in Example 2.10), we geta non-trivial bracket η 2 ∈ h2, η, 2i (modulo 0) in π2 KO. In π4 KO we also have ξ ∈ h2, η, η 2 i (modulo 2ξ).
Exercises
Exercise 5.1. The definition of a symmetric spectrum contains some redundancy. Show that theequivariance condition for the iterated structure map is already satisfied if for every n ≥ 0 the following
two conditions hold:80

I. BASIC DEFINITIONS AND EXAMPLES(i) the structure map σn : Xn ∧ S 1 −→ Xn+1 is Σn -equivariant where Σn acts on the target by restriction
from Σn+1 to the subgroup Σn .
(ii) the composite
σ ∧ Idσn+1nXn ∧ S 2 −−−−→ Xn+1 ∧ S 1 −−−→ Xn+2

is Σ2 -equivariant.
Exercise 5.2. Let X be a symmetric spectrum such that for infinitely many n the action of Σn on Xnis trivial. Show that all stable homotopy groups of X are trivial. (Hint: identify the quotient space of the
Σ2 -action on S 2 .) What can be said if infinitely many of the alternating groups act trivially?
Exercise 5.3. Let S[k] denote the symmetric subspectrum of the sphere spectrum obtained by truncating below level k, i.e.,(∗
for n < k[k](S )n =nS
for n ≥ k.
Show that for every symmetric spectrum X and all k ≥ 0 the inclusion S[k] −→ S induces a π∗ -isomorphism
S[k] ∧ X −→ S ∧ X ∼= X.
Exercise 5.4. Let X be a symmetric spectrum. For each finite set A, choose a bijection κA : A −→ n
where n = |A| is the cardinality of A, insisting that κn is the identity. Define XA = Xn . For each injective
map α : A −→ B of finite sets define a structure map α∗ : XA ∧ S B−α(A) −→ XB as the composite
Id ∧(γκB )∗

σ m−n

γ −1

∗
Xn ∧ S B−α(A) −−−−−−−→ Xn ∧ S m−n −−−−→ Xm −−→ Xm

where n = |A| and m = |B| and γ ∈ Σm is any permutation such that γκB α = ιm−n κA . The firstisomorphism comes from the bijection B − α(A) ∼= m − n = {n + 1, . . . , m} given by the restriction ofγκB : B −→ m.
Show that the above data defines a coordinate free symmetric spectrum and that together with restriction it defines an equivalence between the category of symmetric spectra and the category of coordinate
free symmetric spectra.
Exercise 5.5. Here is yet another perspective on what a symmetric spectrum is. We define a based
topological category Σ as follows. The objects of Σ are the natural numbers 0, 1, 2, . . . and the basedm−n, which is to be interpreted as
space of morphisms from n to m is given by Σ(n, m) = Σ+m ∧1×Σm−n S0+a one-point space if m < n. The identity in Σ(n, n) = Σn ∧ S is the identity of Σn (smashed with thenon-basepoint of S 0 ) and composition ◦ : Σ(m, k) ∧ Σ(n, m) −→ Σ(n, k) is defined by[τ ∧ z] ◦ [γ ∧ y] = [τ (γ × 1) ∧ (y ∧ z)]k−mwhere τ ∈ Σk , γ ∈ Σm , z ∈ S
and y ∈ S m−n .
(i) Given a symmetric spectrum X we define a map X̄ : Σ(n, m) ∧ Xn −→ Xm by[τ ∧ z] ∧ x 7−→ γ∗ (σ m−n (x ∧ z)) .
Verify that this assignment defines a based continuous functor X̄ : Σ −→ T from the category Σ to thecategory of pointed spaces. Show that X 7→ X̄ is an isomorphism between the category of symmetric
spectra and the category of based continuous functors Σ −→ T . How can one modify this to accommodate
symmetric spectra of simplicial sets?
(ii) We define a functor + : Σ × Σ −→ Σ on objects by addition of natural numbers and on morphismsby[τ ∧ z] + [γ ∧ y] = [(τ × γ) ∧ (1 × χn,m̄ × 1)∗ (z ∧ y)] ∈ Σ(n + n̄, m + m̄)for [τ ∧ z] ∈ Σ(n, m) and [γ ∧ y] ∈ Σ(n̄, m̄). Show that ‘+’ is strictly associative and unital, i.e., a strict
monoidal product on the category Σ. Define a symmetry isomorphism to make this into a symmetricmonoidal product.

EXERCISES

81(iii) Show that the construction in (i) can be extended to an isomorphism between the categories ofsymmetric ring spectra and strong monoidal functors from Σ to T such that it takes commutative symmetricring spectra isomorphically onto the full subcategory of symmetric monoidal functors.
Exercise 5.6. Let R be a symmetric ring spectrum. Define mapping spaces (simplicial sets) andfunction symmetric spectra of homomorphisms between two given R-modules. Check that for all k ≥ 0 theendomorphism ring spectrum HomR (k + ∧ R, k + ∧ R) of the R-module k + ∧ R is isomorphic, as a symmetricring spectrum, to the matrix ring spectrum Mk (R) (see Example 2.34).
Exercise 5.7. This exercise is supposed to motivate the term ‘bimorphism’ which we used in the first
way to introduce the smash product of symmetric spectra. Let X, Y and Z be symmetric spectra.
(i) Let bp,q : Xp ∧Yq −→ Zp+q be a collection of Σp ×Σq -equivariant maps. Show that the commutativityof the left part of (3.1) is equivalent to the condition that for every p ≥ 0 the maps bp,q : Xp ∧ Yq −→ Zp+q
form a morphism bp.• : Xp ∧ Y −→ shp Z of symmetric spectra as q varies. Show that the commutativityof the right part of (3.1) is equivalent to the condition that for every q ≥ 0 the composite maps
bp,q

twist

χp,q

Yq ∧ Xp −−−→ Xp ∧ Yq −−→ Zp+q −−−→ Zq+pform a morphism Yq ∧ X −→ shq Z of symmetric spectra as p varies.
(ii) Let b = {bp,q : Xp ∧ Yq −→ Zp+q } be a bimorphism. Define b̄p : Xp −→ map(Y, shp Z) as the adjointof the morphism of symmetric spectra bp.• : Xp ∧ Y −→ shp Z (compare part (i)). Show that as p varies,
the maps b̄p form a morphism of symmetric spectra b̄ : X −→ Hom(Y, Z). Show then that the assignment
Bimor((X, Y ), Z) −→ SpΣ (X, Hom(Y, Z)) ,b 7→ b̄is bijective and natural in all three variables.
Exercise 5.8. The way Hovey, Shipley and Smith introduce the smash product in their original paper [25] is quite different from our exposition, and this exercise makes the link. Thus the paper [25] has thesolutions to this exercise. A symmetric sequence consists of pointed spaces (or simplicial set) Xn , for n ≥ 0,with based, continuous (respectively simplicial) Σn -action on Xn . Morphisms f : X −→ Y are sequencesof equivariant based maps fn : Xn −→ Yn . The tensor product X ⊗ Y of two symmetric sequences X and
Y is the symmetric sequence with nth term_
(X ⊗ Y )n =Σ+n ∧Σp ×Σq Xp ∧ Yq .
p+q=n(i) Make the tensor product into a closed symmetric monoidal product on the category of symmetricsequences.
(ii) Show that the sequence of spheres S = {S n }n≥0 forms a commutative monoid in the category ofsymmetric sequences. Show that the category of symmetric spectra is isomorphic to the category of right
S-modules in the monoidal category of symmetric sequences.
(iii) Given a commutative monoid R in the monoidal category of symmetric sequences and two right
R-modules M and N , show that the coequalizer M ∧R N of the two morphisms
αM ⊗ Id, Id ⊗(αN ◦ τR,N ) : M ⊗ R ⊗ N −→ M ⊗Nis naturally a right R-module. Show that the smash product over R is a closed symmetric monoidal producton the category of right R-modules.
(iv) Show that the smash product over S corresponds to the smash product of symmetric spectra under
the isomorphism of categories of part (ii).
Exercise 5.9. Define a notion of ‘commuting homomorphisms’ between symmetric ring spectra suchthat homomorphism of symmetric ring spectra R∧S −→ T are in natural bijection with pairs of commuting
homomorphisms (R −→ T, S −→ T ). Deduce that the smash product is the categorical coproduct forcommutative symmetric ring spectra.

82

I. BASIC DEFINITIONS AND EXAMPLES

Exercise 5.10. In Remark 4.41 we introduced the ring Z[[M]] of natural operations on the 0-th
homotopy group of symmetric spectra and identified it with a certain completion of the monoid ring Z[M].
(i) An I-set is a functor from the category I of finite sets and injections to the category of sets. Show
that the endomorphism monoid of the ‘colimit over inclusions’ functor(I-sets) −→ (sets) ,F 7→ F (ω)

is isomorphic to M.
(ii) Show that the endomorphism ring of the ‘colimit over inclusions’ functor(I-functors) −→ (ab. groups) ,F 7→ F (ω)

is isomorphic to the ring Z[[M]].
History and credits
I now summarize the history of symmetric spectra and symmetric ring spectra, and the genesis of theexamples which were discussed above, to the best of my knowledge. My point with respect to the examples
is not when certain spectra first appeared as homotopy types or ring spectra ‘up to homotopy’, but rather
when a ‘highly structured’ multiplication was first noticed in one form or another. Additions, corrections
and further references are welcome.
Symmetric spectra and symmetric ring spectra were first introduced under this name in the article [25]by Hovey, Shipley and Smith. However, these mathematical concepts had been used before, in particular inseveral papers related to topological Hochschild homology and algebraic K-theory. For example, symmetricring spectra appeared as strictly associative ring spectra in [22, Def. 6.1] and as FSPs defined on spheresin [23, 2.7].
There is a key observation, however, which is due to Jeff Smith and which was essential for the development of symmetric spectra and related spectra categories. Smith noticed that symmetric ring spectra
are the monoids in a category of symmetric spectra which has a smash product and a compatible stablemodel structure. Smith gave the first talks on this subject in 1993. In the fall of 1995, Hovey, Shipley
and Smith started a collaboration in which many remaining issues and in particular the model structures
were worked out. The results first appeared in a joint preprint on the Hopf algebraic topology server(at hopf.math.purdue.edu), the K-theory preprint server (at www.math.uiuc.edu/K-theory/) and the
ArXiv (under math.AT/9801077) in January 1998. This preprint version has a section about symmetric
spectra based on topological spaces which did not make it into the published version [25] because the referee
requested that the paper be shortened.
Several of the examples which we gave in Section 2.1 had been around with enough symmetries before
symmetric spectra were formally introduced. For example, Bökstedt and Waldhausen introduced functors
with smash product, or FSPs for short, in [6], from which symmetric ring spectra are obtained by restricting
to spheres. Eilenberg-Mac Lane spectra (Example 2.7) and monoid ring spectra (Example 2.32) arise inthis way from FSPs and seem to have first appeared in [6] (or already in Gunnarson’s preprint [22] ?).
Matrix ring spectra (Example 2.34) were also treated as FSP in [6] [first published reference ?].
Cobordism spectra first appeared as highly structured ring spectra in the form of as ‘I∗ -prefunctors’in [42]. I∗ -prefunctors are the same as [commutative ?] orthogonal ring spectra, and the underlying
symmetric ring spectra are what we present in Example 2.8. The construction in Example 2.41 which turns
unitary spectra into orthogonal spectra by looping with the imaginary spheres appears to be new.
The model for the complex topological K-theory spectrum in Example 2.10 is a specialization of a moregeneral construction for C ∗ -algebras by Joachim and Stolz [29]. Earlier, Joachim had given a different model
for real topological K-theory as a commutative symmetric ring spectrum in [28].
Waldhausen notes on p. 330 of [66] that the iterated S· -construction defines a (sequential) spectrum
which is an Ω-spectrum from level 1 upwards. Waldhausen’s construction predates symmetric spectra,
and it was later noticed by Hesselholt [20, Appendix] that iterating the S· -construction in fact has all
the symmetries needed to form a symmetric spectrum. Moreover, bi-exact pairings of input data yields

HISTORY AND CREDITS

83

multiplications of associated K-theory spectra. Our treatment of the algebraic K-theory spectrum on
Example 2.11 follows very closely the Appendix of [20].
Free and semifree symmetric spectra, suspensions, loop and shifts of symmetric spectra were first
discussed in the original paper [25] of Hovey, Shipley and Smith.
The particular method for inverting a homotopy elements in a symmetric ring spectrum described in
Examples 2.35, 2.36 and 4.65 seems to be new. The construction of Example 2.37 for adjoining roots ofunity to a symmetric ring spectrum is due to Schwänzl, Vogt and Waldhausen [51]. They originally wrote
up the construction in the context of S-modules, but their argument only needs that one can form monoid
rings and invert homotopy elements within the given framework of commutative ring spectra. So as soon
as these constructions are available, their argument carries over to symmetric ring spectra.
I learned the model of the periodic complex cobordism spectrum M U P given in Example 2.42 from
Morten Brun, who adapted a construction of Strickland [60, Appendix] from ‘complex S-modules’ to unitary
spectra.
The category of Γ-spaces was introduced by Segal [55], who showed that it has a homotopy category
equivalent to the usual homotopy category of connective spectra. Bousfield and Friedlander [11] considereda bigger category of Γ-spaces in which the ones introduced by Segal appeared as the special Γ-spaces. Their
category admits a closed simplicial model category structure with a notion of stable weak equivalences giving
rise again to the homotopy category of connective spectra. Then Lydakis [34] showed that Γ-spaces admit
internal function objects and a symmetric monoidal smash product with good homotopical properties.
After the discovery of smash products and compatible model structures for Γ-spaces and symmetric
spectra it became obvious that variations of this theme are possible. Simplicial functors were first used forthe purposes of describing stable homotopy types by Bökstedt and Waldhausen when they introduced ‘FSPs’in [6]. Various model structures and the smash product of simplicial functors were systematically studied
by Lydakis in [35]. The paper [39] contains a systematic study of ‘diagram spectra’, their model structuresand smash products, which includes symmetric spectra, Γ-spaces and simplicial functors. Here orthogonal
spectra and continuous functors (defined on finite CW-complexes) make their first explicit appearance. The
category of S-modules is very different in flavor from the categories diagram spectra, and it is defined andstudied in the monograph [19].
The smash product of symmetric spectra was defined by Hovey, Shipley and Smith in their original
paper [25]. However, their exposition of the smash product differs substantially from ours. Hovey, Shipley
and Smith use the category of symmetric sequences (sequences of pointed spaces Xn , for n ≥ 0, with Σn action on Xn ) as an intermediate step towards symmetric spectra and in the construction of the smashproduct, compare Exercise 5.8. I chose to present the smash product of symmetric spectra in a different
way because I want to highlight its property as the universal target for bimorphisms.
The M-action on the homotopy groups of symmetric spectra was first studied systematically by theauthor in [53]. However, several results related to the M-action on homotopy groups are already contained,
mostly implicitly, in the papers [25] and [56]. The definition of semistable symmetric spectra and thecharacterizations (ii)-(v) of Theorem 4.44 appear in Section of [25]; the criterion of trivial M-action on
homotopy groups (Theorem 4.44 (i)) first appears in [53]. I owe the proof of Lemma 4.21 to Neil Strickland.
Multiplications on the ‘classical’ homotopy groups of a symmetric ring spectrum have not previously
been discussed in the literature. One reason may be that the naive approach to defining a product on thehomotopy groups of a symmetric ring spectrum does not always work because it is not in general compatible
with stabilization. The preferred way to bypass this issue has so far been to consider ‘derived homotopy
groups’, i.e., to redefine homotopy groups as the classical homotopy groups of a stably fibrant replacement(which we will discuss in the next chapter). In this approach, a symmetric spectrum is replaced by anΩ-spectrum, which is in particular semistable, and thus has a well-defined multiplication.
The idea to construct various spectra from the Thom spectrum M U by killing a regular sequence andpossibly inverting an element (see Example 4.70) is taken from Chapter V of [19] where this process iscarried out in the world of S-modules. This strategy had previously been adapted to symmetric spectra in
Weiner’s Diplomarbeit [67].

84

I. BASIC DEFINITIONS AND EXAMPLES

The original construction of the Brown-Peterson spectrum in the paper [12] by Brown and Peterson
was quite different. They constructed a spectrum whose mod-p homology realizes a certain polynomial
subalgebra of the dual Steenrod algebra. Later Quillen gave a construction of the spectrum BP using thetheory of formal groups, and Quillen’s approach is still at the heart of most current applications of BP .
Quillen used p-typical formal groups to produce an idempotent endomorphism e : M U(p) −→ M U(p) of
the p-localization of M U in the stable homotopy category (see Section II) which is even a homomorphismof homotopy ring spectra (see Section II.5 below). The ‘image’ of this idempotent is isomorphic, in thestable homotopy category, to the spectrum BP , and Quillen’s construction produces it as a homotopy ring
spectrum. Part II of Adams’ notes [2] are a good exposition of Quillen’s results in this area. [original
paper?]

CHAPTER II

The stable homotopy category
1. Injective Ω-spectra
Our definition of the stable homotopy category uses symmetric spectra of simplicial sets. We define
the stable homotopy category as the homotopy category of injective Ω-spectra. In Chapter III we establish
various stable model category structures for symmetric spectra and can then interpret the stable homotopy
category as introduced here as the homotopy category, in the sense of model category theory, with respect
to the stable model structures.
In order to make sense of the ‘homotopy category of injective Ω-spectra’ we still have to define thehomotopy relation and ‘injective’ symmetric spectra. Recall from Example 2.15 of Chapter I that we cansmash a symmetric spectrum levelwise with a pointed simplicial set K, in such a way that the symmetricgroups and the structure maps do not act on K.
Definition 1.1. Two morphisms of symmetric spectra f0 , f1 : A → X are called homotopic if there isa morphism
H : ∆[1]+ ∧ A −→ X ,called a homotopy, such that f0 = H ◦d0 , and f1 = H ◦d1 , where dj : A −→ ∆[1]+ ∧A for j = 0, 1 is inducedby the face morphisms dj : ∆[0] −→ ∆[1] and A is identified with ∆[0]+ ∧ A. We denote by [A, X] theset of homotopy classes of morphisms of symmetric spectra, i.e., the classes under the equivalence relation
generated by homotopy.
The homotopy relation is not in general an equivalence relation, but it is when the target is injective,
see Proposition 1.6 (ii) below. The notion of homotopy makes just as much sense for symmetric spectra of
topological spaces. The only change is that the simplicial 1-simplex ∆[1] should be replaced by the unit
interval [0, 1] in the definition of a homotopy, and the morphisms dj : ∆[0] −→ ∆[1] are replaced by theendpoint inclusions of the interval. In the context of symmetric spectra of spaces ‘homotopy’ is always anequivalence relation, with no restriction on the target spectra.
A homotopy between spectrum morphisms is really the same as levelwise pointed homotopies between(f0 )n and (f1 )n : An → Xn compatible with the Σn -actions and structure maps. Hence for a morphism ofspectra we have implications
homotopy equivalence =⇒ level equivalence =⇒ π∗ -isomorphism,
but the converse implications are not true in general. Yet another way to look at homotopies is via the
mapping space. Since the spectrum ∆[0]+ ∧ A is canonically isomorphic to A, we can identify the verticesof the morphism space map(A, X) with the spectrum morphisms from A to X. Then a morphism f0 is
homotopic to a morphism f1 if and only if there exists a 1-simplex H ∈ map(A, X)1 satisfying d0 (H) = f0
and d1 (H) = f1 .
Example 1.2. Homotopy groups are represented by homotopy classes of morphisms from the sphere
spectrum. In more detail, if we denote by 1 ∈ π0 S the class of the identity map, then for every integer n
and every Ω-spectrum X which is levelwise Kan, the evaluation map
S, X −→ π0 X , [f ] 7−→ (π0 f )(1) ,is bijective. Then the evaluation map factors as a compositeS, X −→ π0 |X0 | −→ π0 X
85

86

II. THE STABLE HOMOTOPY CATEGORY

where the first map takes the homotopy class of a morphism S −→ X to the component of the image of the
non-basepoint in S0 = S 0 . The second map is the canonical map from π0 |X0 | to the stable homotopy groupin dimension 0. Since the sphere spectrum S is a suspension spectrum, a morphism from S to any otherspectrum is uniquely determined by its restriction to the zeroth level, and similarly for homotopies. So thefirst map in the composition is a bijection for any spectrum X, no matter whether it is an Ω-spectrum ornot. However, if X is an Ω-spectrum, then the second map from π0 |X0 | to π0 X is also bijective.
Definition 1.3. A symmetric spectrum X of simplicial sets is injective if for every monomorphism
which is also a level equivalence i : A −→ B and every morphism f : A −→ X there exists an extensiong : B −→ X with f = gi.
Injective spectra do not arise ‘in nature’ very often, but we prove in Proposition 4.21 (i) below that
injectivity can always be arranged up to level equivalence.
Example 1.4 (Co-free symmetric spectrum). Let Pm : Σm -sset* −→ SpΣ be right adjoint to evaluation
at level m, considered as a Σm -simplicial set. We call Pm L the co-semifree symmetric spectrum generated
by the Σm -simplicial set L in level m. The spectrum Pm L can explicitely be described as follows: it is justa point above level m and for n ≤ m we have
(Pm L)n = map1×Σm−n (S m−n , L) ,the subspace of 1 × Σm−n -equivariant maps in map(S m−n , L), with restricted Σn -action from L. The
structure map σn : (Pm L)n ∧ S 1 −→ (Pm L)n+1 is adjoint to the mapincl.
map1×Σm−n (S m−n , L) −−−→ map1×Σm−n−1 (S m−n , L) ∼= Ω map1×Σm−n−1 (S m−n−1 , L) .
The forgetful functor Σm -sset* −→ sset* also has a right adjoint given by K 7→ map(Σ+m , K), thefunction space from the set Σm into K (i.e., a product of m! copies of K). So the composite forgetful functor
SpΣ −→ sset* which takes X to the pointed simplicial set Xm has a right adjoint Rm : sset* −→ SpΣ
given by Rm K = Pm (map(Σ+m , K)).
Every Kan simplicial set has the right lifting property with respect to all injective weak equivalences ofsimplicial sets. So by adjointness, the co-free symmetric spectrum Rm K is injective for every Kan simplicial
set K. More generally, let L be a pointed Σm -simplicial set with the property that for all subgroups H ≤ Σm
the H-fixed simplicial set LH is a Kan complex. Then H has the right lifting property with respect to all
injective based morphisms of Σm -simplicial sets which are weak equivalences after forgetting the Σm -action.
So again by adjointness, the co-semifree symmetric spectrum Pn L is injective.
Here is the main definition of this chapter.
Definition 1.5. The stable homotopy category SHC is the homotopy category of injective Ω-spectra.
In other words, the objects of SHC are all injective Ω-spectra of simplicial sets and for such spectra, themorphisms from X to Y in SHC are given by [X, Y ], the set of homotopy classes of spectrum morphisms.
As we already mentioned, injective spectra rarely occur in nature. In fact, at this point, trivial spectra(with all levels a point) are the only injective Ω-spectra we can write down explicitly. However, we explain
in Section 4 below how every symmetric spectrum can be replaced, up to a notion called ‘stable equivalence’,by an injective Ω-spectrum. In that way every symmetric spectrum ‘represents a stable homotopy type’,i.e., gives rise to an object in the stable homotopy category.
In Chapter III we will show that the stable equivalences can be complemented by various useful choicesof cofibrations and fibrations, thus arriving at different stable model category structures for symmetric
spectra. For one particular choice (the injective stable model structure), every symmetric spectrum is
cofibrant and the fibrant objects are precisely the injective Ω-spectra. Moreover, the ‘concrete’ homotopy
relation using homotopies defined on ∆[1]+ ∧ A coincides with the model category theoretic homotopy
relation using abstract cylinder objects. Thus the stable homotopy category as introduced above turns out
to be the homotopy category, in the sense of model category theory, with respect to the injective stablemodel structure.

1. INJECTIVE Ω-SPECTRA87

Our next aim in Sections 2 and 3 of this chapter will be to show that the stable homotopy category
just defined has the structure of a triangulated category. We first develop some tools needed in these later
sections.
Proposition 1.6. Let X be an injective spectrum.
(i) For every injective morphism i : A −→ B of symmetric spectra the map map(i, X) :map(B, X) −→ map(A, X) is a Kan fibration of simplicial sets. If in addition i is a level equivalence, then map(i, X) is a weak equivalence.
(ii) For every symmetric spectrum B the function space map(B, X) is a Kan complex and the homotopy
relation for morphisms from B to X is an equivalence relation.
(iii) For every n ≥ 0 the simplicial set Xn is a Kan complex.
Proof. (i) We have to check that map(i, X) has the right lifting property with respect to every injective
weak equivalence j : K −→ L of pointed simplicial sets. By the adjunction between the smash pairing andmapping spaces, a lifting problem in the form of a commutative squareKj/ map(B, X)
map(i,X)L/ map(A, X)

corresponds to a morphism K ∧ B ∪K∧A L ∧ A −→ X, and a lifting corresponds to a morphism L ∧ B −→ X
which restricts to the previous morphism along the ‘pushout product’ map j ∧i : K ∧B∪K∧A L∧A −→ L∧B.
Since j is an injective weak equivalence and i is injective, the pushout product morphism j ∧ i is an injective
level equivalence of symmetric spectra. So the lifting exists since we assumed that X is injective.
The second part is very similar. If i is injective and a level equivalence, then for every injective morphism
j : K −→ L (not necessarily a weak equivalence) of pointed simplicial sets, the pushout product map j ∧ i is
an injective level equivalence of symmetric spectra. So map(i, X) has the right lifting property with respect
to all injective morphisms of pointed simplicial sets.
Part (ii) is the special case of (i) where A is the trivial spectrum so that map(A, X) is a one-point
simplicial set. Vertices of the simplicial set map(B, X) correspond bijectively to morphisms B −→ X insuch a way that 1-simplices correspond to homotopies. So the second claim of (ii) follows since in every
Kan complex, the relation x ∼ y on vertices defined by existence of a 1-simplex z with d0 z = x and d1 z = yis an equivalence relation.
The simplicial set Xn is naturally isomorphic to the mapping space map(Fn S 0 , X) with source the freesymmetric spectrum generated by S 0 in level n. So (iii) is a special case of (ii).
We now get a criterion for level equivalence by testing against injective spectra.
Proposition 1.7. A morphism f : A −→ B of symmetric spectra of simplicial sets is a level equivalence
if and only if for every injective spectrum X the induced map [f, X] : [B, X] −→ [A, X] on homotopy classesof morphisms is bijective.
Proof. Suppose first that f is a level equivalence. We replace f by the inclusion of A into the mapping
cylinder of f , which is homotopy equivalent to B. This way we can assume without loss of generality that f
is injective. By part (i) of Proposition 1.6 the map map(f, X) : map(B, X) −→ map(A, X) is then a weakequivalence of simplicial sets, so in particular a bijection of components. Since π0 map(B, X) ∼= [B, X], andsimilarly for A, this proves the claim.
Now suppose conversely that [f, X] : [B, X] −→ [A, X] is bijective for every injective spectrum X. If K
is a pointed Kan complex and m ≥ 0, then the co-free symmetric spectrum Rm K of Example 1.4 is injective.
The adjunction for morphisms and homotopies provides a natural bijection [A, Rm K] ∼= [Am , K]sset* to thebased homotopy classes of morphisms of simplicial sets. So for every Kan complex K, the induced map[fm , X] : [Bm , K] −→ [Am , K] is bijective, which is equivalent to fn being a weak equivalence of simplicial
sets. Since this holds for all m, the morphism f is a level equivalence.
88

II. THE STABLE HOMOTOPY CATEGORY

The next lemma can be used to recognize certain morphisms as homotopy equivalences, and thus as
isomorphism in the stable homotopy category.
Proposition 1.8.
(i) Every level equivalence between injective spectra is a homotopy equivalence.
(ii) Every π∗ -isomorphism between Ω-spectra is a level equivalence.
Proof. (i) Let f : X −→ Y be a level equivalence between injective spectra. Using the mapping
cylinder construction, f can be factored as a monomorphism followed by a homotopy equivalence. So we
can replace Y by the mapping cylinder and assume without loss of generality that f is also a monomorphism.
By Proposition 1.6 (i) the induced map map(f, X) : map(Y, X) −→ map(X, X) is a weak equivalence andKan fibration, thus surjective on vertices. So there is a morphism g : Y −→ X satisfying gf = IdX .
Also by Proposition 1.6 (i) the induced map map(f, Y ) : map(Y, Y ) −→ map(X, Y ) is a weak equivalence and Kan fibration. Since moreover map(f, Y ) takes the vertices f g and IdY of map(Y, Y ) to thesame vertex (namely f ∈ map(X, Y )), they can be joined by a 1-simplex in map(Y, Y ), i.e., a homotopy ofspectrum morphisms.
(ii) For every Ω-spectrum X and all k, n ≥ 0, the canonical map πk Xn −→ πk−n X is a bijection. So if
f : X −→ Y is a π∗ -isomorphism between Ω-spectra, then for every n ≥ 0, the morphism fn : Xn −→ Yn
induces a bijection of path components and isomorphisms of homotopy groups in positive dimensions,based at the distinguished basepoint of Xn . In particular, fn restricts to a weak equivalence between thecomponents containing the distinguished basepoints. Since Xn and Yn are loop spaces, their various pathcomponents are all weakly equivalent, and so fn restricts to a weak equivalence on every path componentof X, i.e., fn is a weak equivalence of simplicial sets for every n ≥ 0.
Theorem 1.9. For every π∗ -isomorphism f : A −→ B between symmetric spectra and every injectiveΩ-spectrum X the induced map on homotopy classes [f, X] : [B, X] −→ [A, X] is a bijection.
Proof. We use the functor R∞ introduced in (4.43) of Chapter I. Since X is an Ω-spectrum, themorphism λ∗ : X −→ RX = Ω(sh X) is a level equivalence, and so are all other morphisms in the se∞quence (4.43) whose colimit is R∞ X. Thus also the morphism λ∞
X : X −→ R X is a level equivalence, and∞∞R X is again an Ω-spectrum. Since X is an injective spectrum the map [λX , X] : [R∞ X, X] −→ [X, X] isbijective by Proposition 1.7. So there exists a morphism r : R∞ X −→ X such that the composite rλ∞ ishomotopic to the identity of X (the other composite need not be homotopic to the identity of R∞ X).
The functor R∞ preserves the homotopy relation, so we can define a natural transformation[A, X] −→ [R∞ A, X] ,[ϕ] 7→ [r ◦ R∞ ϕ] .

There also is a natural transformation [R∞ A, X] −→ [A, X] in the other direction given by precomposition∞∞
with λ∞A : A −→ R A. Since r is a retraction (up to homotopy) to λX , the composite of the two natural
maps is the identity on [A, X]. In other words, for fixed injective Ω-spectrum X, the functor [−, X] is aretract of the functor [R∞ (−), X].
Now suppose that f : A −→ B is a π∗ -isomorphism. We assume first that both A and B are levelwise
Kan complexes. In [prove] we established a natural isomorphism πk (R∞ A)m ∼= πk−m A (but beware thatR∞ A is not an Ω-spectrum unless A is semistable). So R∞ f : R∞ A −→ R∞ B is a level equivalence[homotopy groups at other basepoints ?]. So by Proposition 1.7 the map [R∞ f, X] : [R∞ B, X] −→ [R∞ A, X]
is bijective. Since this has [f, X] : [B, X] −→ [A, X] as a retract, the latter is bijective.
In general we apply the functors ‘geometric realization’ and ‘singular complex’ to the morphism f :A −→ B to replace it by a level equivalent morphism whose source and target are levelwise Kan. Since[−, X] takes level equivalences to bijections, the general case follows from the special case.

If X is an Ω-spectrum, then so is the shifted spectrum sh X. The left adjoint S 0 .0 to shifting (seeExample I.4.37) preserves level equivalences and level injections, so shifting also preserves the property of
being injective. Moreover, shifting preserves homotopies since sh(∆[1]+ ∧ X) = ∆[1]+ ∧ sh X. If X is anΩ-spectrum and levelwise Kan, then so is the loop spectrum ΩX, and the functor Ω preserves injective
spectra and the homotopy relation. Moreover, for every Ω-spectrum X the natural map
λ∗ : X −→ Ω(sh X)

2. ADDITIVE STRUCTURE

89

is a level equivalence, thus a homotopy equivalence by part (i) of Proposition 1.8. And so on the level ofthe stable homotopy category, Ω and shift are inverse to each other. So we have shown
Proposition 1.10. The shift functor and the functor Ω are quasi-inverse self-equivalences of the stablehomotopy category.
2. Additive structure
Now we prove that the stable homotopy category is additive. We have to define an addition on morphisms sets in SHC for which composition is bilinear.
Theorem 2.1. For all symmetric spectra of simplicial sets A and B the natural morphism A∨B → A×B
is a π∗ -isomorphism.
Proof. This is a direct consequence of the fact that homotopy groups take both finite wedges andfinite products of symmetric spectra to direct sums. More precisely, for every integer k the composite map
πk A ⊕ πk B −→ πk (A ∨ B) −→ πk (A × B) −→ πk A × πk B
is the identity, where the first and last maps are the canonical ones. These canonical maps are isomorphisms
by Corollary I.4.9, hence so is the middle map.
The stable homotopy category has sums (coproducts) and products of arbitraryQsize. For products
this is easy to see: let {X i }i∈I be a family of injective Ω-spectra. Then the product i∈I X i is again an
injectiveΩ-spectrum, so it represents an object of the stable homotopy category. Moreover, a morphism toQiX
is just a family of morphisms, one to each X i , and similarly for homotopies. So the map
i∈I
Y
Y
[A,
X i ] −→[A, X i ]
i∈I

Q

i∈I

i

i

induced by the projections from i∈I X to each X is a bijection for every symmetric spectrum A. In
particular this holds when A is an injective Ω-spectrum, i.e., the pointset level product of symmetric spectra
descends to a product in the stable homotopy category.
The case of sums is slightly more involved. On the level of symmetric spectra, the coproduct of a family
of symmetric spectra {X i }i∈I is given by the levelwise wedge. However, even if all X i are injective Ω-spectra,
the wedge is not an injective Ω-spectrum. Still, the stable homotopy category has sums (coproducts): theproduct X × Y is also a co-product of X and Y in the stable homotopy category, as we will now show.
Infinite coproducts in the stable homotopy are constructed differently, see Proposition 4.14 (iii) below.
Given symmetric spectra A and B we denote by i1 = (Id, ∗) : A −→ A × B respectively i2 = (∗, Id) :B −→ A × B the ‘inclusions’ of the factors into the product.
Proposition 2.2. For every injective Ω-spectrum X and all symmetric spectra A and B the map(2.3)[A × B, X] −→ [A, X] × [B, X]
[f ]

7−→

([f i1 ], [f i2 ])is bijective. Thus if A and B are also injective Ω-spectra, then the morphisms i1 and i2 make A × B intoa co(!)-product of A and B in the stable homotopy category.
Proof. The map (2.3) factors as a composite(2.4)[A × B, X] −→ [A ∨ B, X] −→ [A, X] × [B, X] .

The first map in (2.4) is precomposition with the canonical injection A∨B → A×B; this is a π∗ -isomorphismby Theorem 2.1, so it induces a bijection of homotopy classes of maps into X by Theorem 1.9. The second
map in (2.4) is restriction to the respective wedge summands. A morphism from X ∨ Y is the same as twomorphisms, one from X and one from Y , and similarly for homotopies. So the second map in (2.4) is alsobijection, which finishes the proof.
If A and B are injective Ω-spectra, then the fact that the map (2.3) is bijective is precisely the universal
property of a categorical coproduct in the special case of the stable homotopy category.
90

II. THE STABLE HOMOTOPY CATEGORY

Construction 2.5. We define an operation ‘+’ on the set [A, X] of homotopy classes of morphisms
from an arbitrary symmetric spectrum A to an injective Ω-spectrum X. For two homotopy classes a, b ∈[A, X] we denote by a ∨ b the unique class in [A × A, X] which restricts to the pair (a, b) along the bijectionof Proposition 2.2. Then we define a + b = (a ∨ b)∆ where ∆ : A −→ A × A is the diagonal morphism.
Proposition 2.6. For every symmetric spectrum A and every injective Ω-spectrum X, the binary
operation + makes the set [A, X] of homotopy classes of morphisms into an abelian group. The neutral
element is the class of the trivial morphism. Moreover, the group structure is natural for all morphisms inthe source variable A and all morphisms between injective Ω-spectra in the target variable X. In particular,
the stable homotopy category is an additive category.
The additivity of the stable homotopy category is a fundamental result which deserves two different
proofs.
First proof. The proof is a lengthy, but essentially formal consequence of Proposition 2.2 which saysthat to the eyes of [−, X] coproducts are the same as products. For the associativity of ‘+’ we consider
three morphisms a, b, c : A −→ X. Then a + (b + c) respectively (a + b) + c are the two outer compositesaround the diagram
mm A QQQQQ ∆
∆ mmm
QQQ
m
m
Q(
vmm
A×A
A×A
Id ×∆


A × (A × A)
PPP
PPPa∨(b∨c) PPP( ∆×Id(A × A) × Annnnnn(a∨b)∨cnnvn

X
If we fill in the canonical associativity isomorphism A × (A × A) ∼= (A × A) × A then the upper part of the
diagram commutes because the diagonal morphism is coassociative. The lower triangle then commutes up
to homotopy since the two morphisms a ∨ (b ∨ c), (a ∨ b) ∨ c : A × (A × A) −→ X have the same ‘restrictions’to X, namely a, b respectively c.
The commutativity is a consequence of two elementary facts: first, b ∨ a = (a ∨ b)τ as homotopy classes,
where τ : A × A −→ A × A is the morphism which interchanges the two factors; this follows from τ i1 = i2and τ i2 = i1 . Second, the diagonal map is cocommutative, i.e., τ ∆ = ∆ : A −→ A × A. Altogether we get
a + b = (a ∨ b)∆ = (a ∨ b)τ ∆ = (b ∨ a)∆ = b + a .
We denote by 0 ∈ [A, X] the class of the trivial morphism. Then we have a ∨ 0 = ap1 in [A × A, X]
where p1 : A × A −→ A is the projection onto the first factor. Hence a + 0 = (a ∨ 0)∆ = ap1 ∆ = a; bycommutativity we also have 0 + a = a.
Now we know that the addition makes the set [A, X] into an abelian monoid, and it remains to showthat additive inverses exist. An arbitrary abelian monoid M has additive inverses if and only if the map
M 2 −→ M 2 ,(a, b) 7−→ (a, a + b)

is bijective. Indeed, the inverse of a ∈ A is the second component of the preimage of (a, 0). For the abelianmonoid [A, X] we have a commutative square[A × A, X][i1 ∨∆,X]∼=

[A, X]2/ [A ∨ A, X]∼=(a,b)7→(a,a+b)

/ [A, X]2

The effect of the morphism i1 ∨ ∆ : A ∨ A −→ A × A on homotopy groupsπ∗ (i1 ∨∆)π∗ A ⊕ π∗ A ∼= π∗ (A ∨ A) −−−−−−→ π∗ (A × A) ∼= π∗ A ⊕ π∗ A

2. ADDITIVE STRUCTURE

91

is given by (x, y) 7→ (x + y, y), so i1 ∨ ∆ is a π∗ -isomorphism since homotopy groups have additive inverses.
Thus the map [i1 ∨ ∆, X] is bijective by Theorem 1.9, and so the abelian monoid [A, X] is a group.
Now we verify naturality of the addition on [A, X] in A and X. To check (a + b)c = ac + bc fora, b : A −→ X and c : A0 −→ A we consider the commutative diagram
A0

c

/A

c×c


/ A×A

/ A×A

∆

∆


A0 × A0

∆

a∨b

a∨b


/3 X

ac∨bc

in which the composite through the upper right corner is (a + b)c. The lower vertical composite (a ∨ b)(c × c)
equals ac∨bc since both have the same ‘restrictions’ ac respectively bc to the two factors of A0 ×A0 . Since thecomposite through the lower left corner is ac + bc, we have shown (a + b)c = ac + bc. Naturality in X is eveneasier. For a morphism d : X −→ Y between injective Ω-spectra we have d(a∨b) = da∨db : A×A −→ Y sinceboth sides have the same ‘restrictions’ da respectively db to the two factors of A×A. Thus d(a+b) = da+db
by the definition of ‘+’.
Second proof. If X is an injective Ω-spectrum then λ∗ : X −→ Ω(sh X) is a natural level equivalence,
homotopy equivalence (by Proposition 1.8 (i)) between injective Ω-spectra. So λ∗ induces a homotopy
equivalencemap(A, λ∗ ) : map(A, X) −→ map(A, Ω(sh X)) ∼= Ω map(A, sh X)
on mapping spaces. Since the target is the simplicial loop space, the loop addition defines a group structureon the set of components π0 map(A, Ω(sh X)) which we pull back along the bijection induced by map(A, λ∗ )to a natural group structure on π0 map(A, X). Now we show that the natural bijection[A, X] ∼= π0 map(A, X)
takes the operation ‘+’ to the loop product in the components of the mapping space map(A, X) and we show
simultaneously that the product on the right hand side is abelian. For this we consider the commutative
diagram∼=/ (π map(A, X))2
[A, X]20
OO∗ ∗∼= (i0 ,i1 )[A × A, X]
+





∗
∼
(i∗
0 ,i1 ) =

∼
=

[∆,X]

[A, X]/ π0 map(A × A, X)
π0 map(∆,X)∼=

loop product/ π0 map(A, X)

of sets in which all horizontal and the left upper vertical map are bijections. The left vertical composite
defines ‘+’. The right vertical composite coincides with the loop product in π0 map(A, X) since it is a2
homomorphism which sends (f, ∗) and (∗, f ) to f . Since the group multiplication (π0 map(A, X)) −→π0 map(A, X) is a homomorphism of groups, the group π0 map(A, X), and thus [A, X], is abelian.
For every injective Ω-spectrum X, the identity morphism of the loop spectrum ΩX has an additive
inverse in the group [ΩX, ΩX]. However, there is no natural endomorphism of the symmetric spectrum ΩXwhich represents the negative of the identity in the homotopy category. This is really a consequence of therigidity of simplicial sets: if we work instead with symmetric spectra based on topological spaces, then wecan ‘invert the direction’ of a loop, i.e., precompose a loop with the any selfmap of S 1 of degree −1 andthus realize − IdΩX naturally on the pointset level. It will be convenient later to have something analogous
for symmetric spectra based on simplicial sets, and we can arrange for this by using a simplicial model ofthe circle which is slightly larger than S 1 = ∆[1]/∂∆[1].

92

II. THE STABLE HOMOTOPY CATEGORYWe define a simplicial set S̄ 1 as a quotient of the horn Λ2 [2] (the simplicial subset of ∆[2] generated byd0 ∆[2] and d1 ∆[2]) by identifying the two ‘outer’ vertices, compare the picture.
?•_2

A • ];;;;d0 ∆[2]d1 ∆[2] 
;;

;


0 • identify • 1

e1

2
e0

•

Λ2 [2]0∼1

S̄ 1

We use the common image of the two outer vertices in S̄ 1 as the basepoint and denote the two nondegenerate 1-simplices of S̄ 1 by e0 and e1 . There is an involution τ : S̄ 1 −→ S̄ 1 which interchanges e0 ande1 . The geometric realization of S̄ 1 is homeomorphic to a circle and τ realizes a map of degree −1.
There are two based morphism p0 , p1 : S̄ 1 −→ S 1 = ∆[1]/∂∆[1] where pi sends ei to the generating 1simplex of S 1 and it sends e1−i to the basepoint. We have p1 = p0 τ . Both p0 and p1 are weak equivalences,1
1
so for every symmetric spectrum X which is levelwise Kan the induced maps p∗0 , p∗1 : ΩX = X S −→ X S̄1
are level equivalences. If X is an injective Ω-spectrum, then so is X S̄ (by the same reasoning as for1ΩX = X S ).
1

Lemma 2.7. For every injective Ω-spectrum X the relation [p∗1 ] = −[p∗0 ] holds in the group [ΩX, X S̄ ].
1
1
1
1
and the morphism τ ∗ : X S̄ −→ X S̄ represents the negative of the identity in the group [X S̄ , X S̄ ].
Proof. The diagram of morphisms of symmetric spectra
∼

∗

1
1
/ X S 1 × X S 1 o = X S 1 ∨S 1 o fold X S 1
X S ∨ X S WW
WWWWW
tt
WWWWW
tt
∗
t
pinch
W
t
WWWWW
∗
tt(fold◦pinch)∗
p∗
WWWWW
0 ∨p1
WW+  1 zt
X S̄commutes. Here we use the fold map S 1 ∨ S 1 −→ S 1 and the ‘pinch’ map S̄ 1 −→ S 1 ∨ S 1 which sends e0
1
and e1 to the generating 1-simplex of S 1 . By the very definition of the addition in the group [ΩX, X S̄ ] this∗∗proves that the sum of the homotopy classes of p0 and p1 is represented by the morphism (fold ◦ pinch)∗ :11
X S −→ X S̄ .
The composite fold ◦ pinch : S̄ 1 −→ S 1 factors as the composite
s

incl

0
S̄ 1 = Λ2 [2]/(0 ∼ 1) −−→ ∆[2]/d2 ∆[2] −→∆[1]/∂∆[1] = S 1

where the second map is induced by the morphism s0 : ∆[2] −→ ∆[1] on quotients. Since the simplicial
1
1
set ∆[2]/d2 ∆[2] is contractible, the morphism (fold ◦ pinch)∗ : X S −→ X S̄ is null-homotopic. This proves1[p∗1 ] + [p∗0 ] = 0 in [ΩX, X S̄ ]. Since we have [τ ∗ p∗0 ] = [p∗1 ] = −[p∗0 ] and p∗0 is a homotopy equivalence, we
conclude that [τ ∗ ] = − Id.

Loops of symmetric spectra are defined levelwise, so we have Ω(sh X) = sh(ΩX). We thus have the2
two morphisms λ∗ΩY and Ω(λ∗Y ) from ΩY to Ω2 (sh Y ), and they differ by the involution of Ω2 (sh Y ) = Y S
which flips the two coordinates in S 2 . So we have shown
Lemma 2.8. The relations
λ∗ΩY = −Ω(λ∗Y )andλ∗sh Y = − sh(λ∗Y )hold in the groups [ΩY, Ω2 (sh Y )] respectively [sh Y, Ω(sh2 Y )].

3. TRIANGULATED STRUCTURE

93

3. Triangulated structure
Besides the shift functor, for which we use the notation X[1], a triangulated category has another pieceof extra structure, namely a specified class of distinguished triangles. In the stable homotopy category,these arise from homotopy fibre sequences as follows.
Recall from (4.3) of Chapter I that the homotopy fibre F (ϕ) of a morphism ϕ : X −→ Y between
symmetric spectra is the pullback in the cartesian square
/X

p

F (ϕ)

(∗,ϕ)



Y ∆[1](ev0 ,ev1 )/ Y ×YWe write ‘elements’ of F (ϕ) as pairs (ω, x) where ω is a path in Y starting at the basepoint and x is a pointin X such that ϕ(x) is the endpoint of ω. There are morphisms
p

i

ϕ

ΩY −
→ F (ϕ) −→X−→Ythe first two being given byi(ω) = (ω, ∗)respectivelyp(ω, x) = x .

The composite pi is the trivial map and the composite ϕp comes with a preferred null-homotopy [specify
it].
We call a morphism of symmetric spectra of simplicial sets an injective fibration if it has the right lifting
property with respect to all monomorphisms which are also level equivalences. So X is injective if and only
if the unique morphism from X to a trivial spectrum is an injective fibration. Note that here the adjective‘injective’ refers to a lifting property and does not mean that such maps are monomorphisms.
Lemma 3.1. Let ϕ : X −→ Y be a morphism of symmetric spectra.
(i) If X and Y are Ω-spectra and ϕ is levelwise a Kan fibration, then the fibre of ϕ over the basepointis an Ω-spectrum.
(ii) If X and Y are Ω-spectra which are levelwise Kan complexes, then so is the homotopy fibre F (ϕ).
(iii) Let X be an injective symmetric spectrum and K −→ L a monomorphism of pointed simplicial
sets. Then the induced morphism X L −→ X K is an injective fibration.
(iv) If X and Y are injective, then the projection p : F (ϕ) −→ X is an injective fibration and thehomotopy fibre F (ϕ) is injective.
Proof. (i) Standard. Uses that the geometric realization of a Kan fibration is a Serre fibration, thelong exact sequence of homotopy groups of a Kan fibration and the five lemma.
(ii) The homotopy fibre F (ϕ) is the fibre over the basepoint of the morphism ev0 : Y ∆[1] ×Y X −→ Y .
This morphism is levelwise a Kan fibration The spectrum Y ∆[1] ×Y X is homotopy equivalent to X by‘contracting a path to its endpoint’, so source and target of ev0 are Ω-spectra. The morphism is also
levelwise a Kan fibration: for every morphism ψ : V −→ W between pointed Kan complexes, the lower
horizontal map in the pullback squareW ∆[1] ×W V

(ev0 ,p)(Id,ψ)

W ∆[1]/ W ×V(ev0 ,ev1 )/ W ×Wis a Kan fibration (since W is Kan), and hence so is its base change. Since V is Kan, the projection
W × V −→ W is a Kan fibration, hence also the composite ev0 : W ∆[1] ×W V −→ W . So the morphism
ev0 : Y ∆[1] ×Y X −→ Y satisfies the hypothesis of part (i), and so its (strict) fibre F (ϕ) is an Ω-spectrum.

94

II. THE STABLE HOMOTOPY CATEGORY(iv) Since Y is injective and the boundary inclusion ∂∆[1] −→ ∆[1] is a monomorphism of simplicial
set, the evaluation morphism (ev0 , ev1 ) : Y ∆[1] −→ Y ∂∆[1] ∼= Y × Y is an injective fibration, by part (iii).
So p : F (ϕ) −→ X is an injective fibration since these are stable under basechange. If furthermore X is
injective, then so is F (ϕ) by the composition property.

Now consider a morphism of symmetric spectra ϕ : X −→ Y . If we let F denotes the (strict) fibre of
ϕ at the basepoint, then we have a morphism j : F −→ F (ϕ) given by x 7→ (∗, x) where we write ∗ forthe constant path in Y at the basepoint. If X and Y are levelwise Kan and ϕ : X −→ Y is levelwise a
Kan fibration, then the morphism j : F −→ F (ϕ) is a level equivalence. This holds in particular whenever
X and Y are injective Ω-spectra and ϕ : X −→ Y is an injective fibration, and then the level equivalence
j : F −→ F (ϕ) has injective Ω-spectra as source and target, and so it is a homotopy equivalence, thus an
isomorphism in the stable homotopy category.
An elementary distinguished triangle in the stable homotopy category is a diagram of the form
δϕ

ϕ

incl.

F −−−−→ X −−→ Y −−→ F [1]for an injective fibration ϕ : X −→ Y between injective Ω-spectra. The ‘connecting’ morphism δ : Y −→F [1] is the unique morphism in the stable homotopy category which makes the square
δ

Y
λ∗
Y

∼
=


ΩY [1]/ F [1]∼= j[1]i[1]/ F (ϕ)[1]

commute. Both vertical maps are isomorphism in the stable homotopy category; the left one is the level
equivalence λ∗Y : Y −→ Ω(sh Y ) = ΩY [1]. The lower morphism is the shift of the morphism i : ΩY −→ F (ϕ).
[remark naturality in the fibration and δsh ϕ = − sh(δϕ )]A distinguished triangle is any diagram
f

g

h

A−
→B−
→C−
→ A[1]
in the stable homotopy category which is isomorphic to an elementary distinguished triangle, i.e., such that
there is an injective fibration ϕ : X −→ Y between injective Ω-spectra and isomorphisms α : A −→ F ,β : B −→ X and γ : C −→ Y in SHC such that the diagram
A

/B

f

α


F

g

/C

/ A[1]

γ

β


/X
incl.

h

ϕ


/Y

α[1]

δϕ


/ F [1]

commutes. Our aim is to show that the shift functor and the class of distinguished triangles make the stablehomotopy category into a triangulated category. We first collect some ways to produce new distinguishedtriangles from old ones.
Proposition 3.2. (i) If a triangle (f, g, h) is distinguished, then so is the triangle
−h̄

f

λ∗ ◦g

C
ΩC −−→ A −−→ B −−−→ ΩC[1]where h̄ : ΩC −→ A is the unique morphism in the stable homotopy category such that h̄[1] ◦ λ∗C = h.
(ii) If (f, g, h) is a triangle such that the rotated triangle (g, h, −f [1]) is distinguished, then so is the original
triangle (f, g, h).
(iii) If the triangle (f, g, h) is distinguished, then so is the triangle−f [1]

−g[1]

−h[1]

A[1] −−−→ B[1] −−−→ C[1] −−−→ A[2] .

3. TRIANGULATED STRUCTURE

95

Proof. (i) We can assume without loss of generality that we are dealing with the elementary distinguished triangle (incl, ϕ, δϕ ) of an injective fibration ϕ : X −→ Y between injective Ω-spectra. By
Lemma 3.1 (iv) the projection p : F (ϕ) −→ X is again an injective fibration between injective Ω-spectra.
We claim that the diagram
ΩY

−δ̄ϕ

− Id


ΩY

/F

incl.

∗

/ X λY ◦ϕ / ΩY [1]
− Id

j

i


/ F (ϕ)

p

/X

δp


/ ΩY [1]commutes in the stable homotopy category where δ̄ϕ is the unique morphism such that (δ̄ϕ )[1] ◦ λ∗Y = δ.
Indeed, by the definition of the connective morphism δ we havei[1] ◦ λ∗Y = j[1] ◦ δ = j[1] ◦ (δ̄ϕ )[1] ◦ λ∗Y .
Since λ∗Y is an isomorphism and shifting is faithful this implies i = j ◦ δ̄ϕ , i.e., the left square commutes.
With respect to the right square we first claim that the relation ip = −j ◦ Ω(ϕ) holds in the group[ΩX, F (p)], where j : ΩY −→ F (p) is given by j(ω) = (const∗ , ω). Granted this for a moment, we haveδp = (j −1 ip )[1] ◦ λ∗X = −Ω(ϕ)[1] ◦ λ∗X = −λ∗Y ◦ ϕ(using naturality of λ∗ ), i.e., the right square commutes.
We are now reduced to proving the relation ip = −j ◦ Ω(ϕ) in [ΩX, F (p)]. If we were using symmetric
spectra based on topological spaces we could realize the negative of Ω(ϕ) by ‘inversion of the loop’ andwrite down an explicit homotopy between ip and the ‘loop inverted’ composite j ◦ Ω(ϕ). Since we work
simplicially and S 1 = ∆[1]/∂∆[1] has no selfmap of degree −1, we have to say a bit more to make this idea
1
rigorous. We use the level equivalent model Y S̄ for the loop spectrum ΩY , where S̄ 1 = Λ2 [2]/(0 ∼ 1) isthe ‘large’ simplicial circle (compare Lemma 2.7). A morphismg : F (p) = ∗ ×X X ∆[1] ×X F (ϕ) −→ Y S̄1

is defined levelwise by sending (λ, ν) to the map S̄ 1 −→ Y which is ϕ ◦ λ on the 1-simplex e0 of S̄ 1 , andwhich is ν on 1-simplex e1 . The morphism g is a level equivalence, and thus an isomorphism in SHC, since1
the composite with the level equivalence j : ΩY −→ F (p) is the level equivalence p∗1 : ΩX −→ Y S̄ inducedby the weak equivalence p1 : S̄ 1 −→ S 1 .
The two composites
1
gip , gjΩ(ϕ) : ΩX −→ Y S̄11

differ precisely, on the pointset level, by the involution τ ∗ : Y S̄ −→ Y S̄ induced by the ‘flip’ τ of the‘large’ circle S̄ 1 . By Lemma 2.7 this involution realizes the negative of the identity, so we conclude that1[gip ] = −[gjΩ(ϕ)] in the group [ΩX, X S̄ ]. Since g becomes an isomorphism in the stable homotopy category[justify], this proves the claim.
The lower row of the commutative diagram is the elementary distinguished triangle associated to p :F (ϕ) −→ X and all vertical morphisms are isomorphism. So the upper triangle is distinguished.
(ii) If (g, h, −f [1]) is a distinguished triangle, then so is the lower triangle in the diagram
A

f

/B

g

/C

h

−λ∗
A

/ A[1]
−λ∗
A [1]


ΩA[1]
f [1]

/B

g

/C

λ∗
A[1] ◦h/ ΩA[2]by part (i). The right square commutes by Lemma 2.7 which says that λ∗A[1] = −λ∗A [1]. The left squarecommutes since we havef [1] = f [1][1] ◦ λA[1] = −(f [1] ◦ λA )[1]

96

II. THE STABLE HOMOTOPY CATEGORY

where the first equation is the defining property of f [1]. So (f, g, h) is isomorphic to a distinguished triangle,
and thus itself distinguished.
(iii) If suffices to prove the claim for the elementary distinguished triangle associated to an injective
fibration ϕ : X −→ Y between injective Ω-spectra. Shifting preserves fibres and injective fibrations. However, the two morphisms sh(λ∗Y ) and λ∗sh Y are inverse to each other in the additive group [sh Y, sh(Ω(sh Y ))]by Lemma 2.8. This implies δsh ϕ = − sh(δϕ ) and so in the stable homotopy category we have a commutative
diagramsh F− sh(incl)− sh ϕ/ sh X/ sh Y− sh(δϕ )/ sh2 F

− Id

Fsh ϕ


/ sh Xincl

sh ϕ/ sh Yδsh ϕ/ sh F (sh ϕ)

The lower row is the elementary distinguished triangle associated to the injective fibration sh ϕ : sh X −→sh Y . The upper row is thus distinguished.
Now we can state and prove the main result of this section.
Theorem 3.3. The shift functor and the class of distinguished triangles make the stable homotopy
category into a triangulated category.
Proof. We verify the axiom (T1), (T2) and (T3) as stated in Section 2 of Appendix A. These axioms
seem weaker than Verdier’s original axiom, but we recall in the appendix that they are in fact equivalent.
(T1) This axiom has three parts:(a) For every injective Ω-spectrum X the unique morphism X −→ ∗ is an injective fibration and the triangle
Id

is X −→ X −→ 0 −→ X[1] the associated elementary distinguished triangle.
(b) Let [ϕ] ∈ [X, Y ] be a morphism in the stable homotopy category and let ϕ : X −→ Y be a representing
morphism between injective Ω-spectra. By Lemma 3.1 (iv) the projection p : F (ϕ) −→ X from thehomotopy fibre of ϕ to X is an injective fibration between injective Ω-spectrum, which thus has an associatedelementary distinguished triangle (i, p, δp ). By two applications of (T2) the lower row in the diagramX

ϕ−i[1]◦λ∗Y/Y/ F (ϕ)[1]−p[1]/ X[1]

λ∗
Y

X

δp


/ ΩY [1]−i[1]/ F (ϕ)[1]−p[1]/ X[1]

is distinguished. Since the left square commutes in the stable homotopy category [...] the upper row is
distinguished.
It is worth noting that the triangle which we get this way depends on the choice of representing
morphism ϕ, and is thus not natural in the homotopy class [ϕ]. In fact, if ϕ0 : X −→ Y is homotopic to ϕ,then the homotopy fibre F (ϕ) is homotopy equivalent to F (ϕ0 ), but any construction of such a homotopy
equivalence involves a choice of homotopy between ϕ and ϕ0 . Different choices of homotopies will in generallead to different homotopy classes of homotopy equivalences.
(c) By definition, every triangle isomorphic to a distinguished triangle is itself distinguished.
(T2) If (f, g, h) is a distinguished triangle, then the triangle (−f [1], −g[1], −h[1]) is distinguished by
Proposition 3.2 (iii). Thus the triangle (g, h, −f [1]) is distinguished by two applications of part (ii) of
Proposition 3.2.
(T3) The axiom is usually referred to as the octahedral axiom. Consider distinguished triangles(f1 , g1 , h1 ), (f2 , g2 , h2 ) and (f3 , g3 , h3 ) such that f1 and f2 are composable and f3 = f2 f1 . Then there3. TRIANGULATED STRUCTURE

97

exist morphisms x and y such that (x, y, g1 [1] ◦ h2 ) is a distinguished triangle and the following diagram
commutes
/B

f1

A

g1

/C
x

f2

A


/D

f3

g3


/E

g2

/ A[1]h3yF

f1 [1]


F

h2


B[1]

/ A[1]h1/ B[1]h2
g1 [1]◦h2

g1 [1]

/ C[1]

We will use that for every distinguished triangle (f, g, h) there exists an injective fibration ϕ : X −→ B
and isomorphisms α : A −→ X and γ : C −→ Fϕ in the stable homotopy category such that the diagram
/B

f

A

g

/C

/ A[1]

h

γ

α


X

/B

ϕ

α[1]



δϕ

/ Fϕ [1]−iϕ [1]/ X[1]

commutes, where the bottom row is the rotation of the elementary distinguished triangle associated to ϕ.
Indeed, the triangle (−h̄, f, λ∗C ◦ g) is distinguished by Proposition 3.2; so there is an isomorphism from(−h̄, f, λ∗C ◦ g) to an elementary distinguished triangle (i, ϕ, δϕ ), which we can rotate.
By the above we can assume without loss of generality that the triangle (f2 , g2 , h2 ) is the rotationof the elementary distinguished triangle (inclψ , ψ, δψ ) associated to an injective fibration ψ : Y −→ Z.
We can then assume that the triangle (f1 , g1 , h1 ) equals the rotation of (inclϕ , ϕ, δϕ ) for another injective
fibration ϕ : X −→ Y , where X, Y and Z are injective Ω-spectra. The composite ψϕ : X −→ Z is thenagain an injective fibration and we can finally assume that the triangle (f3 , g3 , h3 ) is the rotated elementary
distinguished triangle of ψϕ. We are now in the situation of the following commutative diagram, where thetwo dotted morphisms have to be constructed:
X

ϕ

/Y

δϕ

/ Fϕ [1]

ψ

X

ψϕ


/Z

/ X[1]

incl[1]δψϕ/ Fψϕ [1]

δψ

−iψϕ

ϕ̄[1]


Fψ [1]

Fψ [1]−iψ [1]Y [1]−iϕ [1]δϕ [1]/ X[1]ϕ[1]−iψ [1]

/ Y [1]−δϕ̄ [1]/ Fϕ [2]We can fill in the required morphisms in the third row taking the shift of the inclusion Fϕ −→ Fψϕrespectively the restriction of ϕ to the strict fibre of ψϕ : X −→ Z, which we denote by ϕ̄ : Fψϕ −→ Fψ .
With these fillers the four squares involving the spectrum Fψϕ commute because the construction of
the triangle from an injective fibration is natural. In order to verify the octahedral axiom (T3) it remains to

98

II. THE STABLE HOMOTOPY CATEGORY

show that the third row is a distinguished triangle. The restriction ϕ̄ : Fψϕ −→ Fψ of the injective fibration
ϕ is again an injective fibration, and the strict fibres of ϕ : X −→ Y and its restriction ϕ̄ are equal. So thetriangle
incl

δϕ

ϕ̄

Fϕ −−−→ Fψϕ −→ Fψ −→ Fϕ [1]is elementary distinguished; by part (iii) of Proposition 3.2, and closure under isomorphisms (to get rid of
two signs) the triangleincl[1]−δϕ [1]

ϕ̄[1]Fϕ [1] −−−−−→ Fψϕ [1] −−→ Fψ [1] −−−−→ Fϕ [2]is distinguished. The relation δϕ̄ = δϕ iψ holds in the group [Fψ , Fϕ [1]] by another instance of the naturalityof elementary distinguished triangles in the injective fibration defining it.

4. Stable equivalences
By our definition, only injective Ω-spectra are objects of the stable homotopy category. However,many constructions which one can perform with symmetric spectra do not preserve the property of being
an injective Ω-spectra, so it would be convenient if we can regard arbitrary symmetric spectra as objectsof the stable homotopy category. In this section we introduce the notion of stable equivalence and showthat up to stable equivalence, every symmetric spectrum can be replaced by an injective Ω-spectrum. The
ultimate consequence will be that the stable homotopy category arises as the localization of the categoryof symmetric spectra obtained by ‘inverting stable equivalences’, compare Theorem 4.12.
Definition 4.1. A morphism f : A −→ B of symmetric spectra of simplicial sets is a stable equivalence
if for every injective Ω-spectrum X the induced map[f, X] : [B, X] −→ [A, X]
on homotopy classes of spectrum morphisms is a bijection.
In Theorem 4.3 below we give a list of several equivalent characterizations of stable equivalences. In
Proposition 4.5 we prove that stable equivalences are closed under various operations. In our new language,
Theorem 1.9 says that every π∗ -isomorphisms of symmetric spectra is a stable equivalence; the converse
does not in general hold, as Example 4.2 below shows. For morphisms of symmetric spectra we thus havethe implications
homotopy equivalence =⇒ level equivalence =⇒ π∗ -isomorphism =⇒ stable equivalence.
Example 4.2. While every π∗ -isomorphism of symmetric spectra is a stable equivalence, the converse
is not true. The standard example of this phenomenon is the following: consider the symmetric spectrum
F1 S 1 freely generated by the circle S 1 in level 1, compare Example I. 2.12. We consider the morphism
λ : F1 S 1 −→ F0 S 0 = S which is adjoint to the identity in level 1.
Since F1 S 1 is freely generated by the circle S 1 in level 1, it ought to be a desuspension of the suspension
spectrum of the circle. And indeed, the morphism λ : F1 S 1 −→ F0 S 0 = S is a stable equivalence. To see
this we consider an injective Ω-spectrum X and consider the commutative square[S, X][λ,X]∼= eval1

eval0 ∼=



[S 0 , X0 ]sset* = π0 X0/ [F1 S 1 , X]ι∗
/ π1 X1 = [S 1 , X1 ]sset*

The vertical maps given by evaluation at levels 0 respectively 1 are adjunction bijections and the lower
horizontal map is the stabilization map. Since X is an Ω-spectrum, the lower vertical map is a bijection,
hence the upper vertical map is, which proves that λ is a stable equivalence.
However, we calculated the 0th homotopy group of the free symmetric spectrum F1 S 1 in Example I.2.12
and, more systematically, in Example I.4.31. The group π0 F1 S 1 is isomorphic to the M-module P1 and4. STABLE EQUIVALENCES

99

in particular free abelian of countably infinite rank. But π0 S is free abelian of rank one, so λ is not aπ∗ -isomorphism.
Theorem 4.3. The following are equivalent for a morphism f : A −→ B of symmetric spectra ofsimplicial sets:(i) f is a stable equivalence;(ii) for every injective Ω-spectrum X the induced map map(f, X) : map(B, X) −→ map(A, X) is aweak equivalence of simplicial sets;(iii) for every injective Ω-spectrum X the induced map Hom(f, X) : Hom(B, X) −→ Hom(A, X) is alevel equivalence of symmetric spectra;(iv) the mapping cone C(f ) of f is stably equivalent to the trivial spectrum.
(v) the suspension S 1 ∧ f : S 1 ∧ A −→ S 1 ∧ B is a stable equivalence;
If A and B are levelwise Kan complexes, conditions (i)-(v) are furthermore equivalent to(vi) the homotopy fibre F (f ) of f is stably equivalent to the trivial spectrum;(vii) the loop Ωf : ΩA −→ ΩB is a stable equivalence.
Proof. (i)⇒(ii) For every simplicial set K and every injective Ω-spectrum X the function spectrum X K
is again injective by Lemma 3.1 (iii) and an Ω-spectrum by Example I.2.22. We have an adjunction bijection[K, map(A, X)] ∼= [A, X K ] where the left hand side means homotopy classes of morphisms of simplicial
sets. So if f is a stable equivalence, then [f, X K ] is bijective, hence [K, map(f, X)] : [K, map(B, X)] −→[K, map(B, X)] is bijective. Since this holds for all simplicial sets K, map(f, X) is a weak equivalence.
(ii)⇒(iii) For every injective Ω-spectrum X and n ≥ 0 the shifted spectrum shn X is again an injectiveΩ-spectrum. So if f : A −→ B satisfies (ii), it also satisfies (iii) since the nth level of the spectrum
Hom(A, X) is defined as map(A, shn X).
(iii)⇒(iv) The morphism spectrum from the mapping cone C(f ) to a symmetric spectrum X is naturally
isomorphic to the homotopy fibre of the morphism Hom(f, X), i.e.,
Hom(C(f ), X) ∼= F (Hom(f, X) : Hom(B, X) −→ Hom(A, X)) .
So if (iii) holds then for every injective Ω-spectrum X the morphism Hom(f, X) is a level equivalence, thus
the homotopy fibre F (Hom(f, X)) is levelwise contractible. So the isomorphic spectrum Hom(C(f ), X)
is levelwise contractible and in particular [C(f ), X] ∼= π0 Hom(C(f ), X)0 has only one element. So themapping cone C(f ) is stably equivalent to the trivial spectrum.
(iv)⇒(v) If p : C(f ) −→ S 1 ∧ A denotes the morphism which collapses B to a point then the mapping
cone of p is homotopy equivalent to ΣB. More precisely there is a diagram of symmetric spectraC(f )/ Z(p)
JJ
JJ
JJ '
p JJJ
% 
S1 ∧ A/ C(p)'

S 1 ∧f/ S1 ∧ B

in which the triangle commutes and the square commutes up to homotopy. If condition (iv) holds then by
the already established implication ‘(i)=⇒(ii)’ the simplicial set map(C(f ), X) is weakly contractible forevery injective Ω-spectrum X. We have a homotopy fibre sequencemap(C(p), X) →− map(Z(p), X) −→ map(C(f ), X)
whose last term is weakly contractible. So the first morphism is a weak equivalence. Since it is homotopy
equivalent to map(S 1 ∧ f, X), that morphism is a weak equivalence. Taking path components shows that[S 1 ∧ f, X] : [S 1 ∧ B, X] −→ [S 1 ∧ A, X] is bijective for every injective Ω-spectrum X, so that S 1 ∧ f is astable equivalence.
(v)⇒(i) Suppose that the suspension S 1 ∧f is a stable equivalence and let X be an injective Ω-spectrum.
Then the shifted spectrum sh X is also an injective Ω-spectrum, so the map [S 1 ∧f, sh X] : [S 1 ∧B, sh X] −→[S 1 ∧ A, sh X] is bijective. By adjunction, the map [f, Ω(sh X)] : [B, Ω(sh X)] −→ [A, Ω(sh X)] is then also

100

II. THE STABLE HOMOTOPY CATEGORY

bijective. Since the map λ∗ : X −→ Ω(sh X) is a homotopy equivalence, it follows that f induces a bijection
on homotopy classes of morphisms to X. So f is a stable equivalence.
(iv)⇔(vi) We assume that A and B are levelwise Kan complexes. The suspension of the homotopy fibreis π∗ -isomorphic, hence stably equivalent, to the mapping cone. By the equivalence of conditions (i) and(v), a symmetric spectrum is stably trivial if and only if its suspension is. So (iv) and (vi) are equivalent.
(iv)⇔(vi) If A is levelwise Kan, then the adjunction counit S 1 ∧ ΩA −→ A is a π∗ -isomorphism, thus
a stable equivalence. So f is a stable equivalence if and only if S 1 ∧ Ω(f ) is. By the already establishedequivalence between conditions (i) and (v) this is equivalent to Ωf being a stable equivalence.

For the next proposition we recall that a commutative square of simplicial sets
α

V

/W

ϕ


X

β


/Y

ψ

is called homotopy cartesian if for some (hence any) factorization of the morphism g as the composite of a
weak equivalence w : W −→ Z followed by a Kan fibration f : Z −→ Y the induced morphism(ϕ,wα)

V −−−−→ X ×Y Z
is a weak equivalence. The definition is in fact symmetric in the sense that the square is homotopy cartesian
if and only if the square obtained by interchanging X and W (and the morphisms) is homotopy cartesian.
So if the square is homotopy cartesian and ψ (respectively β) is a weak equivalence, then so is ϕ (respectivelyα).
Proposition 4.4. Consider a pullback square of symmetric spectra of simplicial sets
A

i

/B
g

f


C

j


/D

in which the morphism g is levelwise a Kan fibration. Then for every injective Ω-spectrum X the commutative square of simplicial setsmap(D, X)map(g,X)map(j,X)
map(C, X)/ map(B, X)
map(i,X)map(f,X)/ map(A, X)

is homotopy cartesian.
Proof. In a first step we prove the proposition under the additional assumption that the morphism
j : C −→ D is a monomorphism. This implies that its basechange i : A −→ B is also a monomorphism. In
this situation the morphism j ∪ g : C ∪A B −→ D is a π∗ -isomorphism [ref], thus a weak equivalence by4. STABLE EQUIVALENCES

101

Theorem 1.9. We fix an injective Ω-spectrum X and consider the commutative diagram of simplicial setsmap(g,X)/ map(B, X)
ggg3
g
g
g
g
g
ggggg
map(C ∪A B, X)
map(j,X)map(i,X)
ss
sss
s
s
s
sss
s
s
 ys

/ map(A, X)
map(C, X)
map(D, X)
SSSS
map(f
SSSS ∪i,X)S)

map(f,X)

The right vertical map map(i, X) is a Kan fibration by Proposition 1.6 (i) and the lower right part of the
diagram is a pullback. The morphism map(f ∪ i, X) is a weak equivalence, so the outer commutative squareis homotopy cartesian.
Now we prove the general case. We factor the morphism j as the mapping cylinder inclusion C −→ Z(j)followed by the projection p : Z(j) −→ D which is a homotopy equivalence. Then the square decomposes
as the composite of two pullback squares
Ai/ Z(j) ×D B

p̃

/B
g

f


/ Z(j)


C

p


/D

For every injective Ω-spectrum X the functor map(−, X) takes the left pullback square to a homotopy
cartesian square of simplicial sets by the special case above. The projection p is a level equivalence, hence
so is its basechange p̃. So both p and p̃ become weak equivalences after applying map(−, X) and thefunctor map(−, X) also takes the right pullback square to a homotopy cartesian square. The composite of
two homotopy cartesian squares is homotopy cartesian, which proves the claim.
Proposition 4.5.
(i) A wedge of stable equivalences is a stable equivalence.
(ii) A finite product of stable equivalences is a stable equivalence.
(iii) Consider a commutative square of morphisms of symmetric spectra
A

ϕA

f0

f


B

/ A0

ϕB


/ B0

and let f 00 : C(ϕA ) −→ C(ϕB ) be the map induced by f and f 0 on mapping cones. Then if two of
the three morphisms f , f 0 and f 00 are stable equivalences, so is the third.
(iv) Consider a commutative of symmetric spectra of simplicial sets
A

i

g

f


C

/B

j


/D

If the square is a pushout, f a stable equivalence and i or f is injective, then g is a stable equivalence. If the square is a pullback, g a stable equivalence and j or g is levelwise a Kan fibration,then f is a stable equivalence.
(v) For every stable equivalence f : A −→ B and every pointed simplicial set K the morphism K ∧ f :K ∧ A −→ K ∧ B is a stable equivalence. If A and B are levelwise Kan complexes and K is finite,then the morphism f K : AK −→ B K is a stable equivalence.

102

II. THE STABLE HOMOTOPY CATEGORY(vi) Let I be a filtered category. If A : I −→ SpΣ is a functor which takes every morphism to an
injective stable equivalence, then for every object i ∈ I the canonical morphism A(i) −→ colimI A
is an injective stable equivalence.
(vii) Let I be a filtered category and let A, B : I −→ SpΣ be functors which take all morphisms in I
to monomorphisms of symmetric spectra. If τ : A −→ B is a natural transformation such that
τ (i) : A(i) −→ B(i) is a stable equivalence for every object i of I, then the induced morphism
colimI τ : colimI A −→ colimI B on colimits is a stable equivalence.
[is the diagonal of stable equivalences a stable equivalence ?]Proof. (i) For every family {Ai }i∈I of symmetric spectra and every injective Ω-spectrum X thatnatural map_
Y[ Ai , X] −→[Ai , X]i∈Ii∈I

is bijective by the universal property of the wedge, applied to morphisms and homotopies.
(ii) Finite products are π∗ -isomorphic to finite wedges (see Theorem 2.1), and π∗ -isomorphisms arestable equivalences (see Theorem 1.9), so the claim follows from part (i).
(iii) We show that if f and f 0 are stable equivalences, then so if f 00 . The other cases are similar.
For every injective Ω-spectrum X we have a commutative diagram [explain][ΣA0 , X][Σf 0 ,X]

[ΣB 0 , X]

[ΣϕA ,X]/ [ΣA, X][p,X][f 00 ,X]

[Σf,X][ΣϕB ,X]/ [ΣB, X]/ [C(ϕA ), X]

[p,X]

[i,X]

/ [A0 , X]

[ϕA ,X][f 0 ,X]/ [C(ϕB ), X]

[i,X]


/ [B 0 , X]

[ϕB ,X]/ [A, X][f,X]


/ [B, X]in which both rows are long exact sequences of abelian groups. If f and f 0 are stable equivalences, thenall except possibly the middle vertical maps are bijective. So the middle map is bijective by the 5-lemma.
[easier to using mapping spaces into X?](iv) Let us first consider the case of a pushout square with f a stable equivalence and i or f injective.
For every injective Ω-spectrum X the commutative square of simplicial sets
(4.6)

map(D, X)map(g,X)map(j,X)
map(C, X)/ map(B, X)
map(i,X)map(f,X)/ map(A, X)

is then a pullback and at least one of the maps map(i, X) or map(f, X) is a Kan fibration by Proposition 1.6 (i). Since moreover the lower horizontal map is a weak equivalence (by Theorem 4.3 (ii)), so is theupper horizontal map. Again by Theorem 4.3 (ii) this means that g is a stable equivalence.
Now we consider the case of a pullback square in which one of the morphisms j or g is levelwise a Kan
fibration. By Proposition 4.4 the commutative square (4.6) is then homotopy cartesian. If g is a stableequivalence, then the upper horizontal map is a weak equivalence (by Theorem 4.3 (ii)). This means that thelower horizontal maps induces an isomorphism of homotopy groups in positive dimensions and an injectionof path components. If we replace X by the injective Ω-spectrum sh X we deduce that map(f, sh X) :map(C, sh X) −→ map(A, sh X) induces an isomorphism on fundamental groups and so Ω(map(f, sh X)) ∼=map(f, Ω(sh X)) induces a bijection on path components. Since the simplicial set map(A, X) is naturally
weakly equivalent to map(A, Ω(sh X)), this proves that map(f, X) is a weak equivalence for all injectiveΩ-spectra X. Again by Theorem 4.3 this means that f is a stable equivalence.
(v) The first statement follows from the adjunction bijection [K ∧A, X] ∼= [A, X K ] and the fact that X K
is an injective Ω-spectrum whenever K is. For the second statement we observe that the functor A 7→ AK
commutes with homotopy fibres and preserves the property of being levelwise Kan. So passage to homotopy

4. STABLE EQUIVALENCES

103

fibres and the equivalence of conditions (i) and (vi) in Theorem 4.3 reduce us to showing that if A is levelwise
Kan and stably contractible, then so is AK . We prove this by induction over the dimension of K. If K is
0-dimensional, then AK is a finite product of copies of A, this stably contractible by part (ii). Now supposethat K has positive dimension n. We do another induction on the number of non-degenerate n-simplices of
K. We write K = L ∪∂∆[n]+ ∆[n]+ for a simplicial subset L with one non-degenerate n-simplex less than
K. We obtain pullback square of symmetric function spectra
/ AL
AK

+
A∆[n]


/ A∂∆[n]++in which the horizontal morphisms are restrictions, thus levelwise Kan fibrations. The spectrum A∂∆[n]is stably contractible since ∂∆[n]+ has smaller dimension. Since ∆[n]+ is weakly equivalent to S 0 the+0spectrum A∆[n] is level equivalent to AS ∼= A, thus stably contractible. In particular, the lower horizontal
morphism is a stable equivalence, hence so is the upper one by part (iv). Since AL is stably contractible byinduction, so is AK .
(vi) For every injective Ω-spectrum X the simplicial set map(colimI A, X) is isomorphic to the inverse
limit of the functor map(A, X) : I op −→ sset* . Since A consists of injective stable equivalences, eachmorphism in the inverse system map(A, X) is a weak equivalence (by Theorem 4.3 (ii)) and a Kan fibration(by Proposition 1.6 (i)). Since I is filtered, the map from the inverse limit map(colimI A, X) to each stagemap(A(i), X) is then also a weak equivalence of simplicial set, which means that A(i) −→ colimI A is astable equivalence.
(vii) For every injective Ω-spectrum X the simplicial set map(colimI A, X) is isomorphic to the inverse
limit of the functor map(A, X) : I op −→ sset* , and similarly for the functor B. Since A and B consists ofinjective morphisms, all morphisms in the inverse systems map(A, X) and map(B, X) are Kan fibrations(by Proposition 1.6 (i)). Filtered inverse limits of weak equivalences along Kan fibrations are again weakequivalences, so the map map(colimI B, X) −→ map(colimI A, X) is a weak equivalence of simplicial set,
which means that colimI A −→ colimI B is a stable equivalence.

Now that we mentioned many constructions which preserves stable equivalences we also mention one
which does not, namely shifting (this should be contrasted with the fact that shifting does preserve π∗ isomorphisms because πk+1 (sh X) equals πk X as abelian groups). An example is the fundamental stableequivalence λ : F1 S 1 −→ S of Example 4.2 which is adjoint to the identity of S 1 . The symmetric spectrumsh(F1 S 1 ) is isomorphic to the wedge of F0 S 1 and F1 S 2 , while sh S ∼= F0 S 1 ; the map sh λ : sh(F1 S 1 ) −→ sh S
is the projection to the wedge summand. The complementary summand F1 S 2 ∼= S 1 ∧ F1 S 1 is stably
1∞1
equivalent, via the suspension of λ, to S ∧ S ∼= Σ S , and is thus not stably contractible.
4.1. The stable homotopy category as a localization. We recall that a general symmetric spectrum is not π∗ -isomorphic to an injective Ω-spectrum. By Theorem I.4.44 a necessary (and in fact sufficient)condition for that is semistability. In contrast, we will now see that every symmetric spectrum is stably
equivalent to an injective Ω-spectrum. This is the key point in constructing a functor from the categoryof symmetric spectra to the stable homotopy category, thus associating a ‘stable homotopy type’ to every
symmetric spectrum. In the language of model structures which we will discuss later, this amounts to theexistence of fibrant replacements in the injective stable model structure.
Theorem 4.7. For every symmetric spectrum A there exists a stable equivalence from A to an injectiveΩ-spectrum.
We postpone the technical proof to the end of this section, where we give a functorial construction,using the ’small object argument’.
Construction 4.8. We now construct a functor γ : SpΣ −→ SHC which by Theorem 4.12 below
is a localization of the category of symmetric spectra at the class of stable equivalences. First, for each

104

II. THE STABLE HOMOTOPY CATEGORY

symmetric spectrum A we choose a stable equivalence pA : A −→ γA with target an injective Ω-spectrum,
which is possible by Theorem 4.7. We insist that if A is already an injective Ω-spectrum, then γA = A andpA is the identity. This is not really necessary, but will make some arguments slightly easier.
There is then a preferred way to make these choices into the object part of functor: given a morphism
f : A −→ B of symmetric spectra, we consider the diagram of morphisms of symmetric spectra (without
the dotted morphism)
(4.9)

A

pA

f


B

/ γA
γf

pB


/ γB

By definition of ‘stable equivalence’ the map [pA , γB] : [γA, γB] −→ [A, γB] is bijective. Thus there exists
a unique homotopy class of morphism γf : γA −→ γB which makes the entire square commute up tohomotopy.
Functoriality is a formal consequence of the uniqueness property. Since IdγA pA = pA , the uniquenessof the filler guarantees that γ(IdA ) = IdγA . If g : B −→ C is another morphism, then γ(g)γ(f )pA 'γ(g)pB f ' pC (gf ). By uniqueness we conclude that γ(g)γ(f ) = γ(gf ) as homotopy classes in [γA, γC].
Remark 4.10. The choice pA : A −→ γA of stable equivalence to an injective Ω-spectrum could in fact
be made functorially at the pointset level (and not just up to homotopy), see Proposition 4.21. However,
if we want this extra functoriality, we cannot simultaneously arrange things so that γA = A if A is alreadyan injective Ω-spectra. The pointset level functoriality of γ is irrelevant for the current discussion, and so
we continue without it.
The next theorem says that the functor γ : SpΣ −→ SHC is a localization of the category of symmetric
spectra at the class of stable equivalence. Since the ‘collection’ of all categories forms a 2-category (withrespect to functors as morphisms and natural transformations), one should not expect such localizations to
be unique up to isomorphism, but rather only unique up to equivalence. The following definition capturesthe ‘correct’ universal property of such a 2-categorical localization. We use the following notation: if C and
D are categories, then Hom(C, D) is the category whose objects are the functors from C to D and whosemorphisms are natural transformations [when is this a category, i.e., small Hom-sets?].
Definition 4.11. Let C be a category and W a class of morphisms in C. A localization of C at theclass W is a functor L : C −→ D with the following two properties:• The functor L takes all morphisms in W to isomorphisms in D.
• For every category E, precomposition with the functor L induces an equivalence of categories− ◦ L : Hom(D, E) −→ HomW (C, E)
where the target is the full subcategory of Hom(C, E) of functors which take all morphisms in W
to isomorphisms.
A direct consequence of the definition of ‘localization’ is the following. If L : C −→ D is a localizationof C and W then for every functor ε : D −→ E which takes W to isomorphisms, there exists a functor
ε : C −→ E, unique up to natural isomorphism, such that ε ◦ L is naturally isomorphic to ε. The existencepart is the statement that − ◦ L as in Definition 4.11 is essentially surjective on objects (i.e., functors); theuniqueness part is the statement that − ◦ L is fully faithful.
The universal property of a localization L : C −→ D makes localizations unique up to equivalence,
whenever they exist. Indeed, if L0 : C −→ D0 is another localization, then there exist functors F : D −→ D0
and G : D0 −→ D such that F ◦ L is naturally isomorphic to L0 and G ◦ L0 is naturally isomorphic to L.
Then G ◦ F ◦ L is naturally isomorphic to L and by the ‘fully faithful’ part of the universal property, anysuch natural isomorphism is of the form τ ◦ L for a unique natural isomorphism τ : G ◦ F ∼= IdD . Similarly,
F ◦ G is naturally isomorphic to the identity functor of D0 .

4. STABLE EQUIVALENCES

105

Theorem 4.12. The functor γ : SpΣ −→ SHC is a localization of the category of symmetric spectra at
the class of stable equivalences. In particular, for every functor ε : SpΣ −→ C which takes stable equivalences
to isomorphisms, then there exists a functor ε : SHC −→ C, unique up to preferred natural isomorphism,
such that ε ◦ γ is naturally isomorphic to ε.
Proof. We start by showing that γ takes stable equivalences to isomorphisms. By definition of γf
we have the homotopy commutative diagram (4.9). So if f : A −→ B is a stable equivalence, then γf is astable equivalence between injective Ω-spectra and therefore a homotopy equivalence. In other words, γf
is an isomorphism in SHC.
Next make some observations about functors ε : SpΣ −→ E which invert stable equivalences. For every
spectrum A, the projection π : ∆[1]+ ∧ A −→ A is a homotopy equivalence, hence a stable equivalence, so
ε(π) is an isomorphism. The two end inclusions i0 , i1 : A −→ ∆[1]+ ∧ A satisfy π ◦ i0 = IdA = π ◦ i1 , so wehave
ε(π) ◦ ε(i0 ) = Idε(A) = ε(π) ◦ ε(i1 ) .
Since ε(π) is an isomorphism, we deduce ε(i0 ) = ε(i1 ).
Suppose now that f, g : A −→ B are homotopic morphisms via some homotopy H : ∆[1]+ ∧ A −→ B.
Thenε(f ) = ε(H) ◦ ε(i0 ) = ε(H) ◦ ε(i1 ) = ε(g) .
In other words, every functor ε : SpΣ −→ E which takes stable equivalences to isomorphisms also takes
homotopic maps to the same morphisms.
Now we show that for every functor ε : SpΣ −→ E which takes stable equivalences to isomorphisms
there is a functor ε : SHC −→ E such that εγ is naturally isomorphic to ε. This proves that the functor− ◦ γ : Hom(SHC, E) −→ Homst. equi. (SpΣ , E) is dense (essentially surjective on objects). We simply define
ε : SHC −→ E on objects by ε(A) = ε(A) and on morphisms via representatives by ε[f : A −→ B] = ε(f ).
This will automatically be a functor. If we apply the functor ε to the stable equivalence pA : A −→ γA we
get a natural isomorphism in E
ε(pA ) : ε(A) −→ ε(γA) = (εγ)(A) .
It remains to show that precomposition with γ is fully faithful. So we consider two functors F, G :SHC −→ E and have to show that− ◦ γ : Nat(F, G) −→ Nat(F γ, Gγ)
is bijective. We define the inverse map K : Nat(F γ, Gγ) −→ Nat(F, G) as follows. Given a naturaltransformation τ : F γ −→ Gγ of functors SpΣ −→ E we define the natural transformation K(τ ) : F −→ Gof functors SHC −→ E as the restriction of τ to injective Ω-spectra. This makes sense because we had
insisted earlier that γX = X and pX = Id whenever X is an injective Ω-spectrum.
We have K(τ ) ◦ γ = τ as natural transformations because γ(γX) = γX and pγX = Id (again because γ
is the identity on injective Ω-spectra). We also have K(ϕ ◦ γ) = ϕ for a natural transformation ϕ : F −→ G,again because γX = X for every injective Ω-spectrum. So K is indeed inverse to precomposition with γ,
which finishes the proof.
The next proposition makes precise in which way the suspension of a symmetric spectrum ‘is’ the shift
in the stable homotopy category and how homotopy cofibre and homotopy fibre sequences give rise todistinguished triangles in SHC.
For any symmetric spectrum A we have a diagram of morphisms of symmetric spectra
S1 ∧ ApS 1 ∧AS 1 ∧pA
S 1 ∧ γA

/ γ(S 1 ∧ A)
ΦA

λγA


/ sh(γA)

106

II. THE STABLE HOMOTOPY CATEGORY

in which all solid arrows are stable equivalences. Since sh(γA) is an injective Ω-spectrum, there exists unique
homotopy class of morphisms ΦA : γ(S 1 ∧ A) −→ sh(γA) which makes the entire square commute up tohomotopy. The morphism ΦA is a stable equivalence between injective Ω-spectra, thus an isomorphism(4.13)
ΦA : γ(S 1 ∧ A) ∼= (γA)[1]
in the stable homotopy category.
Dually, the functor γ commutes with taking functions out of a finite simplicial set, so in particular withloops. Indeed, if K is a finite pointed simplicial set and A a symmetric spectrum which is levelwise a Kancomplex, then the morphism (pA )K : AK −→ (γA)K is a stable equivalence by part (v) of Proposition 4.5.
Since (γA)K is an injective Ω-spectrum, there is a unique homotopy class of morphisms ΨA,K : γ(AK ) −→(γA)K such that ΨA,K ◦pAK is homotopic to (pA )K . This morphism is a stable equivalence between injectiveΩ-spectra, thus a homotopy equivalence. An important special case is K = S 1 , which yields a preferredhomotopy class of homotopy equivalence ΨA,S 1 : γ(ΩA) −→ Ω(γA).
The isomorphisms ΦA : γ(S 1 ∧ A) −→ (γA)[1] and ΨA,K : γ(AK ) −→ (γA)K are natural in SHC as
functors of A, which is a consequence of the uniqueness properties.
Proposition 4.14. (i) Let f : A −→ B be a morphism of symmetric spectra. Then the functor
γ : SpΣ −→ SHC takes the sequence
f

p

i

A−
→B−
→ C(f ) −→ S1 ∧ A
to a distinguished triangle in the stable homotopy category after identifying γ(S 1 ∧ A) with (γA)[1] via φA .
(ii) Let f : A −→ B be a morphism of symmetric spectra which are levelwise Kan complexes. Then thefunctor γ : SpΣ −→ SHC takes the sequence
p

i

f

ΩB −
→ F (f ) −→A−→B
to a distinguished triangle in the stable homotopy category after making the identifications
λ∗γB

ΨB,S 1 [1](γ(ΩB))[1] −−−−−→ Ω(γB)[1] ←−−− γB .
(iii) The functor γ commutes with arbitrary coproducts and finite products. Thus in particular the stablehomotopy category has arbitrary coproducts.
Proof. (iii) Given a family {Ai }i∈I of symmetric spectra, then theWwedge of the stable equivalencespi : Ai −→ γ(Ai ) is a stable equivalence by Proposition
W 4.5 (i). SinceWγ( i Ai ) is an injective Ω-spectrum,W
there is a unique homotopy class of morphism g : i γ(Ai ) −→ γ( i Ai ) whose restriction with i pi ishomotopic to the stable equivalence pW Ai . The morphism g is then also a stable equivalence, and so forevery injective Ω-spectrum X the induced map on homotopy classes__[g, X] : [γ( Ai ), X] −→ [ γ(Ai ), X]ii

Q

is bijective.
The target is isomorphic to the product i [γ(Ai ), X], which shows that the injective Ω-spectrumWγ( i Ai ) has the universal property of a coproduct of the objects γ(Ai ).
The proof that γ preserves finite products is similar but slightly easier because products in the stablehomotopy category are given by pointset level products. We consider the case of two factors. By the same
reasoning as before there is a unique homotopy class of morphism h : γA × γB −→ γ(A × B) satisfyingh(pA × pB ) = pA×B , and h is a stable equivalence. But now both sides are injective Ω-spectra, so the stableequivalence is even a homotopy equivalence, i.e., an isomorphism in SHC.

4.2. The homotopy groups of γA. We now have a way of associating to every symmetric spectrum
an object of the stable homotopy category, via the functor γ : SpΣ −→ SHC. However, the functor depends
on an abstract construction which produces a stable equivalence to an injective Ω-spectrum. This does notmake it transparent what ‘happens’ to a symmetric spectrum during this passage, and it not clear how
basic invariants like stable homotopy groups change in this process.

4. STABLE EQUIVALENCES

107

By the ‘true’ homotopy groups of a symmetric spectrum A we mean the homotopy groups of thespectrum γA. One can think of these true homotopy groups as ‘right derived functors’ of the homotopy
groups, since they are the homotopy groups of a fibrant replacement (in any of the stable model structures
to be introduced in Section III.2). If A is semistable, then the stable equivalence p : A −→ γA is a π∗ isomorphism and the true homotopy groups are naturally isomorphic to the classical homotopy groups π∗ A.
In fact, semistable symmetric spectra are characterized by this property.
For spectra which are not semistable it would thus be interesting to describe the true homotopy groups
in terms of the homotopy groups of A, which are often more readily computable from an explicit presentationof the symmetric spectrum. The bad news is that the true homotopy groups are not a functor of the classicalhomotopy groups, not even if one takes the M-action into account. But the next best thing is true: there
is a natural spectral sequence whose E 2 -term depends on π∗ A as a graded M-module and which converges
to the true homotopy groups of A. We shall now construct this spectral sequence and discuss it in someexamples.
In the next theorem and in what follows we use the notation − ⊗M − as short hand for the tensor
product over the monoid ring Z[M].
Theorem 4.15. There is a strongly convergent half-plane spectral sequence2
Ep,q= TorpZ[M] (Z, πq A) =⇒ πp+q (γA) .

The spectral sequence is natural in the symmetric spectrum A with dr -differential of bidegree (−r, r − 1).
The edge homomorphism
2
Z ⊗M πq A = E0,q−→ πq (γA)
is induced by the stable equivalence p : A −→ γA.
We will see below that the spectral sequence of Theorem 4.15 collapses in many cases, for example forsemistable symmetric spectra and for free symmetric spectra (see Example 4.17), and it always collapses
rationally (see Example 4.19). The spectral sequence typically does not collapse for semifree symmetric
spectra, see Example 4.20.
We need a preparatory Lemma.
Lemma 4.16. For every symmetric spectrum A there is a morphism of symmetric spectra P −→ A withthe following properties:(i) the induced map π∗ P −→ π∗ A is surjective;(Z, π∗ P ) are trivial;(ii) for all p ≥ 1 the groups TorZ[M]p(iii) the map Z ⊗M (π∗ P ) −→ π∗ (γP ) induced by the stable equivalence pP : P −→ γP is bijective.
Proof. For each k ∈ Z we choose a set of generators of πk A as an M-module, represent each generator
by a pointed map S k+n −→ An for large enough n and consider the adjoint morphism Fn S k+n −→ A. We
take P as the wedge of these spectra Fn S k+n for varying k and varying generators, with the induced map toA, which is then surjective on homotopy groups. The Tor groups of this wedge vanish by Lemma 4.26 (ii),sthe isomorphism π∗ (Fn S k+n ) ∼(see (4.33) of Chapter I) and because homotopy takes wedges= Pn ⊗ π∗−k
to sums.
To see that property (iii) holds we can similarly restrict to a single wedge summand of the form Fn S k+n .
For k ≥ 0 the morphism λ : Fn S k+n −→ F0 S k = Σ∞ S k is a stable equivalence (compare Example 4.2) withtarget a semistable symmetric spectrum. Since the target is semistable, its classical homotopy groups are thetrue homotopy groups and we may show that λ induces an isomorphism Z ⊗M (π∗ Fn S k+n ) −→ π∗ (Σ∞ S k )By (4.33) of Chapter I, the effect of the morphism λ on homotopy groups is isomorphic to the morphismof M-modules
Pn ⊗ π∗s −→ P0 ⊗ π∗s ∼= π∗sinduced by the surjection π : Pn −→ P0 adjoint to the preferred generator of P0 ∼= Z. Since Z ⊗M π :Z ⊗M Pn −→ Z ⊗M P0 is an isomorphism, this proves property (iii)108

II. THE STABLE HOMOTOPY CATEGORY

Proof of Theorem 4.15. We inductively define symmetric spectra An and Pn starting with A0 = A.
In each step we choose a level fibration fn : Pn −→ An with the properties of Lemma 4.16, define An+1 as
the fibre of fn , denote by in : An+1 −→ Pn the inclusion, and iterate the construction. Then the homotopy
groups of the sequence of symmetric spectrain fn+1

f0· · · −→ Pn+1 −−−−→ Pn −→ · · · −→ P0 −→ A0 = A
give a resolution of π∗ A by tame M-modules which are Z ⊗M − acyclic. Since the strict fibre An+1 is level
equivalent to the homotopy fibre F (fn ) , Proposition 4.14 (ii) provides a distinguished triangle in the stablehomotopy categoryγ(in )

γ(fn )γ(An+1 ) −−−→ γ(Pn ) −−−→ γ(An ) −→ γ(An+1 )[1]
which gives rise to a long exact sequence of homotopy groups. These homotopy groups thus assemble into
an exact couple with
1
1
Ep,q
= πq (γ(Pp )) and Dp,q
= πq (γ(Ap ))and morphisms
1
1
j : Dp+1,q−→ Ep,q

k :
i :

1
Ep,q
1
Dp,q

induced by γ(ip ) : γ(Ap+1 ) −→ γ(Pp ),1−→ Dp,qinduced by γ(fp ) : γ(Pp ) −→ γ(Ap ),1−→ Dp+1,q−1

given by the boundary map πq (γ(Ap )) −→ πq−1 (γ(Ap+1 )) of the distinguished triangle.
By property (iii) of Lemma 4.16 we have
E 1 = πq (γ(Pp )) ∼= Z ⊗M (πq (Pp )) ;p,q1
1−→ Ep−1,qbecomes the map obtained by applying
under this isomorphism, the differential d1 = jk : Ep,qZ ⊗M − to the above resolution of π∗ A. Since the homotopy groups of the spectra Pp are acyclic for the(Z, πq A).
functor Z ⊗M −, the E 2 -term calculates the Tor groups TorZ[M]p
It remains to discuss convergence of the spectral sequence. The pth filtration subgroup F p of theabutment π∗ (γA) is the kernel of the map
11
ip : π∗ (γA) = D0,∗−→ Dp,∗−p= π∗−p (γ(Ap )) .

To prove that the spectral S
sequence converges to the homotopy groups of γA we show that the filtration is
exhaustive, i.e., πq (γA) = p Fqp .
By construction, the connecting maps Ak −→ ΣAk+1 induce the trivial map on homotopy groups, so
the mapping telescope of the sequence
A = A0 −→ ΣA1 −→ Σ2 A2 −→ · · ·has trivial homotopy groups and is thus stably contractible. A mapping telescope of stable equivalences isa stable equivalence, so the mapping telescope of the sequence
γ(A0 ) −→ γ(A1 )[1] −→ γ(A2 )[2] −→ · · ·of stably fibrant replacements is also stably contractible. But this is a mapping telescope of semistable
spectra, thus itself semistable. Thus the homotopy groups of the mapping telescope of the sequence γ(Ap )[p],which are isomorphic to the colimit of homotopy groups, are trivial. Since each instance of the map
1
1
i : Dp,q−→ Dp+1,q−1
is induced by a connecting homomorphism γ(Ak ) −→ γ(Ak+1 )[1] this shows that thekernels of the maps ip exhaust all of π∗ (γA). The spectral sequence is concentrated in a half-plane and has
exiting differentials in the sense of [5, II.6], so it is strongly convergent.

Example 4.17 (Semistable and free symmetric spectra). When X is a semistable symmetric spectrum
or a free symmetric spectrum, then the higher Tor groups for the homotopy of X vanish by part (ii) of
2Lemma I.4.26. Thus in the spectral sequence of Theorem 4.15 we have Ep,q
= 0 for p 6= 0, and so the edge
homomorphism
Z ⊗M (π∗ X) −→ π∗ (γX)

4. STABLE EQUIVALENCES

109

is an isomorphism. For the free symmetric spectrum generated by a pointed space (or simplicial set) K inlevel m, the map K ∧ λ : K ∧ Fn S n −→ K ∧ S ∼= Σ∞ K is a stable equivalence, hence so is its n-fold loop
and thus the composite
Fn K −→ Ωn (K ∧ Fn S n ) −→ Ωn (Σ∞ K)
since the first map is a π∗ -isomorphism. So the free symmetric spectrum Fn K represents the same stablehomotopy type as γ(Σ∞ K)[−n].
Example 4.18 (Eilenberg-Mac Lane spectra). In Example I.4.27 we associate an Eilenberg-Mac Lane
spectrum HW to every tame M-module W . The homotopy groups of HW are concentrated in dimension 0,
where we get the module W back. So the spectral sequence of Theorem 4.15 for HW collapses onto theaxis q = 0 to isomorphisms
πp (γ(HW )) ∼= TorpZ[M] (Z, W ) .
In particular, the true homotopy groups of HW need not be concentrated in dimension 0. One can showthat HW is in fact stably equivalent to the product of the Eilenberg-Mac Lane spectra associated to thegroups TorZ[M](Z, W ), shifted up p dimensions.
pHere is an example which shows that for non-trivial W the Eilenberg-Mac Lane spectrum HW can bestably contractible: we let W be the kernel of a surjection Pn −→ Z. Lemma I.4.26 and the long exact
sequence of Tor groups show that the groups TorZ[M](Z, W ) vanish for all p ≥ 0. Thus the homotopy groupspof γ(HW ) are trivial, i.e., HW is stably contractible.
Example 4.19 (Rational collapse). We claim that for every tame M-module W and all p ≥ 1, we have
(Q, W ) = 0. So the spectral sequence of Theorem 4.15 always collapses rationally and the edge
TorZ[M]phomomorphism is a rational isomorphism
Q ⊗M (π∗ X) −→ Q ⊗ π∗ (γX) .
The rational vanishing of higher Tor groups is special for tame M-modules.
To prove the claim we consider a monomorphism i : V −→ W of tame M-modules and show that thekernel of the map Z ⊗M i : Z ⊗M V −→ Z ⊗M W is a torsion group. The inclusions W (n) −→ W induce
an isomorphism∼=

colimn Z ⊗Σn W (n) −−→ Z ⊗M W .
For every n ≥ 0, the kernel of Z ⊗Σn i(n) : Z ⊗Σn V (n) −→ Z ⊗Σn W (n) is annihilated by the order of thegroup Σn . Since the kernel of Z ⊗M i is the colimit of the kernels of the maps Z ⊗Σn i(n) , it is torsion.
Thus the functor Q ⊗M − is exact on short exact sequences of tame M-modules and the higher Tor groups
vanish as claimed.
Example 4.20 (Semifree symmetric spectra). For semifree symmetric spectra (see Example 2.13) the
spectral sequence of Theorem 4.15 typically does not degenerate. As an example we consider the semifree
symmetric spectrum G2 S 2 , where S 2 is a Σ2 -space by coordinate permutations.
We first identify the stable equivalence type of G2 S 2 . The spectrum G2 S 2 is isomorphic to the quotient
spectrum of Σ2 permuting the smash factors of (F1 S 1 )∧2 . Since the Σ2 -action on (F1 S 1 )∧2 is free [not yet
shown], the map
1 (2)
EΣ+
−→ (F1 S 1 )∧2 /Σ2 = G2 S 2
2 ∧Σ2 (F1 S )which collapses EΣ2 to a point is a level equivalence. On the other hand, the stable equivalence λ(2) :(F1 S 1 )(2) −→ S is Σ2 -equivariant, so it induces a stable equivalence
+
1 (2)
∞
EΣ+
−→ EΣ+
2 ∧Σ2 (F1 S )2 ∧Σ2 S = Σ BΣ2

on homotopy orbit spectra. Altogether we conclude that G2 S 2 is stably equivalent to Σ∞ BΣ+2.
The spectral sequence of Theorem 4.15 for G2 S 2 has as E 2 -term the Tor groups of π∗ (G2 S 2 ). Acs
cording to (4.32) of Chapter I these homotopy groups are isomorphic to P2 ⊗Σ2 (π∗+2S 2 )(sgn). The sign2
representation cancels the sign action induced by the coordinate flip of S , so we have an isomorphism

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II. THE STABLE HOMOTOPY CATEGORY

of M-modules πq (G2 S 2 ) ∼= P2 ⊗Σ2 πqs S 0 , this time with trivial action on the stable homotopy groups ofspheres. Using part (iii) of Lemma I.4.26, the spectral sequence of Theorem 4.15 for G2 S 2 takes the form
2
s
∼
Ep,q
(BΣ+
= Gp (Σ2 ; πqs S 0 ) =⇒ πp+q2) .

This spectral sequence has non-trivial differentials and it seems likely that it coincides with the AtiyahHirzebruch spectral sequence for the stable homotopy of the space BΣ+
2 . [In general Gm L is stably equivalent
to the semistable symmetric spectrum L ∧Σn Ωn (S|S|)]
4.3. Proof of Theorem 4.7. We have to construct, for every symmetric spectrum A, a stable equivalence p : A −→ γA such that the target γA is an injective Ω-spectrum. We divide this construction into
two steps.
Proposition 4.21. (i) There exists a endofunctor (−)inj on the category of symmetric spectra and anatural level equivalence A −→ Ainj such that Ainj is an injective spectrum.
(ii) There exists a endofunctor Q on the category of symmetric spectra and a natural stable equivalenceA −→ QA such that QA is an Ω-spectrum.
We get Theorem 4.7 by setting γA = (QA)inj and as morphism pA : A −→ γA the composite of
the stable equivalence of part (ii) of Proposition 4.21 with the level equivalence, for the spectrum QA, ofpart (i).
In both cases we use the small object argument (see Theorem 1.6 of Appendix) with respect to a certainclass of morphisms of symmetric spectra. As usual with small object arguments we have to limit the sizeof objects. We call a symmetric spectrum of simplicial sets countable if the cardinality of the disjoint unionof all simplices in all levels is countable.
Lemma 4.22. A morphism of symmetric spectra is an injective fibration if and only if it has the right
lifting property with respect to all injective level equivalences between countable symmetric spectra.
Proof. Suppose that f : X −→ Y has the right lifting property with respect to all injective level
equivalences with countable target (and hence source). Consider a lifting problem
A
i


B

/X

/Y

f

in which i : A −→ B is an injective level equivalence, with no restriction on the cardinality of B.
We denote by P the set of ‘partial lifts’: an element of P is a pair (U, h) consisting of a symmetricsubspectrum U of B which contains the image of A and such that the inclusion U −→ B (and hence themorphism A −→ U ) is a level equivalence and a morphism h : U −→ X making the following diagram
commute
/
A
nn7 X
n
n
hnnn
f
i
nnn
n
n
 nn

/Y
U incl. / B
The set P can be partially ordered by declaring (U, h) ≤ (U 0 , h0 ) if U is contained in U 0 and h0 extends h.
Then every chain in P has an upper bound, namely the union of all the subspectra U with the common
extension of the morphisms h. So by Zorn’s lemma, the set P has a maximal element (V, k). We show thatV = B, so k provides the required lifting showing that f is an injective fibration.
We argue by contradiction and suppose that V is strictly smaller than B. Then we can find a countable
subspectrum W of B which is not contained in V and such that the morphism V ∩ W −→ W is a level4. STABLE EQUIVALENCES

111

equivalence [justify; see Lemmas 5.1.6 and 5.1.7 of [25]]. Since W is countable, f has the right lifting
property with respect to the inclusion V ∩ W −→ W . We have a pushout square
/V
V ∩W

W


/ V ∪Wso f also has the right lifting property with respect to the inclusion V −→ V ∪ W . But that means thatthe morphism k : V −→ X can be extended to V ∪ W , which contradicts the assumption that (V, k) is a
maximal element in the set P extensions.

Proof of part (i) of Proposition 4.21. Let I be a set containing one morphism of each isomorphism class of injective level equivalences i : A −→ B for which B is a countable symmetric spectrum.
The class of injective level equivalences of symmetric spectra is closed under wedges, cobase change andcomposition, possibly transfinite. So every I-cell complex is an injective level equivalence.
We apply the small object argument (see Theorem 1.6) to the unique morphism from a given symmetric
spectrum A to the trivial spectrum. We obtain a functor A 7→ Ainj together with a natural transformationj : A −→ Ainj which is an I-cell complex, hence an injective level equivalence. Moreover, the morphismfrom Ainj to the trivial spectrum is I-injective. Lemma 4.22 shows that Ainj is an injective spectrum. Proof of part (ii) of Proposition 4.21. We use the small object argument with respect to a certain set J of injective stable equivalences. First we let λn : Fn+1 S 1 −→ Fn S 0 denote the morphism which1
is adjoint to the wedge summand inclusion S 1 −→ (Fn S 0 )n+1 = Σ+n+1 ∧ S indexed by the identity element.
The morphism λn factors through the mapping cylinder as λn = rn cn where cn : Fn+1 S 1 −→ Z(λn ) isthe ‘front’ mapping cylinder inclusion and rn : Z(λn ) −→ Fn S 0 is the projection, which is a homotopy
equivalence. We then define K as the set of all pushout product maps
im ∧ cn : ∆[m]+ ∧ Fn+1 S 1 ∪∂∆[m]+ ∧Fn+1 S 1 ∂∆[m]+ ∧ Z(λn ) −→ ∆[m]+ ∧ Z(λn )for n, m ≥ 0, where im : ∂∆[m] −→ ∆[m] is the inclusion. We let F IΛ be the set of all morphisms
Fn Λk [m]+ −→ Fn ∆[m]+ induced by the horn inclusions for n, m ≥ 0 and 0 ≤ k ≤ m.
By adjointness, a symmetric spectrum X has the right lifting property with respect to the set F IΛ
if and only if for all n ≥ 0 the simplicial set Xn has the right lifting property for all inclusions of horns
into simplices, i.e., if Xn is a Kan simplicial set. By adjointness, a symmetric spectrum X has the right
lifting property with respect to the set K if and only if for all n ≥ 0 the map of simplicial sets map(cn , X) :map(Z(λn ), X) −→ map(Fn+1 S 1 , X) ∼= ΩXn+1 has the right lifting property for all inclusions of boundaries
into simplices, which is equivalent to map(cn , X) being an acyclic Kan fibration of simplicial set. Since themapping cylinder Z(λn ) is homotopy equivalent to Fn S 0 , the simplicial set map(Z(λn ), X) is homotopy
equivalent to map(Fn S 0 , X) ∼= Xn . So altogether the right lifting property with respect to the set K implies
that the map σ̃n : Xn −→ ΩXn+1 is a weak equivalence of simplicial sets.
So if X has the right lifting property with respect to the union J = K ∪ F IΛ then X is levelwise Kanand σ̃n : Xn −→ ΩXn+1 is a weak equivalence, so X is an Ω-spectrum.
Now we apply the small object argument (see Theorem 1.6) for the set J to the unique morphism froma given symmetric spectrum A to the trivial spectrum. We obtain a functor A 7→ QA together with anatural transformation j : A −→ QA which is an J-cell complex and such that QA is J-injective. By theabove, this means that QA is an Ω-spectrum. By Proposition 4.5 the class of injective stable equivalencesof symmetric spectra is closed under wedges, cobase change and composition, possibly transfinite. So every
J-cell complex such as j : A −→ QA is an injective stable equivalence, which finishes the proof.

In Proposition 1.8 we proved that every level equivalence between injective Ω-spectra is a homotopy
equivalence and that every π∗ -isomorphism between Ω-spectra is a level equivalence. Along similar lines wehaveLemma 4.23. Every stable equivalence between semistable symmetric spectra is a π∗ -isomorphism.

112

II. THE STABLE HOMOTOPY CATEGORY

Proof. Suppose f : A −→ B is a stable equivalence between semistable spectra. We first treat
the special case where A and B are Ω-spectra and show that then f is even a level equivalence. In thecommutative square/ Ainj
A
f inj

f



/ B injBthe horizontal morphisms are level equivalences with injective targets of Proposition 4.21 (i). Since A and B
are Ω-spectra, so are Ainj and B inj . Thus the spectra Ainj and B inj are injective Ω-spectra. The morphismf inj is a stable equivalence between injective Ω-spectra, hence a homotopy equivalence. In particular, allthree morphisms except possibly f are π∗ -isomorphisms, hence f is a π∗ -isomorphism.
Now we treat the general case where A and B are arbitrary semistable symmetric spectra. In thecommutative squareAλ∞A

R∞ (S|f |)fB/ R∞ (S|A|)λ∞B/ R∞ (S|B|)

the horizontal morphisms are π∗ -isomorphisms and the spectra R∞ A and R∞ B are Ω-spectra, by parts (iv)and (v) of Theorem I.4.44. Since π∗ -isomorphisms are stable equivalences and f is one, the morphism R∞ f
is a stable equivalence. By the first part R∞ f is a π∗ -isomorphism, and hence so is f .

5. Derived smash product
The main result of this section is that the pointset level smash product of symmetric spectrum (seeSection I.3) descends to a closed symmetric monoidal product on the stable homotopy category. Recall thatγ : SpΣ −→ SHC denotes the universal functor from symmetric spectra to the stable homotopy category
which inverts stable equivalences (see Theorem 4.12).
Theorem 5.1. The smash product of symmetric spectra has a total left derived functor. More precisely,
there exists a functor∧L : SHC × SHC −→ SHC
and a natural transformation
ψA,B : (γA) ∧L (γB) −→ γ(A ∧ B)
with the following universal property. If F : SHC × SHC −→ SHC is any functor and β : F (γA, γB) −→γ(A∧B) a natural transformation, then there exists a unique natural transformation κ : F (X, Y ) −→ X ∧L Y
such that ψ ◦ κ(γ × γ) = β.
We refer to the functor ∧L as the derived smash product . We postpone the proof of Theorem 5.1 to theend of this section. What makes it work is, roughly speaking, that every symmetric spectrum A admits a‘flat resolution’, i.e., a level equivalence A[ −→ A from a symmetric spectrum such that smashing with A[
preserves level and stable equivalences. The proof will also show that if A or B is flat, than the universal
natural transformation ψA,B : (γA) ∧L (γB) −→ γ(A ∧ B) is an isomorphism in the stable homotopy
category. In other words, as soon as one of the factors is flat, the pointset level smash product has the‘correct’ stable homotopy type.
Theorem 5.2. For every injective Ω-spectrum Z the functor Hom(−, Z) has a total right derived
functor. More precisely, there exists a functor
RHom(−, Z) : SHC op −→ SHC
and a natural transformation
φ : γ(Hom(A, Z)) −→ RHom(γA, Z)

5. DERIVED SMASH PRODUCT

113

of functors from (SpΣ )op to SHC with the following universal property. If G : SHC op −→ SHC is any
functor and α : γ(Hom(A, Z)) −→ G(γA) a natural transformation, then there exists a unique naturaltransformation λ : RHom(−, Z) −→ G such that λ(γ)φ = α.
We draw an important corollary from Theorem 5.1.
Theorem 5.3. The stable homotopy category is closed symmetric monoidal with respect to the derived
smash product. The unit object is γS, the stably equivalent replacement of the sphere spectrum.
[check: there is a unique way to choose natural coherence isomorphisms such that ψ : γA ∧L γB −→γ(A ∧ B) and the identity of γS make γ : SpΣ −→ SHC into a lax symmetric monoidal functor]
Proof. We have to construct coherence isomorphisms for the derived smash product; of course, these
don’t just come out of the blue, but they are ‘left derived’ from the coherence isomorphisms for the pointset
level smash product. We start with the associativity. We consider the functor∧L ◦ (∧L × Id) : SHC × SHC × SHC −→ SHC
together with the natural transformation
ψA,B ∧L Id

ψA∧B,C

γ(αA,B,C )(γA ∧L γB) ∧L γC −−−−−−→ γ(A ∧ B) ∧L γC −−−−−→ γ((A ∧ B) ∧ C) −−−−−−−→ γ(A ∧ (B ∧ C)) .
The universal property of the pair (∧L , ψ) implies that the above pair is a total left derived functor of∧ ◦ (Id ×∧) : (SpΣ )3 −→ SpΣ .
On the other hand there is the functor∧L ◦ (Id ×∧L ) : SHC × SHC × SHC −→ SHC
together with the natural transformation
Id ∧L ψB,C

ψA,B∧CγA ∧L (γB ∧L γC) −−−−−−−→ γA ∧L γ(B ∧ C) −−−−−→ γ(A ∧ (B ∧ C)) .
The universal property of the pair (∧L , ψ) similarly implies that this pair is another total left derived functorof ∧ ◦ (Id ×∧). By the uniqueness of universal objects there is a preferred natural isomorphismᾱX,Y,Z : (X ∧L Y ) ∧L Z −→ X ∧L (Y ∧L Z)
of functors from SHC 3 to SHC such that for all symmetric spectra A, B and C the diagram(γA ∧L γB) ∧L γC

ψA,B ∧L Id/ γ(A ∧ B) ∧L γC

ψA∧B,C

ᾱγA,γB,γC
γA ∧L (γB ∧L γC)/ γ((A ∧ B) ∧ C)
γ(αA,B,C )Id ∧ψB,C/ γA ∧L γ(B ∧ C)

ψA,B∧C/ γ(A ∧ (B ∧ C))

commutes. Similarly, the universal properties of total left derived functors make sure that the pentagon
axiom for the pointset level smash product of symmetric spectra is inherited by the derived smash product.
The construction of the symmetry isomorphism τ̄ : X ∧L Y −→ Y ∧L X is completely analogous, andthere is a unique such natural isomorphism such that for all symmetric spectra A and B theγA ∧L γB

ψA,B

τ̄γA,γB
γB ∧L γA

/ γ(A ∧ B)
γ(τA,B )ψB,A/ γ(B ∧ A)

commutes. [define unit isomorphism r̄X : X ∧L γ(S) ∼= X, coherence diagrams]



114

II. THE STABLE HOMOTOPY CATEGORY

Now that we constructed the derived smash product (modulo the proof of Theorem 5.1), we canconsider monoid objects in the stable homotopy category with respect to the derived smash product. For
us a homotopy ring spectrum or ring spectrum up to homotopy is an injective Ω-spectrum S together withmorphisms µ : S ∧L S −→ S and ι : γS −→ S in the stable homotopy category which are associative andunital in the sense that the following diagrams commute(S ∧L S) ∧L S

ᾱS,S,S/ S ∧L (S ∧L S)

Id ∧L µ/ S ∧L Sµµ∧L IdS ∧L Sµ/S

Id ∧ ι
ι∧ Id /
S ∧L SMS ∧L S o
S ∧L S
MMM
q
q
MMM
qq
MMM µ
qqr̄qSql̄Sq
MM&  xqqq
S
L

L

A homotopy ring spectrum (S, µ, ι) is homotopy commutative if the multiplication is unchanged when
composed with the symmetric isomorphism, i.e., if the relation µ ◦ τ̄S,S = µ holds in the stable homotopy
category.
The definition of the derived smash product was such that the universal functor γ : SpΣ −→ SHC islax symmetric monoidal (with respect to the universal transformation ψ : ∧L ◦ (γ × γ) −→ γ ◦ ∧). A formal
consequence is that γ takes symmetric ring spectra to homotopy ring spectra. Indeed, if (R, µ : R ∧ R −→R, ι : S −→ R) is a symmetric ring spectrum, then γR becomes a ring spectrum up to homotopy withrespect to the multiplication map
ψR,R

γ(µ)

(γR) ∧L (γR) −−−→ γ(R ∧ R) −−−→ γR
and the unit map γ(ι) : γS −→ γR.
The converse is far from being true. More precisely, given a ring spectrum up to homotopy S one
can ask if there is a symmetric ring spectrum R such that γR is isomorphic to S as a homotopy ring
spectrum. There is an infinite sequence of coherence obstructions for the associativity to get a positive
answer. [Illustrate the pentagon coherence condition?] The question of when a homotopy commutative
homotopy ring spectrum is represented by a commutative symmetric ring spectrum is even more subtle.
We hope to get back to this later, and discuss some of the obstruction theories available to tackle such‘rigidification’ questions.
A concrete example is the mod-p Moore spectrum SZ/p for a prime p ≥ 5. Indeed, SZ/p has a homotopy
associative and homotopy commutative multiplication in the stable homotopy category for p ≥ 5, but there
is no symmetric ring spectrum whose underlying spectrum is a mod-p Moore spectrum.
[derived smash product commutes with suspension and preserves distinguished triangles]
5.1. Flat symmetric spectra. To motivate the following definition, recall that a module over acommutative ring is called flat if tensoring with it preserves monomorphisms.
Definition 5.4. A symmetric spectrum A of simplicial sets is flat if the functor A ∧ − preservesmonomorphisms.
There is a corresponding notion of flatness for symmetric spectra of topological spaces [reference], but
simply requiring that A ∧ − preserves monomorphisms is not the right condition in the topological context.
We will show in Chapter III that flat symmetric spectra are the cofibrant objects in suitable ‘flat model
structures’.
Example 5.5. For every m ≥ 0 and every pointed Σm -simplicial set L the semifree symmetric spectrum
Gm L is flat and moreover, the functor Gm L ∧ − preserves level equivalences. As special cases, this applies
to free symmetric spectra Fn K and suspension spectra Σ∞ K.
Indeed, if X is another symmetric spectrum then Gm L ∧ X is isomorphic to the twisted smash productL .m X (see Proposition I.3.5) and so it is trivial in levels below m and we have a natural isomorphism(5.6)(Gm L ∧ X)m+n ∼= Σ+
m+n ∧Σm ×Σn L ∧ Xn ,

5. DERIVED SMASH PRODUCT115

As a pointed simplicial set, the right hand side is a wedge of m+n
copies of L ∧ Xn . So if f : X −→ Ymis a monomorphism respectively a level equivalence of symmetric spectra of simplicial sets, then so is
Id ∧f : Gm L ∧ X −→ Gm L ∧ Y .
Some other properties of flat spectra are fairly straightforward from the definition:
Proposition 5.7. (i) A wedge of flat symmetric spectra is flat.
(ii) A filtered colimit of flat symmetric spectra is flat.
(iii) The smash product of two flat symmetric spectra is flat.
Proof. Properties (i) and (ii) follow from the two facts that the smash product commutes with colimits
and that a filtered colimit or a wedge of monomorphisms is a monomorphism.
(iii) Let A and be B flat symmetric spectra of simplicial sets. If f : X −→ Y is a monomorphism, thenId ∧f : B ∧ X −→ B ∧ Y is a monomorphism since B is flat; then Id ∧ Id ∧f : A ∧ B ∧ X −→ A ∧ B ∧ Y is amonomorphism since A is flat (where we have implicitly used the associativity isomorphisms). Thus A ∧ B
is flat.
An example of a non-flat symmetric spectrum is S̄, the subspectrum of the sphere spectrum given by(∗
for n = 0
(5.8)
S̄n =
n
S
for n ≥ 1.
So the difference between S̄ and S is only one missing vertex in level 0, but that missing vertex makes ahuge difference for the flatness property. Indeed, since S̄ is trivial in level 0 we have+2(S̄ ∧ S̄)2 = Σ+
2 ∧ S̄1 ∧ S̄1 = Σ2 ∧ S

while (S̄ ∧ S)2 ∼= S̄2 = S 2 . So S̄ ∧ − does not take the inclusion S̄ −→ S to a monomorphism, hence S̄ is not
flat.
Now we develop a convenient criterion for recognizing flat symmetric spectra which involves latching
spaces.
Definition 5.9. The nth latching space Ln A of a symmetric spectrum A is the Σn -simplicial set(A ∧ S̄)n where S̄ is the subspectrum of the sphere spectrum defined in (5.8). The nth level of the morphism
Id ∧i : A ∧ S̄ −→ A ∧ S ∼= A, for i : S̄ −→ S the inclusion, provides a natural map of pointed Σn -spaces
νn : Ln A −→ An .
Since the latching spaces play important roles in what follows, we make their definition more explicit.
Specializing the construction of the smash product (compare Section I.3) to A ∧ S̄ displays Ln A as thecoequalizer, in the category of pointed Σn -simplicial sets, of two maps
n−2
_

1
n−p−1
Σ+
−→
n ∧Σp ×Σ1 ×Σn−p−1 Ap ∧ S ∧ S

p=0

n−1
_

n−p
Σ+
.
n ∧Σp ×Σn−p Ap ∧ S

p=0(in the target we have discarded the wedge summand which would contribute An ∧ S̄0 , since that is just a
point, and similarly in the source). One of the maps takes the wedge summand indexed by p to the wedge
summand indexed by p + 1 using the map
σp ∧ Id : Ap ∧ S 1 ∧ S n−p−1 −→ Ap+1 ∧ S n−p−1
and inducing up. The other map takes the wedge summand indexed by p to the wedge summand indexed
by p using the canonical isomorphism∼=

Ap ∧ S 1 ∧ S n−p−1 −→ Ap ∧ S n−p
and inducing up.
For example, L0 A is a one-point simplicial set, L1 A = A0 ∧ S 1 and L2 A is the pushout of the diagram
act on S 2

Id ∧σ ∧Id

2
1
A0 ∧ S 2 ←−−−−−−− Σ+−−−−0−−→ Σ+
2 ∧ A0 ∧ S
2 ∧ A1 ∧ S .

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II. THE STABLE HOMOTOPY CATEGORY

1
Thus L2 A is the quotient of Σ+
2 ∧ A1 ∧ S by the equivalence relation generated byγ ∧ σ0 (a ∧ x) ∧ y ∼ (γ(1, 2)) ∧ σ0 (a ∧ y) ∧ x
1for a ∈ A0 and x, y ∈ S 1 . In general, Ln A is a quotient of Σ+n ∧Σn−1 An−1 ∧ S by a suitable equivalence
relation.

Proposition 5.10. A symmetric spectrum of simplicial sets A is flat if and only if for every n ≥ 1 themap of Σn -simplicial sets νn : Ln A −→ An is injective.
The proof of Proposition 5.10 uses a certain natural ‘filtration’ for symmetric spectra which shows
how a general symmetric spectrum is built from semifree ones. We have put the term ‘filtration’ in quotes
since in general this only is a natural sequence of symmetric spectra and morphisms with colimit the given
spectrum. In the special case of flat symmetric spectra, the morphisms are injective.
For any integer k we denote by S[k] the sphere spectrum truncated below level k, i.e., the symmetricsubspectrum of S with level(∗for n < k[k](S )n =nS
for n ≥ k.
For example we have S[1] = S̄ and S[k] = S for all k ≤ 0. Note that two consecutive truncated sphere spectra
are related by the equation S 1 ∧ S[k] = sh(S[1+k] ). Given a symmetric spectrum A we define a sequence
F m A of symmetric spectra by(F m A)n = (S[n−m] ∧ A)n
as a Σn -space. The structure map (F m A)n ∧ S 1 −→ (F m A)n+1 is given by the composite
twist(S[n−m] ∧ A)n ∧ S 1 −−−→ S 1 ∧ (S[n−m] ∧ A)n = (S 1 ∧ S[n−m] ∧ A)n = (sh(S[1+n−m] ) ∧ A)nξS[1+n−m] ,Aχ1,n−−−−−−−−→ (sh(S[1+n−m] ∧ A))n = (S[1+n−m] ∧ A)1+n −−−→ (S[n+1−m] ∧ A)n+1 .
For example we haveF 0 A = Σ∞ A ,(F n−1 A)n = Ln A and (F m A)n = An for m ≥ n.

In general the spaces of (F m A)n for m < n are a kind of ‘generalized latching objects’. The inclusions
S[k+1] −→ S[k] induce morphisms jm : F m−1 A −→ F m A and in a fixed level n, the system stabilizes to(A ∧ S)n which is isomorphic to An . So the colimit of the sequence of symmetric spectra F m A over themorphisms jm is isomorphic to A.
Proposition 5.11. For every symmetric spectrum A and every m ≥ 0 the commutative square(5.12)

Gm Lm A
F m−1 AGm νm

/ Gm Am/ F mAjm

is a pushout square, where the vertical morphisms are adjoint to the identity Lm A = (F m−1 A)m respectively
the isomorphism (F m A)m ∼= Am .
Proof. The commutative square of symmetric spectra
S k .k S̄

/ S k .k S = Gk S kS[k+1]

/ S[k]

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117

is a pushout, where .k is the twisted tensor product, see Example I.2.20. So after smashing with A weobtain a pushout square/ S k .k A
S k .k (S̄ ∧ A)
/ S[k] ∧ A
S[k+1] ∧ A
Evaluating at level k + m we obtain a pushout square of Σk+n -spaces (or simplicial sets)
/ Σ+

k
Σ+
k+m ∧Σk ×Σm S ∧ Lm A

k+m ∧Σk ×Σm S

(F m−1 A)k+m

k

∧ Am


/ (F m A)k+m

which is precisely the part of the square (5.12) in level k + m.

Proof of Proposition 5.10. One direction is essentially clear by definition: the inclusion i : S̄ −→ S
is certainly a monomorphism, so if A is flat, then Id ∧i : A ∧ S̄ −→ A ∧ S is a monomorphism, hence eachlevel νn : Ln A −→ An is injective.
[Alternative: define ‘flat cofibrations’; show they are preserves under cobase change, composition; show
inductively that F m−1 A −→ F m A is a flat cofibration] Now suppose conversely that the maps Ln A −→ An
are injective and let f : X −→ Y be a monomorphism of symmetric spectra of simplicial sets. We use thefiltration of A by the spectra F m A and show inductively that the map Id ∧f : F m A ∧ X −→ F m A ∧ Y isa monomorphism. Since A is the colimit of the F m A, the smash product commutes with colimits and thesequential colimit of monomorphisms is a monomorphism, this proves that Id ∧f : A ∧ X −→ A ∧ Y is amonomorphism, i.e., A is flat.
For the inductive step we use the pushout square of Proposition 5.11. Since smashing is a left adjoint
the spectrum F m A ∧ X is (isomorphic to) the pushout of the upper row in the commutative diagram(5.13)

F m−1 A ∧ X o

Gm Lm A ∧ X

Id ∧f

Id ∧f


F m−1 A ∧ Y o

Gm νm ∧Id

Gm Lm A ∧ Y/ Gm Am ∧ XGm νm ∧Id

Id ∧f

/ Gm Am ∧ Y

and F m A∧Y is the pushout of the lower row. The left vertical morphism is injective by induction hypothesis,
and we claim that in addition the pushout product map of the right square in (5.13)
Gm νm ∧ f : Gm Lm A ∧ Y ∪Gm Lm A∧X Gm Am ∧ X −→ Gm Am ∧ Y
is injective. We can verify this claim levelwise and we can use the description of the levels of a smashproduct Gm L ∧ X, for L any Σm -simplicial set,
(Gm L ∧ X)m+n ∼= Σ+ ∧Σ ×Σ L ∧ Xn
m+n

m

n

of Proposition I.3.5. The claim in level m + n then follows from the fact that the map of pointed simplicialsets
νm ∧ fn : Lm A ∧ Yn ∪Lm A∧Xn Am ∧ Xn −→ Am ∧ Yn
is injective since both νm : Lm A −→ Am and fn : Xn −→ Yn are injective.
It is a general fact about simplicial sets (actually about set) that given a commutative diagram
Ao

B

/C


A0 o


B0


/ C0

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II. THE STABLE HOMOTOPY CATEGORY

in which the map from A −→ A0 and the map B 0 ∪B C −→ C 0 are injective, then the induced map on
pushouts A ∪B C −→ A0 ∪B 0 C 0 is injective.
If we apply this levelwise to the diagram (5.13) we can conclude that the induced morphism on pushouts
F m A ∧ X −→ F m A ∧ Y is also a monomorphism, which finishes the proof.
Proposition 5.14. Let A be a flat symmetric spectrum.
(i) Smashing with A preserves level equivalences, π∗ -isomorphisms and stable equivalences.
(ii) For every injective spectrum X the symmetric spectrum Hom(A, X) is injective.
Proof. We first prove the statement in (i) referring to level equivalences. For a level equivalence
f : X −→ Y we show by induction that the morphisms f ∧ Id : X ∧ F m A −→ Y ∧ F m A are level equivalence
for all m ≥ 0. For the inductive step we use the pushout square of Proposition 5.11. Since smashing is aleft adjoint the spectrum F m A ∧ X is (isomorphic to) the pushout of the upper row in the commutative
diagramνm .m Id
/ Am .m X
Lm A .m X
F m−1 A ∧ X o
Id ∧fF m−1 A ∧ Y o

Id .m fLm A .m Y

Id .m f

νm .m Id
/ Am .m Y

and F m A ∧ Y is the pushout of the lower row. The left vertical morphism is a level equivalence by
induction hypothesis and the other two vertical morphism are level equivalences by Example 5.5. Because(L .m X)m+n = Σ+m+n ∧Σm ×Σn L ∧ Xn the functor − .m X takes injective maps of Σm -simplicial sets tomonomorphisms of symmetric spectra, and similarly for Y . Since A is flat the map νm : Lm A −→ Am is
injective, and so the two horizontal morphisms labeled νm .m Id are monomorphisms. The gluing lemma
for simplicial sets allows us to conclude that the induced morphism on pushouts F m A ∧ X −→ F m A ∧ Y
is also a level equivalence, which finishes the proof.
We now prove (i) for π∗ -isomorphisms. Smash product with A commutes with the mapping cone
construction, so using the long exact sequence of homotopy groups of Proposition I.4.7 (i) it suffices toshow that if a symmetric spectrum C has trivial stable homotopy groups, then so does A ∧ C. By [...]
the symmetric spectrum L ∧ C has trivial stable homotopy groups for every pointed simplicial set L. The
isomorphism
πk (L .m C) ∼= Z[M]+m ⊗Σm ×M πk+m (L ∧ C)
(see (4.29) in Chapter I) combined with the isomorphism L .m C ∼= Gm L ∧ C shows that the homotopy
groups of Gm L ∧ C are trivial. In other words, the claim holds for semifree symmetric spectra.
If A is an arbitrary flat symmetric spectrum we again use induction and show that F m A ∧ C has trivial
homotopy groups for all m ≥ 0. Since homotopy groups commute with filtered colimits this show that thegroups π∗ (A ∧ C) vanish and it finished the argument. In the inductive step we use the pushout square of
Proposition 5.11. Since A is flat the morphism νm : Lm A −→ Am is injective, hence jm ∧Id : F m−1 A∧C −→F m A ∧ C is a monomorphism. We know by induction that the homotopy groups of F m−1 A ∧ C vanish,
and we know that the homotopy groups of(F m A ∧ C)/(F m−1 A ∧ C) ∼= Gm (Am /Lm A) ∧ Cvanish by the special case. So the long exact sequence of homotopy groups shows that the homotopy groupsof F m A ∧ C are trivial.
Now we prove statement (ii). Given an level equivalence i : K −→ L of symmetric spectra which is also amonomorphism and morphism g : K −→ Hom(A, X) we have to produce an extension ḡ : L −→ Hom(A, X)
satisfying ḡ ◦ f = g. By the definition of flatness and part (i) the morphism i ∧ Id : K ∧ A −→ L ∧ A
is an injective level equivalence. Since X is injective, the adjoint G : K ∧ A −→ X if g has an extensionḠ : L ∧ A −→ X satisfying Ḡ(f ∧ Id) = G. The adjoint L −→ Hom(A, X) of Ḡ is then the required
extension of g.
Now we prove the part of (i) referring to stable equivalences. If Z is an injective Ω-spectrum, thenHom(A, X) is injective by part (ii) and an Ω-spectrum by [ref.] So for every stable equivalence f : X −→ Y

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119

the induced map [f, Hom(A, Z)] : [Y, Hom(A, Z)] −→ [X, Hom(A, Z)] is bijective. By adjunction, [f ∧A, Z] : [Y ∧ A, Z] −→ [X ∧ A, Z] is bijective. Since this holds for all injective Ω-spectra Z, the morphismf ∧ Id : X ∧ A −→ Y ∧ A is a stable equivalence.

Example 5.15. Here is an example which shows that smashing with an arbitrary symmetric spectrum
does not preserve level equivalences. Let X be the symmetric spectrum with X0 = S 0 , X1 = CS 1 and
Xn = ∗ for n ≥ 2. Here CS 1 = S 1 ∧ ∆[1] is the cone on S 1 , where ∆[1] is pointed at the 0th vertex.
The only nontrivial structure map σ0 : X0 ∧ S 1 −→ X1 is the cone inclusion S 1 −→ CS 1 . Let Y be thesymmetric spectrum with Y0 = S 0 and Yn = ∗ for n ≥ 1. Then the unique morphism f : X −→ Y whichis the identity in level 0 is a level equivalence, but we claim that f ∧ S̄ : X ∧ S̄ −→ Y ∧ S̄ is not a level
equivalence. Indeed, in level 2 we have
1

act
2 i∧S
1
1
(X ∧ S̄)2 = L2 X ∼−−→ Σ+= pushout(S 2 ←−− Σ+2 ∧S −2 ∧ (CS ) ∧ S )which is the suspension of the double cone on S 1 , i.e., a simplicial 3-sphere. In contrast,act
2(Y ∧ S̄)2 = L2 Y ∼→ ∗)= pushout(S 2 ←−− Σ+2 ∧S −

is a point, so f ∧ S̄ is not a weak equivalence in level 2. [give an examples where π∗ -isos or stable equivalences
are not preserved]
Proposition 5.16 (Flat resolutions). There exists a functor (−)[ : SpΣ −→ SpΣ with values in flat
symmetric spectra and a natural level equivalence A[ −→ A. Moreover, there is an isomorphism (K ∧ A)[ ∼=K ∧ A[ which is natural in pointed simplicial sets K and symmetric spectra A.
Proof. Given a symmetric spectrum A we construct the ‘flat resolution’ A[ and the level equivalencer : A[ −→ A level by level, starting with A[0 = A0 and r0 = Id. Suppose now that A[ and r have been
constructed up to level n − 1. The definition of the nth latching only involves the data of a symmetric
spectrum in levels strictly smaller than n. So we have a latching object Ln A[ and the morphism r induces a
Σn -equivariant map Ln r : Ln A[ −→ Ln A. We define A[n as the pointed mapping cylinder of the composite
map
Ln r
νn
Ln A[ −−
→ Ln A −→An .
[This inherits a Σn -action from the actions on Ln A and An , and the trivial action on the cylinder coordinate.
The structure map
σn−1 : A[n−1 ∧ S 1 −→ A[n = Z(νn ◦ Ln r : Ln A[ −→ An )is the composite of the map
1∧Id

proj

[
1
A[n−1 ∧ S 1 −−−→ Σ+−−→ Ln A[
n ∧Σn−1 ×Σ1 An−1 ∧ S

with the inclusion into the mapping cylinder. The nth level of the morphism r is the projection A[n =Z(νn ◦ Ln r) −→ An of the mapping cylinder onto the target; this is a homotopy equivalence so in particulara weak equivalence.
After the dust settles we have constructed a symmetric spectrum A[ and a morphism r : A[ −→ A
which is levelwise a simplicial homotopy equivalence, so altogether a level equivalence (but in general not a
homotopy equivalence of symmetric spectra). The construction is made so that the map νn : Ln A[ −→ A[nis the mapping cylinder inclusion, thus injective. So by the criterion of Proposition 5.10 the symmetric
spectrum A[ is indeed flat.

Theorem 5.17. Let X and Y be two semistable spectra one of which is flat. Then the smash productX ∧ Y is semistable.
Proof. Suppose X is flat and semistable. We first prove the proposition when Y has a special form,
namely Y = Ωn L0 (Σ∞ K) for a pointed simplicial set K, where L0 is a level fibrant replacement functor.
Smashing with a flat symmetric spectrum preserves level equivalences (Proposition 5.14) so X ∧ L0 (Σ∞ K)
is level equivalent to X ∧ Σ∞ K, which is isomorphic to the symmetric spectrum K ∧ X and thus semistable
by Example I.4.52.

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II. THE STABLE HOMOTOPY CATEGORY

The counit of the adjunction between loop and suspension is a π∗ -isomorphism ε : S n ∧Ωn L0 (Σ∞ K) −→L0 (Σ∞ K), so by Proposition 5.14 the map
Id ∧ε : X ∧ S n ∧ Ωn L0 (Σ∞ K) −→ X ∧ L0 (Σ∞ K)
is a π∗ -isomorphism. Since the target is semistable, so is X ∧ S n ∧ Ωn L0 (Σ∞ K). A symmetric spectrum issemistable if and only if its suspension is, so we conclude that X ∧ Ωn L0 (Σ∞ K) is semistable.
To prove the general case we use Shipley’s detection functor [56, Sec. 3] which associates to every
symmetric spectrum Y the semistable symmetric spectrum DY . Here DY is the homotopy colimit of afunctor DY : I −→ SpΣ from the category I to the category of symmetric spectra with values DY (n) =Ωn L0 (Σ∞ Yn ). By the above the symmetric spectrum X ∧ DY (n) = X ∧ Ωn L0 (Σ∞ Yn ) is semistable for eachn ≥ 0. Hence the homotopy colimit of the functor X ∧ DY : I −→ SpΣ , which is isomorphic to X ∧ DY , issemistable by Example 4.53.
By [56, Cor. 3.1.7] the semistable spectrum Y is related by a chain of π∗ -isomorphisms to the symmetric
spectrum DY . By Proposition 5.14 X ∧ Y is thus related by a chain of π∗ -isomorphisms to the symmetric
spectrum X ∧ DY , which we just recognized as semistable. Hence X ∧ Y is semistable, which finishes theproof.
We recall from Example I.2.39 that every Γ-space A of simplicial sets can be extended to a simplicial
functor from the category of pointed simplicial sets. The extended functor then comes with a natural
associative and unital assembly map A(K) ∧ L −→ A(K ∧ L). Evaluating A on simplicial spheres givesymmetric spectrum A(S). More generally, we can evaluate a Γ-space A on any symmetric spectrum andobtain a new symmetric spectrum A(X) by A(X)n = A(Xn ) with structure maps
assemblyσnA(Xn ) ∧ S 1 −−−−−→ A(Xn ∧ S 1 ) −−→A(Xn+1 ) .

In this situation the collection of assembly maps
A(twist)assemblyσnA(χm,n )A(S n ) ∧ Xm −−−−−→ A(S n ∧ Xm ) −−−−−→ A(Xm ∧ S n ) −−→ A(Xm+n ) −−−−−→ A(Xn+m )form a bimorphism and thus assemble into a morphism of symmetric spectra
A(S) ∧ X −→ A(X)
which we also refer to as the assembly map.
Proposition 5.18. Let A be is a Γ-space of simplicial sets and X a symmetric spectrum of simplicial
sets.
(i) The symmetric spectrum A(S) obtained by evaluating A on spheres is flat.
[is A(X) flat if X is flat?](ii) The assembly morphism A(S) ∧ X −→ A(X) is a π∗ -isomorphism.
(iii) If X is semistable then the symmetric spectrum spectrum A(X) is again semistable.
Proof. (i) For every Γ-space of simplicial sets X and n ≥ 0 we define a new Γ-space Ln X by
(Ln X)(K) = colimA6=⊂{1,...,n} X(K A ) ∧ K cA
for pointed sets K, where cA = {1, . . . , n} − A is the complement of A. The assembly maps assemble
into a morphism of Γ-spaces λn : Ln X −→ X(−∧n ). We then have X(S)n = X(S n ) = X((S 1 )∧n ) andLn (X(S)) = (Ln X)(S 1 ) and the map λn : Ln (X(S)) −→ X(S n ) is given by evaluation the transformation
λn : Ln X −→ X(−∧n ) at S 1 .
So it suffices to show that for every Γ-space the morphism Ln X −→ X(−∧n ) is injective.
For all pointed simplicial sets A and B and all Γ-spaces X the natural map
X(A) ∧ B ∪A∧X(1+ )∧B A ∧ X(B) −→ X(A ∧ B)
induced by the assembly maps is injective. [...] For A = B = K this is the above.
(ii) We choose a strict (i.e., objectwise) weak equivalence of Γ-spaces Ac −→ A such that Ac is cofibrantin the strict Quillen model structure [ref.] Then for every simplicial set K, the map Ac (K) −→ A(K) is a

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121

weak equivalence and thus the morphisms of symmetric spectra Ac (S) −→ A(S) and Ac (X) −→ A(X) arelevel equivalences. In the commutative square
Ac (S) ∧ X/ Ac (X)
A(S) ∧ X

/ A(X)

the left vertical morphism is a level equivalence since Ac (S) and A(S) are flat. So in order to show that thelower assembly map is a π∗ -isomorphism we can show that the upper assembly map is one; in other words,
we can assume without loss of generality that the Γ-space A is Q-cofibrant.
Now use induction over the skeleta of the Γ-space. This reduced to the special case A = Γn of arepresentable Γ-space where the assembly map becomes (S × · · · × S) ∧ X = Γn (S) ∧ X −→ Γn (X) = X nwhich is a π∗ -isomorphism.
(iii) The symmetric spectrum A(S) is flat (by Proposition 5.7 (ii)) and semistable, so by Theorem 5.17
the smash product A(S) ∧ X is semistable. By Proposition 5.18 the assembly map A(S) ∧ X −→ A(X) isa π∗ -isomorphism, hence its target is also semistable.

Example 5.19. Let A be an abelian group. We consider the Γ-space HA which assigns to a finite
pointed set k + the simplicial abelian group A ⊗ Z̃[k + ]. Then HA(S) equals the Eilenberg-Mac Lane
spectrum HA as defined in Example I.2.7. Proposition 5.18 shows that the Eilenberg-Mac Lane spectrum
HA is flat and the assembly morphism
HA ∧ X −→ HA(X)
is a π∗ -isomorphism for every symmetric spectrum of simplicial sets X. The nth level of the target spectrum
is A ⊗ Z̃[Xn ] whose homotopy groups are the reduced homology groups of the pointed simplicial set Xn
with coefficients in A. So we get natural isomorphisms of abelian groups(5.20)

πk (HA ∧ X) ∼= colimn H̃k+n (X; A) .

We still owe the proof of Theorem 5.1, i.e., we have to construct the derived smash product. Given
injective Ω-spectra X and Y define the derived smash product on objects by
X ∧L Y = γ(X [ ∧ Y )where r : X [ −→ X is the ‘flat resolution’ of Proposition 5.16, i.e, a functorial level equivalence withflat source. We could also use flat resolutions of both factors, but that makes no real difference since themorphism X [ ∧Y [ −→ X [ ∧Y is a level equivalence by Proposition 5.14 (i), and so becomes an isomorphismin the stable homotopy category after applying the universal functor γ : SpΣ −→ SHC.
This construction is evidently functorial in morphisms (i.e., homotopy classes) Y −→ Y 0 since smashingcommutes with ∆[1]+ ∧ − and hence preserves the homotopy relation. Functoriality in X need an extra
argument. Since the flat resolutions of Proposition 5.16 commutes with smash product with ∆[1]+ it induces
a well-defined map (−)[ : [X, X 0 ] −→ [X [ , (X 0 )[ ] on homotopy classes of morphisms, and then we can define[f ] ∧L Y : X ∧L Y −→ X 0 ∧L Y to be [f [ ∧ Y ].
Having defined a functor ∧L : SHC × SHC −→ SHC we now need universal natural transformation
ψA,B : (γA) ∧L (γB) −→ γ(A ∧ B) in symmetric spectra A and B. We have a stable equivalence (pA )[ :A[ −→ (γA)[ obtained by taking flat resolutions of the stable equivalence pA : A −→ γA. Since source
and target of (pA )[ are flat, the map (pA )[ ∧ pB : A[ ∧ B −→ (γA)[ ∧ γB is again a stable equivalence. So
this last map become an isomorphism in the stable homotopy category after applying the universal functor
γ : SpΣ −→ SHC. Now we define ψA,B as the composite γ((p )[ ∧p )−1γ(rA ∧Id)
A
B(γA) ∧L (γB) = γ (γA)[ ∧ γB −−−−−−−−−−→ γ(A[ ∧ B) −−−−−−→ γ(A ∧ B) .

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II. THE STABLE HOMOTOPY CATEGORY

If A or B is flat, then the morphism rA ∧ Id : A[ ∧ B −→ A ∧ B is a level equivalence [justify for A flat],so it becomes an isomorphism after applying γ. Thus if at least one of A or B is flat, then ψA,B is an
isomorphism in the stable homotopy category.
Having defined the functor ∧L and the transformation ψ : ∧L ◦ (γ × γ) −→ γ ◦ ∧, it remains to showthat the pair (∧L , ψ) has the universal property of a total left derived functor. So we consider anotherfunctor F : SHC × SHC −→ SHC and a natural transformation β : F (γA, γB) −→ γ(A ∧ B). We define anatural transformation κ : F (X, Y ) −→ X ∧L Y as the composite∼=

βX [ ,Y∼=F (X, Y ) −−→ F (γX, γY ) ←−− F (γ(X [ ), γY ) −−−−→ γ(X [ ∧ Y ) = X ∧L Y
and then we have ψ ◦ κ(γ × γ) = β [...]. Moreover, κ is uniquely determined by this property.
Exercises
Exercise 6.1. Show that for every injective Ω-spectrum X and all n, m ≥ 0 the bijection[Fn S m , X] −→ πm−n X ,[f ] 7−→ (πm−n f )(j) ,is additive, hence a group isomorphism. Here j ∈ πm−n (Fn S m ) is the ‘fundamental class’.
Exercise 6.2. This exercise generalizes Lemmas 2.7 and 2.8. Let K be a pointed simplicial set whose
reduced integral homology is concentrated in one dimension, where it is free abelian of rank 1. The degreeof a based self map τ : K −→ K is the unique integer deg(τ ) such that τ induces multiplication by deg(τ )on reduced integral homology.
Show that for every injective Ω-spectrum X the induced morphism τ ∗ : X K −→ X K equals deg(τ ) · Id
in the group [X K , X K ].
Exercise 6.3. [check if this works] Let R be a symmetric ring spectrum. An R-module M is strongly
injective is it has the extension property for all homomorphisms of R-modules which are levelwise injective
and a weak equivalence of underlying simplicial sets. We define the derived category D(R) of the ring
spectrum R as the homotopy category of those strongly injective R-modules whose underlying symmetric
spectra are Ω-spectra.
(i) Suppose that R is flat as a symmetric spectrum. Show that then the underlying symmetric spectrumof a strongly injective R-modules is injective. Give an example showing that the converse is not true.
(ii) Show that the derived category D(R) has the structure of a triangulated category with shift anddistinguished triangles defined after forgetting the R-action.
(iii) Show that D(R) is the target of a universal functor from R-modules which takes stable equivalences
to isomorphisms.
(iv) Let f : R −→ S be a homomorphism of symmetric ring spectra which makes R a flat right
S-module. Show that restriction of scalars from S-modules to R-modules passes to an exact functor oftriangulated categories f ∗ : D(S) −→ D(R).
(v) Suppose that the underlying symmetric spectrum of R is semistable. Show that then R, considered
as a module over itself, has a strongly injective Ω-spectrum replacement γR as an R-module. Show thatthe map
D(R) ∼[γR, γR]k= πk (γR) ∼= πk Ris an isomorphism of graded rings, where the first map is evaluation at the unit 1 ∈ π0 (γR) ∼= πk (γR)[k].
Show that the map
D(R) ∼[γR, M ]k= πk Mis an isomorphism of graded modules over π∗ R for every strongly injective Ω-R-module M . Show that γR
is a compact weak generator of the triangulated category D(R).
We shall see later that for R = HA the Eilenberg-Mac Lane ring spectrum associated to a ring A (compare
Example I.2.7) the derived category D(HA) is triangle equivalent to the unbounded derived category of

HISTORY AND CREDITS

123

the ring A. In fact, the equivalence of triangulated categories will come out as a corollary of a Quillen
equivalence of model categories.

History and credits
The stable homotopy category as we know it today is usually attributed to Boardman, who introduced
it in his thesis [3] including the triangulated structure and the symmetric monoidal (derived !) smashproduct. Boardman’s stable homotopy category is obtained from a category of CW-spectra by passing tohomotopy classes of morphisms. Boardman’s construction was widely circulated as mimeographed notes [4],but he never published these. Accounts of Boardman’s construction appear in [62], [65], and [2, Part III].
Strictly speaking the ‘correct’ stable homotopy category had earlier been introduced by Kan [30] based on
his notion of semisimplicial spectra. Kan and Whitehead [31] defined a smash product in the homotopy
category of semisimplicial spectra and proved that it is homotopy commutative, but neither they, nor
anyone else, ever addressed the associativity of that smash product. Before Kan and Boardman there had
been various precursors of the stable homotopy category, and I recommend May’s survey article [43] for a
detailed discussion and an extensive list of references to these.
I am not aware of a complete published account that Boardman’s category is really equivalent to thestable homotopy category as defined in Definition 1.5 using injective Ω-spectra. However, here is a shortguide through the literature which outlines a comparison. In a first step, Boardman’s stable homotopy
category can be compared to Kan’s homotopy category of semisimplicial spectra, which is done in Chapter
IV of Boardman’s unpublished notes [4]. An alternative source is Tierney’s article [62] where he promotes
the geometric realization functor to a functor from Boardman’s category of CW-spectra to Kan’s categoryof semisimplicial spectra. Tierney remarks that the singular complex functor from spaces to simplicial set
does not lift to a pointset level functor in the other directions, but Section 3 of [62] then ends with thewords “(. . . ) it is more or less clear – combining various results of Boardman and Kan – that the singularfunctor exists at the level of homotopy and provides an inverse to the stable geometric realization, i.e. thetwo homotopy theories are equivalent. The equivalence of homotopy theories has also been announced by
Boardman.” I am not aware that the details have been carried out in the published literature.
Kan’s semisimplicial spectra predate model categories, but Brown [13, Thm. 5] showed later that theπ∗ -isomorphisms used by Kan are part of a model structure on semisimplicial spectra. In the paper [11]Bousfield and Friedlander introduce a model structure on a category of ‘sequential spectra’ which are justlike symmetric spectra, but without the symmetric group actions. In Section 2.5 of [11], Bousfield andFriedlander describe a chain of Quillen equivalences between semisimplicial and sequential spectra, whichthen in particular have equivalent homotopy categories. Hovey, Shipley and Smith show in [25, Thm. 4.2.5]that the forgetful functor is the right adjoint of a Quillen equivalence from symmetric spectra (with thestable absolute projective model structure in the sense of Chapter III) to the Bousfield-Friedlander stablemodel structure of sequential spectra. Since the weak equivalences used for symmetric spectra are thestable equivalences in the sense of Definition 4.1 we can conclude that altogether that Boardman’s stablehomotopy category is equivalent to the localization of the category of symmetric spectra at the class of
stable equivalences, which coincides with the stable homotopy category in our sense by Theorem 4.12.
A word of warning: the comparison which I just summarized passes through the intermediate homotopy
category of sequential spectra for which no intrinsic way to define a derived smash product has been studied.
As a consequence, it is not clear to me if the combined equivalence takes Boardman’s derived smash productto the derived smash product as discussed in Section 5. However, I would be surprised if the composite
equivalence were not strongly symmetric monoidal.
The spectral sequence of Theorem 4.15 was first constructed by Shipley in [56], by completely different
method; more precisely, Shipley obtains a spectral sequence with isomorphic E 2 -term and isomorphic
abutment, and so it seems very likely that the spectral sequences are isomorphic. Shipley constructs aspectral sequence of the form
2
Ep,q
= colimpI (π sq X) =⇒ πp+q DX

124

II. THE STABLE HOMOTOPY CATEGORY

which also converges to the true homotopy groups of a symmetric spectrum X and whose E 2 -term depends
on the I-functor π sq X of stable homotopy groups of the spectrum X. Here D is Shipley’s detection functor [56, Def. 3.1.1]. The proof that Shipley’s E 2 -term (the derived functors of colimit) is isomorphic to the
Tor groups which arise as the E 2 -term in Theorem 4.15 was given by the author in [53].
The argument used in Lemma 4.22 to reduce the lifting property to a set of morphisms with boundedcardinality is taken from [25, Lemma 5.1.4 (6)] and ultimately goes back to Bousfield, who used it in [8] toestablish a ‘local’ model structure for simplicial sets with respect to a homology theory.

CHAPTER III

Model structures
Symmetric spectra support many useful model structures and we will now develop several of these.
We will mainly be interested in two kinds, namely level model structures (with weak equivalences the level
equivalences) and stable model structures (with weak equivalences the stable equivalences). The level model
structures are really an intermediate steps towards the more interesting stable model structures. We willdevelop the theory for symmetric spectra of simplicial sets first, and later say how to adapt things tosymmetric spectra of topological spaces.
We have already seen pieces of some of the model structures at work. Our definition of the stablehomotopy category in Section 1 of Chapter II is implicitly relying on the absolute injective stable model
structure in which every object is cofibrant (as long as we use simplicial sets, not topological spaces) and thefibrant objects are the injective Ω-spectra. However, this model structure does not interact well with thesmash product, so when we constructed the derived smash product in Section 5 of Chapter II we implicitly
worked in the flat model structures. So it should already be clear although the homotopy category of amodel category only depends on the class of weak equivalences, it can be useful to play different model
structures off against each other.
Besides the injective and flat model structures there is another useful kind of cofibration/fibration pairwhich we will discuss, giving the projective model structures. Moreover, we will later need ‘positive’ versionsof the model structures which discard all homotopical information contained in level 0 of a symmetric
spectrum.
So each of the model structures which we discuss has four kinds of ‘attributes’:• a kind of space (simplicial set or topological space)• a kind of cofibration/fibration pair (injective, flat or projective)• a type of equivalence (level or stable)• which levels are used (absolute or positive)
Since all of these attributes can be combined, this already makes 2×3×2×2 = 24 different model structureson the two kinds of symmetric spectra. More variations are possible: one can also take π∗ -isomorphisms as
weak equivalences, or even isomorphisms in some homology theory (giving model structures which realize
Bousfield localizations), or one could study ‘more positive’ model structures which disregard even morethan the level 0 information. And this is certainly not the end of the story. . .

1. Level model structures
1.1. Types of cofibrations. The latching space Ln A of a symmetric spectrum A was defined in
Definition II.5.9 as the nth level of the symmetric spectrum A ∧ S̄, where S̄ is the subspectrum of the sphere
spectrum with S̄0 = ∗ and S̄n = S n for positive n. We also gave a more explicit presentation of Ln A as1
a quotient of Σ+n ∧Σn−1 An−1 ∧ S . The latching space has a based Σn -action and comes with a natural
equivariant map νn : Ln A −→ An .
125

126

III. MODEL STRUCTURES

For a morphism f : A −→ B of symmetric spectra and n ≥ 0 we have a commutative square of
Σn -simplicial sets
Ln A

Ln f

νn


An

/ Ln B
νn

fn


/ Bn

We thus get a natural morphism of Σn -simplicial setsνn (f ) : An ∪Ln A Ln B −→ Bn .
Definition 1.1. A morphism f : A −→ B of symmetric spectra of simplicial sets is• a projective cofibration if for every n ≥ 0 the morphism νn (f ) is injective and the symmetric groupΣn acts freely on the complement of its image;• a flat cofibration if for every n ≥ 0 the morphism νn (f ) is injective;• a level cofibration if it is a categorical monomorphism, i.e., if for every n ≥ 0 the morphism
fn : An −→ Bn is injective.
By the criterion for flatness given in Proposition II.5.10 a symmetric spectrum A is flat in the original
sense (i.e., A∧− preserves monomorphisms) if and only if the unique morphism ∗ −→ A is a flat cofibration.
We call a symmetric spectrum A projective if the unique morphism ∗ −→ A is a projective cofibration or,equivalently, if for every n ≥ 0 the morphism νn : Ln A −→ An is injective and the symmetric group Σn
acts freely on the complement of its image. Every symmetric spectrum of simplicial sets is level cofibrant.
Now we define the analogues of the three kinds of cofibrations for symmetric spectra of pointed topological spaces. We refer to the standard model structure on the category of pointed compactly generated
weak Hausdorff spaces as described for example in [24, Thm. 2.4.25]. In this model structure, the weakequivalences are the weak homotopy equivalences and fibrations are the Serre fibrations. The cofibrations
are the retracts of ‘generalized CW-complexes’, i.e., cell complexes in which cells can be attached in any
order and not necessarily to cells of lower dimensions. The term ‘Σn -cofibration’ in the next definition refers
to the model structure on pointed Σn -spaces which is created by the forgetful functor to pointed spaces.
These cofibrations are the retracts of ‘generalized free Σn -CW-complexes’, i.e., equivariant cell complexes
in which only free Σn -cells are attached.
Definition 1.2. A morphism f : A −→ B of symmetric spectra of topological spaces is• a projective cofibration if for every n ≥ 0 the morphism νn (f ) is a Σn -cofibration;• a flat cofibration if for every n ≥ 0 the morphism νn (f ) is a cofibration of spaces;• a level cofibration if for every n ≥ 0 the morphism fn : An −→ Bn is a cofibration of spaces.
To see the analogy with the earlier definitions for symmetric spectra of simplicial sets one should remember that in the standard model structure for pointed simplicial set the cofibrations are the monomorphisms.
By definition every projective cofibration is also a flat cofibration. Flat cofibrations are level cofibrations
by the following lemma.
Lemma 1.3. Let f : A −→ B be a morphism of symmetric spectra of simplicial sets or topologicalspaces. Then f is a flat cofibration if and only if for every level cofibration g : X −→ Y the pushout productmapf ∧ g : B ∧ X ∪A∧X A ∧ Y −→ B ∧ Y
is a level cofibration. In particular, every flat cofibration is a level cofibration.
Proof. The inclusion S̄ −→ S is a level cofibration and in level n the pushout product of f with thisinclusion is the morphism νn (f ) : An ∪Ln A Ln B −→ Bn . So the pushout product condition for all level
cofibrations implies that f : A −→ B is a flat cofibration.

1. LEVEL MODEL STRUCTURES

127

For the other direction we first consider the special case where f is of the form Gm i : Gm K −→ Gm L
for a Σm -equivariant map i : K −→ L of pointed simplicial sets or spaces which is a cofibration in theunderlying category. We use the isomorphism Gm L ∧ X ∼= L .m X (compare Proposition I.3.5) to rewrite
the pushout product (Gm i) ∧ g as
L .m X ∪K.m X K .m Y −→ L .m Y .
In level m+n this morphism is given by first forming the pushout product of i∧gn : L∧Xn ∪K∧Xn K ∧Yn −→L ∧ Yn as pointed simplicial set (or pointed spaces), and then inducing up from Σm × Σn to Σm+n . The mapi ∧ gn is a cofibration by the pushout product property of simplicial sets respectively spaces, and inducing
up preserves cofibrations. So in this special case the pushout product map is a level cofibration. [...]
If we apply this to the injective map ∗ −→ S the pushout product is isomorphic to the map f . So as a
special csae we obtain that every flat cofibration is a level cofibration.

Lemma 1.4. Let f : A −→ B be a morphism of symmetric spectra of simplicial sets. Then f : A −→ B
is a projective cofibration if and only if it is a flat cofibration and the cokernel B/A is projective. [Is amorphism f : A −→ B is a flat cofibration if and only if it is an injective cofibration (i.e., monomorphism)
and the cokernel B/A is flat?]
Proof. This is direct consequence of the definitions since a group acts freely on the complement of
the image of an equivariant map A −→ B if and only if the induced action on the quotient B/A is freeaway from the basepoint.
Thus we have the following implications for the various kinds of cofibrations:projective cofibration =⇒ flat cofibration =⇒ level cofibration
All these containments are strict, as the following examples show. The symmetric spectrum S̄ is not flat
since its second latching object L2 S̄ is isomorphic to S 1 ∨ S 1 and the map L2 S̄ −→ S̄2 = S 2 is the fold map,
which is not injective. Semifree symmetric spectra Gm L are flat for all pointed Σm -simplicial sets L, butthey are projective only if Σm acts freely away from the basepoint (compare the following proposition).
Proposition 1.5.
(i) Let K be a pointed Σm -simplicial set for some m ≥ 0. Then the nth
latching spaces of the semifree symmetric spectrum Gm K is trivial for n ≤ m and for n > m themap νn : Ln (Gm K) −→ (Gm K)n is an isomorphism.
(ii) Given m ≥ 0 and an injective morphism f : K −→ L of Σm -simplicial sets, the induced map
Gm f : Gm K −→ Gm L on semifree symmetric spectra is a flat cofibration, and it is a projective
cofibration if and only if Σm acts freely on the complement of the image of f . In particular, every
semifree symmetric spectrum Gm L is flat and it is projective if and only if Σm acts freely on L
away from the basepoint.
(iii) If f : K −→ L is a monomorphism of simplicial sets, then for every m ≥ 0, the induced map
Fm f : Fm K −→ Fm L is a projective cofibration. In particular, all free symmetric spectra areprojectively cofibrant.
Proof. (i) For n < m, the latching space Ln (Gm K) consists only of the basepoint. For n ≥ m,
substitution of the definitions and some rewriting gives
Lm+n (Gm K) = (Gm K ∧ S̄)m+n ∼= (K .m S̄)m+n = Σ+m+n ∧Σm ×Σn K ∧ S̄n(∗
for n = 0, and=+n ∼Σm+n ∧Σm ×Σn K ∧ S = (Gm K)m+n for n ≥ 1.
where we used the identification of Gm K ∧X with the twisted smash product K .m X (see Proposition I.3.5).

128

III. MODEL STRUCTURES(ii) We use part (i) to identify the terms in the commutative square of Σn -simplicial sets
Ln (Gm K)
νn

Ln (Gm f )(Gm K)n/ Ln (Gm L)


fn

νn

/ (Gm L)n

For n < m all four terms are just points. For n = m the two upper objects are points and the lower vertical
map is injective. For n > m both vertical maps are isomorphisms. So the map
νn (Gm f ) : (Gm K)n ∪Ln (Gm K) Ln (Gm L) −→ (Gm L)n
is an isomorphism for n 6= m and injective for n = m. So Gm f is always a flat cofibration. The only casein which νn (Gm f ) is not an isomorphism is n = m, and then νn (Gm f ) is isomorphic to f : K −→ L. So
Gm f is a projective cofibration if and only if Σm acts freely away from the image of f .
The natural isomorphism between Fm K and Gm (Σ+m ∧ K) makes (iii) a special case of (ii).
Proposition 1.6. Let A be a flat symmetric spectrum and n ≥ 2. Then the symmetric power spectrum(A∧n )/Σn is again flat.
[is the product of flat spectra flat ? how about AK and sh A ?]Theorem 1.7. Let f : X −→ Y be an injective morphism of Γ-spaces of simplicial sets. Then theassociated morphism f (S) : X(S) −→ Y (S) is a flat cofibration of symmetric spectra. In particular, forevery Γ-space of simplicial sets X, the associated symmetric spectrum X(S) is flat.
[how do the BF- and Q-cofibrations of Γ-spaces relate to the various cofibrations ?]Proof.

Definition 1.8. A morphism f : K −→ L of Σn -simplicial sets is a Σn -fibration (respectively Σn equivalence ) if the induced map on H-fixed points f H : K H −→ LH is a Kan fibration (respectively weakequivalence) of simplicial sets for all subgroups H of Σn .
Theorem 1.9. The category of symmetric spectra of simplicial sets admits the following three level
model structures in which the weak equivalences are those morphisms f : X −→ Y such that for all n ≥ 0
the map fn : Xn −→ Yn is a weak equivalence of simplicial sets.
(i) In the projective level model structure the cofibrations are the projective cofibrations and a morphism f : X −→ Y is a projective level fibration if and only if for every n ≥ 0 the map
fn : Yn −→ Xn is a Kan fibration of simplicial sets.
(ii) In the flat level model structure the cofibrations are the flat cofibrations, and a morphism f :X −→ Y is a flat level fibration if and only if for every n ≥ 0 the map fn : Yn −→ Xn satisfies
the following two equivalent conditions– the map fn has the right lifting property for all injective morphisms of Σn -simplicial sets
which are weak equivalences on underlying simplicial set;
– map fn is a Σn -fibration and the commutative square
Xn
fn


Yn

/ map(EΣn , Xn )map(EΣn ,fn )/ map(EΣn , Yn )is Σn -homotopy cartesian. Here map(EΣn , X) is the simplicial mapping space of all maps
from the contractible free Σn -simplicial set to X, with Σn -action by conjugation.
A morphism f : X −→ Y is an acyclic fibration in the flat model structure if and only if for alln ≥ 0 the map fn : Xn −→ Yn is a Σn -equivariant acyclic fibration.

1. LEVEL MODEL STRUCTURES

129(iii) In the injective level model structure the cofibrations are the level cofibrations (i.e., monomorphisms) and the injective fibrations are those morphisms which have the right lifting property withrespect to all morphisms which are simultaneously level equivalences and monomorphisms.
Moreover we have:• All three level model structures are proper, simplicial and cofibrantly generated.
• The flat and projective level model structures are even finitely generated and monoidal with respect
to the smash product of symmetric spectra.
Proof. The category of symmetric spectra of simplicial sets has all set-indexed limits and colimits, thelevel equivalences satisfy the 2-out-of-3 property and in all three cases the classes of cofibrations, fibrations
and weak equivalences are closed under retracts. So it remains to prove the factorization and lifting axioms.
As usual we construct the factorizations using Quillen’s small object argument. We first defined the
lv
lv
lv
lv
respective classes Iproj, Iflvlat and Iinj
of generating cofibrations and Jproj, Jflvlat and Jinjof generating
acyclic cofibrations. As generating projective cofibrations we take

lv
Iproj
= Fn ∂∆[m]+ −→ Fn ∆[m]+ n,m≥0where Fn is the free symmetric spectrum generated by a pointed simplicial set in level n, see Example I.2.12.
Since Fn is left adjoint to evaluation at level n, a morphism f : X −→ Y of symmetric spectra has the rightlv
lifting property with respect to Iproj
if and only if for every n ≥ 0 the map fn : Yn −→ Xn has the RLP
for the boundary inclusions ∂∆[n] −→ ∆[n], i.e., if it is a Kan fibration and a weak equivalence. In otherwords, precisely the morphisms which are both level equivalences and projective level fibrations enjoy thelvright lifting property with respect to Iproj.
As generating flat cofibrations we takeno++
Iflvlat = Gm (Σm /H × ∂∆[n]) −→ Gm (Σm /H × ∆[n])n,m≥0, H≤Σnwhere Gm is the semifree symmetric spectrum generated by a pointed Σm -simplicial set in level m, see
Example I.2.13. Since Gm is left adjoint to evaluation at level m with values in Σm -simplicial sets, amorphism f : X −→ Y of symmetric spectra has the right lifting property with respect to Iflvlat if and only
if for every m ≥ 0 and every subgroup H of Σn the map (fm )H : (Ym )H −→ (Xm )H on H-fixed pointsof fm is a Kan-fibration and weak equivalence of simplicial sets. By Proposition 1.9 of Appendix A thisis equivalent to the property that fn is simultaneously a Σn -fibration, a weak equivalence on underlying
simplicial sets and the square above is Σn -homotopy cartesian; in other words, precisely the flat level
fibrations defined in (ii) above enjoy the right lifting property with respect to Iflvlat .
Let f : X −→ Y be a flat level fibration. If H is a subgroup of Σn then the semifree symmetric spectrum
Gn (Σn /H)+ is flat. Since the flat model structure is simplicial, the induced map on mapping spaces(Xn )H ∼= map(Gn (Σn /H)+ , X) −→ map(Gn (Σn /H)+ , Y ) ∼= (Yn )H
is a Kan fibration. Since this holds for all subgroup, the map fn : Xn −→ Yn is a Σn -fibration.
Suppose that X is fibrant in the flat level model structure. Let L be any (unbased) Σm -simplicial set.
Then the projection EΣn × L −→ L is Σn -equivariant and a weak equivalence of underlying simplicial sets.
So the induced map of semifree symmetric spectra
Gn (EΣn × L)+ −→ Gn L+
is a level equivalence between flat symmetric spectra. Since the flat model structure is simplicial, theinduced map on mapping spaces
map(Gn L+ , X) −→ map(Gn (EΣn × L)+ , X)
is a weak equivalence. By adjointness that map is isomorphic tomapΣn (L, Xn ) −→ mapΣn (EΣn × L, Xn ) ∼= mapΣn (L, map(EΣn , Xn ))130III. MODEL STRUCTURES

where map(EΣn , Xn ) is the space (i.e., simplicial set) of all morphisms from EΣn to X with conjugation
action by Σn . If we specialize to L = Σn /H for a subgroup H of Σn we see that the map
Xn −→ map(EΣn , Xn )is a weak equivalence on H-fixed points, so it is an equivariant equivalence.
To define the generating injective cofibrations we choose one representative for each isomorphism classof pairs (B, A) consisting of a countable symmetric spectrum B and a symmetric subspectrum A.

We still have to show that the level model structures are simplicial and that the flat and projective
level model structures are monoidal with respect to the internal smash product of symmetric spectra. So
we have to verify various forms of the pushout product property. We recall that the pushout product ofa morphism i : K −→ L of pointed simplicial sets or symmetric spectra and a morphism j : A −→ B ofsymmetric spectra is the morphismi ∧ j : L ∧ A ∪K∧A K ∧ B −→ L ∧ B .
The first proposition below is about smash products of simplicial sets with symmetric spectra, and it saysthat various model structures of symmetric spectra are simplicial model structures. The next proposition
is about internal smash products of symmetric spectra, and it says that various flat and projective (but not
injective) model structures of symmetric spectra are monoidal model structures.
Proposition 1.10. Let i : K −→ L be a morphism of pointed simplicial sets and j : A −→ B amorphism of symmetric spectra.
(i) If i is injective and j a level cofibration, flat cofibration respectively projective cofibration, then the
pushout product i ∧ j is also a level cofibration, flat cofibration respectively projective cofibration.
(ii) If i is an injective weak equivalence of simplicial sets, and j is a level cofibration (i.e, monomorphism), then i ∧ j is also a level equivalence of symmetric spectra.
(iii) If i is injective and j a level cofibration (i.e, monomorphism) and a level equivalence, π∗ isomorphism respectively stable equivalence of symmetric spectra, then i ∧ j is also a level equivalence, π∗ -isomorphism respectively stable equivalence.
Thus the injective, flat and projective level model structures are simplicial model categories.
Proof. For every pointed simplicial set K and symmetric spectrum A the smash product K ∧ L isnaturally isomorphic to the smash product of the suspension spectrum Σ∞ K with A. The suspension
spectrum functor takes injective maps of simplicial sets to projective cofibrations (see Proposition 1.5 (iii)
for m = 0) and it takes weak equivalences to level equivalences. So this proposition is a special case of
Proposition 1.11 below.
Proposition 1.11. Let i : K −→ L and j : A −→ B be morphisms of symmetric spectra.
(i) If i is a level cofibration and j is a flat cofibration, then i ∧ j is a level cofibration.
(ii) If both i and j are flat cofibrations, then so is i ∧ j.
(iii) If both i and j are projective cofibrations, then so is i ∧ j.
(iv) If i is a level cofibration, j a flat cofibration and one of i or j a level equivalence, π∗ -isomorphism
respectively stable equivalence, then i ∧ j is also a level equivalence, π∗ -isomorphism respectively
stable equivalence.
Thus the flat and projective level model structures are monoidal model categories with respect to the smashproduct of symmetric spectra.
Proof. Check on generators.

[State all adjoint forms of the simplicial and monoidal axiom]
Definition 1.12. A morphism f : K −→ L of Σn -spaces is a Σn -fibration (respectively Σn -equivalence)
if the induced map on H-fixed points f H : K H −→ LH is a Serre fibration (respectively weak equivalence)
for all subgroups H of Σn .

2. STABLE MODEL STRUCTURES

131

Theorem 1.13. The category of symmetric spectra of topological spaces admits the following two level
model structures in which the weak equivalences are those morphisms f : X −→ Y such that for all n ≥ 0
the map fn : Xn −→ Yn is a weak equivalence of spaces.
(i) In the projective level model structure the cofibrations are the projective cofibrations and a morphism f : X −→ Y is a projective level fibration if and only if for every n ≥ 0 the map
fn : Yn −→ Xn is a Serre fibration.
(ii) In the flat level model structure the cofibrations are the flat cofibrations, and a morphism f :X −→ Y is a flat level fibration if and only if for every n ≥ 0 the map fn : Yn −→ Xn satisfies
the following two equivalent conditions– the map fn has the right lifting property for all cofibrations of pointed Σn -spaces which are
weak equivalences on underlying spaces;– map fn is a Σn -fibration and the commutative square/ map(EΣn , Xn )Xn
fn

map(EΣn ,fn )/ map(EΣn , Yn )Yn

is Σn -homotopy cartesian. Here map(EΣn , X) is the space of all maps from the contractible
free Σn -space to X, with Σn -action by conjugation.
A morphism f : X −→ Y is an acyclic fibration in the flat model structure if and only if for alln ≥ 0 the map fn : Xn −→ Yn is a Σn -equivariant acyclic fibration.
Moreover, both level model structures are proper, topological and finitely generated, and monoidal withrespect to the smash product of symmetric spectra.
[Is there are an injective level model structure for symmetric spectra of spaces ?][positive model structures]
2. Stable model structures
Recall from Definition II.4.1 that a morphism f : A −→ B if symmetric spectra of simplicial sets is astable equivalence if for every injective Ω-spectrum X the induced map[f, X] : [B, X] −→ [A, X]
on homotopy classes of spectrum morphisms is a bijection.
For every morphism f : X −→ Y the natural morphism λ∗X : X −→ Ω(sh X) adjoint to λX : S 1 ∧ X −→sh X gives rise to a commutative square of symmetric spectra
(2.1)

X

λ∗
X

f


Y

/ Ω(sh X)Ω(sh f )λ∗Y/ Ω(sh Y )Theorem 2.2. The category of symmetric spectra of simplicial sets admits the following three stablemodel structures in which the weak equivalences are the stable equivalences.
(i) In the projective stable model structure the cofibrations are the projective cofibrations and thefibrations are those projective level fibrations f : X −→ Y for which the commutative square (2.1)
is levelwise homotopy cartesian.
(ii) In the flat stable model structure the cofibrations are the flat cofibrations, and the fibrations arethose flat level fibrations f : X −→ Y for which the commutative square (2.1) is levelwise homotopy
cartesian.

132

III. MODEL STRUCTURES(iii) In the injective stable model structure the cofibrations are the level cofibrations (i.e., monomorphisms) and the fibrations are those those injective fibrations f : X −→ Y for which the commutative square (2.1) is levelwise homotopy cartesian.
Moreover we have:• All three stable model structures are proper, simplicial and cofibrantly generated.
• The flat and projective stable model structures are even finitely generated and monoidal with respect
to the smash product of symmetric spectra.
• In all three cases a morphism is a stable acyclic fibration if and only if it is a level acyclic fibration.
Proof. We reduce the proof of the stable model structures to the level model structures by applying
a general localization theorem of Bousfield, see Theorem 1.8 of Appendix A. In Proposition II.4.21 weconstructed a functor Q : SpΣ −→ SpΣ with values in Ω-spectra and a natural stable equivalence α : A −→QA. We note that a morphism f : A −→ B of symmetric spectra is a stable equivalence if and only if
Qf : QA −→ QB is a level equivalence. Indeed, since αA : A −→ QA and αB : B −→ QB are stableequivalences, f is a stable equivalence if and only if Qf is. But Qf is a morphism between Ω-spectra, so it
is a stable equivalence if and only if it is a level equivalence.
We now apply Bousfield’s Theorem A.1.8 to the injective, flat and projective level model structures. All
three level model structures are proper by Theorem 1.9. Axiom (A1) holds since we have a commutativesquare
(2.3)

X

/ QX

α

f

Qf


Y


/ QY

α

If f is a level equivalences, then Qf is a stable equivalence between Ω-spectra, hence a level equivalence.
Axiom (A2) holds: αQX is a stable equivalence between Ω-spectra, hence a level equivalence. Then QαX :QX −→ QQX is a level equivalence since Q takes all stable equivalences, in particular αX , to level
equivalences.
We prove (A3) in the projective level model structure. Since the projective fibrations include the flatand injective fibrations, it then also holds in the flat and injective level model structures. So we are givena pullback square
V

i

g

f


W

/X

j


/Y

of symmetric spectra in which X and Y are Ω-spectra (possibly not levelwise Kan), f is levelwise a Kan
fibration and j is a stable equivalence. We showed in part (iv) of Proposition II.4.5 that then i is also a stableequivalence. This proves (A3), and thus Bousfield’s theorem provides three model structures with stableequivalences as weak equivalence and with cofibrations the projective, flat or level cofibrations respectively.
Bousfield’s theorem characterizes the fibrations as those level fibrations f : X −→ Y for which thecommutative square (2.3) is homotopy cartesian. So it remains to shows that for a morphism f : X −→Y which is levelwise a Kan fibration the square (2.1) is levelwise homotopy cartesian if and only if thesquare (2.3) is levelwise homotopy cartesian.

Corollary 2.4. The following categories are equivalent• the stable homotopy category, i.e., the homotopy category of injective Ω-spectra of simplicial sets;• the homotopy category of those flat Ω-spectra of simplicial sets for which all Xn are Σn -fibrantand the maps Xn −→ map(EΣn , Xn ) are Σn -equivalences;• the homotopy category of projective Ω-spectra which are levelwise Kan complexes.

3. MODEL STRUCTURES FOR MODULES

133

3. Model structures for modules
With the symmetric monoidal smash product and a compatible model structure in place, we are readyto explore ring and module spectra. In this section we construct model structures on the category ofmodules over a symmetric ring spectrum. We restrict our attention to stable model structures and showthat the forgetful functor to symmetric spectra ‘creates’ various such model structure. The forgetful functor
also creates various level model structures, but we have no use for that and so will not discuss level model
structures for R-modules.
The various stable model structures are also ‘stable’ in the technical sense that the suspension functor
on the homotopy category is an equivalence of categories. As consequence of this is that stable homotopy
category of modules over a ring spectrum is a triangulated category. The free module of rank one is a small
generator.
We originally defined a symmetric ring spectrum in Definition I.1.3 in the ‘explicit’ form, i.e., as a family{Rn }n≥0 of pointed simplicial sets with a pointed Σn -action on Rn and Σp × Σq -equivariant multiplication
maps µp,q : Rp ∧ Rq −→ Rp+q and two unit maps subject to an associativity, unit and centrality condition.
Using the internal smash product of symmetric spectra we saw in Theorem I.3.8 that a symmetric ring
spectrum can equivalently be defined as a symmetric spectrum R together with morphisms µ : R ∧ R −→ Rand ι : S −→ R, called the multiplication and unit map, which satisfy certain associativity and unit
conditions. In this ‘implicit’ picture a morphism of symmetric ring spectra is a morphism f : R −→ S ofsymmetric spectra commuting with the multiplication and unit maps, i.e., such that f ◦ µ = µ ◦ (f ∧ f ) andf ◦ ι = ι.
Similarly, if R is a symmetric ring spectrum, a right R-module was originally defined explicitly, but it
can also be given in an implicit form as a symmetric spectrum M together with an action map M ∧R −→ Msatisfying associativity and unit conditions. A morphism of right R-modules is a morphism of symmetric
spectra commuting with the action of R. We denote the category of right R-modules by mod- R.
The unit S of the smash product is a ring spectrum in a unique way, and S-modules are the sameas symmetric spectra. The smash product of two ring spectra is naturally a ring spectrum. For a ring
spectrum R the opposite ring spectrum Rop is defined by composing the multiplication with the twist map
R ∧ R −→ R ∧ R (so in terms of the bilinear maps µp,q : Rp ∧ Rq −→ Rp+q , a block permutation appears).
The definitions of left modules and bimodules is hopefully clear; left R-modules and R-T -bimodule can also
be defined as right modules over the opposite ring spectrum Rop , respectively right modules over the ring
spectrum Rop ∧ T .
A formal consequence of having a closed symmetric monoidal smash product of symmetric spectra isthat the category of R-modules inherits a smash product and function objects. The smash product M ∧R N
of a right R-module M and a left R-module N can be defined as the coequalizer, in the category ofsymmetric spectra, of the two maps
M ∧R∧N// M ∧ N

given by the action of R on M and N respectively. Alternatively, one can characterize M ∧R N as theuniversal example of a symmetric spectrum which receives a bilinear map from M and N which is Rbalanced, i.e., all the diagrams(3.1)

Mp ∧ R q ∧ N r
αp,q ∧Id

Id ∧αq,r/ Mp ∧ Nq+r
ιp,q+r

Mp+q ∧ Nrιp+q,r/ (M ∧ N )p+q+r

commute. If M happens to be a T -R-bimodule and N an R-S-bimodule, then M ∧R N is naturally a
T -S-bimodule. If R is a commutative ring spectrum, the notions of left and right module coincide andagree with the notion of a symmetric bimodule. In this case ∧R is an internal symmetric monoidal smashproduct for R-modules. There are also symmetric function spectra HomR (M, N ) defined as the equalizer

134

III. MODEL STRUCTURESof two maps
Hom(M, N ) −→ Hom(R ∧ M, N ) .
The first map is induced by the action of R on M , the second map is the composition of R ∧ − :Hom(M, N ) −→ Hom(R ∧ M, R ∧ N ) followed by the map induced by the action of R on N . The internal function spectra and function modules enjoy the ‘usual’ adjointness properties with respect to thevarious smash products. [spell out]
Theorem 3.2. Let R be a symmetric ring spectrum of topological spaces or simplicial sets. The categoryof right R-modules admits the following four stable model structures in which the weak equivalences arethose morphisms of R-modules which are stable equivalences on underlying symmetric spectra.
(i) In the absolute projective stable model structure the fibrations are those morphisms of R-modules
which are absolute projective stable fibrations on underlying symmetric spectra.
(ii) In the positive projective stable model structure the fibrations are those morphisms of R-modules
which are positive projective stable fibrations on underlying symmetric spectra.
(iii) In the absolute flat stable model structure the fibrations are those morphisms of R-modules which
are absolute flat stable fibrations on underlying symmetric spectra.
(iv) In the positive flat stable model structure the fibrations are those morphisms of R-modules which
are positive flat stable fibrations on underlying symmetric spectra.
Moreover we have:• All four stable model structures are proper, simplicial and cofibrantly generated.
• If R is commutative then all four stable model structures are monoidal with respect to the smashproduct over R.
If underlying symmetric spectrum of R is flat, then the category of right R-modules admits the following
two injective stable model structures in which the weak equivalences are those morphisms of R-modules which
are stable equivalences on underlying symmetric spectra.
(v) In the absolute injective stable model structure the fibrations are those morphisms of R-modules
which are absolute injective stable fibrations on underlying symmetric spectra.
(vi) In the positive injective stable model structure the fibrations are those morphisms of R-modules
which are positive injective stable fibrations on underlying symmetric spectra.
Moreover, both injective stable model structures are proper, simplicial and cofibrantly generated.
In all six model structures, a cofibration of R-modules is a monomorphism of underlying symmetric
spectra.
Proof. In the language of Definition 1.3 of Appendix A we claim that in all of the six cases the forgetful
functor from R-modules to symmetric spectra creates a model structure on R-modules. In Theorem A.1.4
we can find sufficient conditions for this, which we will now verify.
The category of R-modules is complete, cocomplete and simplicial; in fact all limits, colimits, tensors
and cotensors with simplicial sets are created on underlying symmetric spectra. In particular the forgetful
functor preserves filtered colimits. The forgetful functor has a left adjoint free functor, given by smashingwith R. [Smallness]
It remains to check the condition which in practice is often the most difficult one, namely that every(J ∧ R)-cell complex is a weak equivalence. We claim that in all six cases the free functor X 7→ X ∧ R takes
stable acyclic cofibrations of symmetric spectra of the respective kind to stable equivalences of R-modules
which are monomorphisms. In the first four cases (where we have no assumption on R) this uses that every
generating acyclic cofibration i : A −→ B is in particular a flat cofibration, so i ∧ Id : A ∧ R −→ B ∧ R
is injective and a stable equivalence by parts (i) and (iv) of Proposition 1.11. In the ‘injective’ cases (v)and (vi) the argument is slightly different; then the assumption that R is flat assures that for every injective
stable equivalence i : A −→ B the morphism i ∧ Id : A ∧ R −→ B ∧ R is again injective (by the definitionof flatness) and a stable equivalence (by Proposition II.5.14).
So in all the six cases, the free functor − ∧ R takes the generating stable acyclic cofibrations to injective
stable equivalences of R-modules. Since colimits of R-modules are created on underlying symmetric spectra,

3. MODEL STRUCTURES FOR MODULES

135

the class of injective stable equivalences is closed under wedges, cobase change and transfinite composition.
So every (J ∧ R)-cell complex is a stable equivalence. So we have verified the hypothesis of Theorem 1.4,
which thus shows that the forgetful functor creates the six model structure. It also shows that the model
structures are simplicial and right proper.
[left proper] [monoidal if R commutative.] [preservation of cofibrations][Is there an ‘strongly injective’ stable model structure in which cofibrations are the monomorphisms of
R-modules ? make exercise?]
Proposition 3.3. A morphism f : M −→ N of right R-modules is a flat cofibration if and only if forevery morphism g : V −→ W of left R-modules the pushout product map
f ∧R g : M ∧R W ∪M ∧R V N ∧R W −→ N ∧R W
is an injective morphism of symmetric spectra.
There are also characterizations of flat and projective cofibrations in terms of ‘R-module latching
objects’, see Exercise 4.1.
As we just proved, cofibrations of R-modules are always monomorphisms of underlying symmetric
spectra, but sometimes more is true. As the special case S = S of Theorem 3.4 (iii) below we will see that
if R is flat as a symmetric spectrum, then every flat cofibration of R-modules is also a flat cofibration onunderlying symmetric spectra. Similarly, if R is projective as a symmetric spectrum, then every projective
cofibration of R-modules is also a projective cofibration on underlying symmetric spectra.
For a morphism f : S −→ R of symmetric ring spectra, there is are two adjoint functor pairs relating themodules over S and R. The functors are analogous to restriction and extension respectively coextensionof scalars. Every R-module becomes an S-module if we let S act through the homomorphism f ; more
precisely, given an R-module M we define an S-module f ∗ M as the same underlying symmetric spectrum
as M and with S-action given by the composite
Id ∧f

α

(f ∗ M ) ∧ S = M ∧ S −−−→ M ∧ R −−→ M .
We call the resulting functor f ∗ : mod- R −→ mod- S restriction of scalars along f and note that it has botha left and right adjoint. We call the left adjoint extension of scalars and denote it by f∗ : mod- S −→ mod- R.
The left adjoint takes an S-module N to the R-module f∗ N = N ∧S R, where S is a left R-module via f ,and with right R-action through the right multiplication action of R on itself. We call the right adjoint off ∗ the coextension of scalars and denote it by f! : mod- S −→ mod- R. The right adjoint takes an S-module
N to the R-module f! N = Hommod- S (R, N ), where S is a right R-module via f , and with right R-action
through the left multiplication action of R on itself.
Theorem 3.4. Let f : S −→ R be a homomorphism of symmetric ring spectra.
(i) The functor pairmod- Sf∗o/mod- Rf∗

is a Quillen adjoint functor pair with respect to the absolute projective, the positive projective, theabsolute flat and the positive flat stable model structures on both sides.
(ii) If S and R are flat as symmetric spectra then (f∗ , f ∗ ) is a Quillen adjoint functor pair with respect
to the absolute injective and the positive injective stable model structures on both sides.
(iii) Suppose that the morphism f : S −→ R makes R into a flat (respectively projective) right Smodule. Then the functor pairmod- R

o

f∗
f!

/

mod- Sis a Quillen adjoint functor pair with respect to the absolute and positive flat stable (respectively
absolute and positive projective stable) model structures on both sides. In particular, the restriction

136

III. MODEL STRUCTURES

of scalars f ∗ then takes flat (respectively projective) cofibrations of R-modules to flat (respectively
projective) cofibrations of S-modules.
(iv) If the homomorphism f : S −→ R is a stable equivalence, then the adjoint functor pairs (f∗ , f ∗ )and (f ∗ , f! ) are a Quillen equivalences in all the cases when they are Quillen adjoint functors.
Proof. (i) In each case, the weak (i.e., stable) equivalences and the various kinds of fibrations aredefined on underlying symmetric spectra, hence the restriction functor preserves fibrations and acyclic
fibrations. By adjointness, the extension functor preserves cofibrations and trivial cofibrations.
(iv) If f : S −→ R is a stable equivalence, then for every flat right S-module N the morphismN ∼= N ∧S S −→ N ∧S R = f∗ N
is a stable equivalence. Thus if Y is a fibrant left R-module, an S-module map N −→ Y is a weakequivalence if and only if the adjoint R-module map f∗ N −→ Y is a weak equivalence.

Example 3.5 (Modules over Eilenberg-Mac Lane spectra). For every ring A we have an associatedEilenberg-Mac Lane ring spectrum, see Example I.2.7. This symmetric spectrum arises from a Γ-spaceby evaluation on spheres, so it is flat as a symmetric spectrum (Proposition II.5.18). Hence all six model
structure of Theorem 3.2 are defined on the category of HA-modules, and they are Quillen-equivalent toeach other.
The homotopy category of HA-modules can be described purely algebraically in terms of A-modules.
More precisely, the stable model structures of HA-modules are Quillen equivalent to the category of chain
complexes of A-modules in any of the model structures which have the quasi-isomorphisms as weak equivalences. In particular, we get an equivalence of triangulated categories
Ho(mod- HA) ∼= D(A)
to the unbounded derived category of the ring A.
Exercises
Exercise 4.1. Let R by a symmetric ring spectrum. We define an R-bimodule R̄ by(∗
for n = 0R̄n =Rn for n ≥ 1.
R
We define the n-latching object LR
n M of a right R-module M by Ln M = (M ∧R R̄)n . The latching object
has a left action of the symmetric group Σn and a right action of the pointed monoid R0 . The inclusionR̄ −→ R is a morphism of R-bimodules and thus induces a morphism of Σn -R0 simplicial bisetsνn : LR M = (M ∧R R̄)n −→ (M ∧R R)n ∼= Mn .
nShow:(i) A morphism f : M −→ N is a flat cofibration of R-modules if and only if the maps νn (f ) :Mn −→ Nn are cofibrations of right R0 -simplicial sets.
LRn N ∪LRnM(ii) A morphism f : M −→ N is a projective cofibration of R-modules if and only if the maps
νn (f ) : LR
Mn −→ Nn are cofibrations of Σn -R0 -simplicial bisets.
n N ∪LRnM(Hint: define a suitable R-module analog of the filtration F m A of a symmetric spectrum A so that theproof of Proposition II.5.10 can be adapted.)
History and credits
The projective and injective level and stable model structures for symmetric spectra are constructed in
the original paper [25] of Hovey, Shipley and Smith. The flat model structures show up in the literature
under the name of S-model structure. (the ‘S’ refers to the sphere spectrum). The cofibrant objects in thismodel structure (which we call ‘flat’ and Hovey, Shipley and Smith call ‘S-cofibrant’) and parts of the model
structures show up in [25] and in [52], but the first verification of the full model axioms appears in Shipley’s
paper [58]. I prefer the term ‘flat’ model structure because the cofibrant objects are very analogous to flat

HISTORY AND CREDITS

137

modules in algebra and because we can then also use the term ‘flat model structure’ for modules over asymmetric ring spectrum. Shipley [58] calls the flat model structure for modules over a symmetric ring
spectrum R the ‘R-model structure’.

APPENDIX A

1. Tools from model category theory
1.1. Cofibrantly generated model categories and a lifting theorem. In this section we review
cofibrantly generated model categories and a general method for creating model category structures. If a
model category is cofibrantly generated, its model category structure is completely determined by a set of
cofibrations and a set of acyclic cofibrations. The transfinite version of Quillen’s small object argument
allows functorial factorization of maps as cofibrations followed by acyclic fibrations and as acyclic cofibrations followed by fibrations. Most of the model categories in the literature are cofibrantly generated, e.g.
topological spaces and simplicial sets, as are all model structures involving symmetric spectra which we
discuss in this book.
The only complicated part of the definition of a cofibrantly generated model category is formulating
the definition of relative smallness. For this we need to consider the following set theoretic concepts. The
reader might keep in mind the example of a compact topological space which is ℵ0 -small relative to closed
inclusions.
Ordinals and cardinals. An ordinal γ is an ordered isomorphism class of well ordered sets; it can be
identified with the well ordered set of all preceding ordinals. For an ordinal γ, the same symbol will denotethe associated poset category. The latter has an initial object ∅, the empty ordinal. An ordinal κ is a
cardinal if its cardinality is larger than that of any preceding ordinal. A cardinal κ is called regular if forevery set of sets {Xj }j∈J indexed by a set J of cardinalitySless than κ such that the cardinality of each Xj
is less than that of κ, then the cardinality of the union J Xj is also less than that of κ. The successor
cardinal (the smallest cardinal of larger cardinality) of every cardinal is regular.
Transfinite composition. Let C be a cocomplete category and γ a well ordered set which we identify
with its poset category. A functor V : γ −→ C is called a γ-sequence if for every limit ordinal β < γ thenatural map colimV |β −→ V (β) is an isomorphism. The map V (∅) −→ colimγ V is called the transfinite
composition of the maps of V . A subcategory C1 ⊂ C is said to be closed under transfinite composition
if for every ordinal γ and every γ-sequence V : γ −→ C with the map V (α) −→ V (α + 1) in C1 for every
ordinal α < γ, the induced map V (∅) −→ colimγ V is also in C1 . Examples of such subcategories are thecofibrations or the acyclic cofibrations in a closed model category.
Relatively small objects. Consider a cocomplete category C and a subcategory C1 ⊂ C closed undertransfinite composition. If κ is a regular cardinal, an object C ∈ C is called κ-small relative to C1 if forevery regular cardinal λ ≥ κ and every functor V : λ −→ C1 which is a λ-sequence in C, the map
colimλ HomC (C, V ) −→ HomC (C, colimλ V )is an isomorphism. An object C ∈ C is called small relative to C1 if there exists a regular cardinal κ suchthat C is κ-small relative to C1 .
I-injectives, I-cofibrations and I-cell complexes. Given a cocomplete category C and a class I of maps,
we denote• by I-inj the class of maps which have the right lifting property with respect to the maps in I.
Maps in I-inj are referred to as I-injectives.
• by I-cof the class of maps which have the left lifting property with respect to the maps in I-inj.
Maps in I-cof are referred to as I-cofibrations.
139

140

A

• by I-cell ⊂ I-cof the class of the (possibly transfinite) compositions of pushouts (cobase changes)of maps in I. Maps in I-cell are referred to as I-cell complexes.
In [46, p. II 3.4] Quillen formulates his small object argument, which immediately became a standard
tool in model category theory. In our context we will need a transfinite version of the small object argument,
so we work with the ‘cofibrantly generated model category’, which we now recall. Note that here I has tobe a set, not just a class of maps. The obvious analogue of Quillen’s small object argument would seem torequire that coproducts are included in the I-cell complexes. In fact, any coproduct of an I-cell complex isalready an I-cell complex, see [24, 2.1.6].
Lemma 1.1. Let C be a cocomplete category and I a set of maps in C whose domains are small relativeto I-cell. Then
• there is a functorial factorization of any map f in C as f = qi with q ∈ I-inj and i ∈ I-cell andthus• every I-cofibration is a retract of an I-cell complex.
Definition 1.2. A model category C is called cofibrantly generated if it is complete and cocomplete
and there exists a set of cofibrations I and a set of acyclic cofibrations J such that• the fibrations are precisely the J-injectives;• the acyclic fibrations are precisely the I-injectives;• the domain of each map in I (resp. in J) is small relative to I-cell (resp. J-cell).
Moreover, here the (acyclic) cofibrations are the I (J)-cofibrations.
For a specific choice of I and J as in the definition of a cofibrantly generated model category, themaps in I (resp. J) will be referred to as generating cofibrations (resp. generating acyclic cofibrations).
In cofibrantly generated model categories, a map may be functorially factored as an acyclic cofibration
followed by a fibration and as a cofibration followed by an acyclic fibration.
Definition 1.3. Let C be a model category
R : D −→ C
a functor. We say that R creates a model structure on the category D if the following definitions make D
into a model category: a morphism f in D is a
• weak equivalence if the morphism R(f ) is a weak equivalence in C,• fibration if the morphism R(f ) is a fibration in C,• cofibration if it has the left lifting property with respect to all morphisms in D which are both
fibrations and weak equivalences.
Theorem 1.4. Let C be a model category, D a category which is complete and cocomplete and let
R : D −→ C : L
be a pair of adjoint functors such that R commutes with filtered colimits. Let I (J) be a set of generating
cofibrations (resp. acyclic cofibrations) for the cofibrantly generated model category C. Let LI (resp. LJ) be
the image of these sets under the left adjoint L. Assume that the domains of LI (LJ) are small relative to
LI-cell (LJ-cell). Finally, suppose every LJ-cell complex is a weak equivalence. Then R : D −→ C creates amodel structure on D which is cofibrantly generated with LI (LJ) a generating set of (acyclic) cofibrations.
If the model category C is right proper, then so is the model structure on D.
If C and D are simplicially enriched, the adjunction (L, R) is simplicial, and the model structure of C
is simplicial, then the model structure on D is again simplicial.
If C and D are topologically enriched, the adjunction (L, R) is continuous, and the model structure of
C is topological, then the model structure on D is again topological.

1. TOOLS FROM MODEL CATEGORY THEORY

141

Proof. Model category axiom MC1 (limits and colimits) holds by hypothesis. Model category axioms
MC2 (saturation) and MC3 (closure properties under retracts) are clear. One half of MC4 (lifting properties)
holds by the definition of cofibrations in D.
The proof of the remaining axioms uses the transfinite small object argument (Lemma 1.1), whichapplies because of the hypothesis about the smallness of the domains. We begin with the factorization
axiom, MC5. Every map in LI and LJ is a cofibration in D by adjointness. Hence every LI-cofibration or
LJ-cofibration is a cofibration in D. By adjointness and the fact that I is a generating set of cofibrations
for C, a map is LI-injective precisely when the map becomes an acyclic fibration in C after application ofR, i.e., an acyclic fibration in D. Hence the small object argument applied to the set LI gives a (functorial)
factorization of any map in D as a cofibration followed by an acyclic fibration.
The other half of the factorization axiom, MC5, needs the hypothesis. Applying the small object
argument to the set of maps LJ gives a functorial factorization of a map in D as an LJ-cell complex
followed by a LJ-injective. Since J is a generating set for the acyclic cofibrations in C, the LJ-injectives are
precisely the fibrations among the D-morphisms, once more by adjointness. We assume that every LJ-cellcomplex is a weak equivalence. We noted above that every LJ-cofibration is a cofibration in D. So we seethat the factorization above is an acyclic cofibration followed by a fibration.
It remains to prove the other half of MC4, i.e., that any acyclic cofibration A −→ B in D has the left
lifting property with respect to fibrations. In other words, we need to show that the acyclic cofibrations arecontained in the LJ-cofibrations. The small object argument provides a factorization
A −→ W −→ B
with A −→ W a LJ-cofibration and W −→ B a fibration. In addition, W −→ B is a weak equivalencesince A −→ B is. Since A −→ B is a cofibration, a lifting in
/W
A
>

B

∼


B

exists. Thus A −→ B is a retract of a LJ-cofibration, hence it is a LJ-cofibration.



In cofibrantly generated model categories fibrations can be detected by checking the right lifting property
against a set of maps, the generating acyclic cofibrations, and similarly for acyclic fibrations. This is incontrast to general model categories where the lifting property has to be checked against the whole class ofacyclic cofibrations. Similarly, in cofibrantly generated model categories, the pushout product axiom andthe monoid axiom only have to be checked for a set of generating (acyclic) cofibrations:Lemma 1.5. Let C be a cofibrantly generated model category endowed with a closed symmetric monoidal
structure. If the pushout product axiom holds for a set of generating cofibrations and a set of generating
acyclic cofibrations, then it holds in general.
Proof. For the first statement consider a map i : A −→ B in C. Denote by G(i) the class of maps
j : K −→ L such that the pushout productA ∧ L ∪A∧K B ∧ K −→ B ∧ L
is a cofibration. This pushout product has the left lifting property with respect to a map f : X −→ Y if
and only if j has the left lifting property with respect to the map
p : [B, X] −→ [B, Y ] ×[A,Y ] [A, X].
Hence, a map is in G(i) if and only if it has the left lifting property with respect to the map p for all
f : X −→ Y which are acyclic fibrations in C.
G(i) is thus closed under cobase change, transfinite composition and retracts. If i : A −→ B isa generating cofibration, G(i) contains all generating cofibrations by assumption; because of the closure
properties it thus contains all cofibrations, see Lemma 1.1. Reversing the roles of i and an arbitrary

142

A

cofibration j : K −→ L we thus know that G(j) contains all generating cofibrations. Again by the closure
properties, G(j) contains all cofibrations, which proves the pushout product axiom for two cofibrations.
The proof of the pushout product being an acyclic cofibration when one of the constituents is, follows in
the same manner.

We now spell out the small object argument for symmetric spectra.
Theorem 1.6 (Small object argument). Let I be a set of morphisms of symmetric spectra based on
simplicial sets. Then there exists a functorial factorization of morphisms as I-cell complexes followed by
I-injective morphisms.
Proof. In the first step we construct a functor F from the category of morphisms of symmetric spectra
to symmetric spectra as follows. Given a morphism f : X −→ Y and a morphism i : Si −→ Ti in the set I
we let Di denote the set of all pairs (a : Si −→ X, b : Ti −→ Y ) of morphisms satisfying f a = bi, i.e., whichmake the square
a /
Si
X
f

i


Ti

b


/Y

commute. We define F (f ) as the pushout in the diagram
W W∨ai∈I Di Si

/X
j

∨i

W

i∈I


W


/ F (f )Di T i

The morphisms b : Ti −→ Y and f : X −→ Y glue to a morphism p : F (f ) −→ Y such that pj = f .
The factorization we are looking for is now obtained by iterating this construction infinitely often, possiblytransfinitely many times.
jnpn
We define functors F n : Ar(SpΣ ) −→ SpecΣ and natural transformations X −→ F n (f ) −→ Y for every
ordinal n by transfinite induction. We start with F 0 (f ) = X, j0 = Id and p0 = f . For successor ordinal weset F n+1 (f ) = F (pn : F n (f ) −→ Y ) with the morphisms jn+1 = j ◦ jn respectively pn+1 = p(pn ). For limit
ordinals λ we set F λ (f ) = colimµ<λ F µ (f ) with morphisms induced by the jµ and pµ . By construction, allmorphisms jn : X −→ F n (f ) are I-cell complexes.
We claim that there exists a limit ordinal κ, depending on the set I, such that for every morphism f
the map pκ : F κ (f ) −→ Y is I-injective. Then f = pκ jκ is the required factorization.
We prove the claim under the simplifying hypothesis that for each morphism i ∈ I the source Si isfinitely presented as a symmetric spectrum, i.e., for every sequence Z0 −→ Z1 −→ Z2 −→ . . . the naturalmapcolimn SpΣ (Si , Zn ) −→ SpΣ (Si , colimn Zn )is bijective. In that case, the first infinite ordinal ω will do the job. Indeed, F ω (f ) is the colimit over thesequencej1

j2

X = F 0 (f ) −→ F 1 (f ) −→ F 2 (f ) · · · .
Given a morphism i ∈ I and a lifting problem
(1.7)

Si

/ F ω (f )

a

pω

i


Ti

b


/Y

1. TOOLS FROM MODEL CATEGORY THEORY

143

there exists a factorization a = can for some n ≥ 0 and some morphism an : Si −→ F n (f ) since Si is finitely
presented (where c : F n (f ) −→ F ω (f ) is the canonical morphism to the colimit). The commutative squareanSi/ F n (f )pω c=pn

i


Ti


/Y

b

is an element in the set Di which is used to define F n+1 (f ) = F (pn ). Thus the canonical morphism
C : Ti −→ F n+1 (f ) makes the diagram/ F n+1 (f )
;
ww
ww
pn+1
i
w
w
 www C

/Y
Ti
b
an

Si

commute. Then the composite of C with the canonical morphism F n+1 (f ) −→ F ω (f ) solves the lifting
problem (1.7).

1.2. Bousfield’s localization theorem.
Theorem 1.8 (Bousfield). Let C be a proper model category with a functor Q : C −→ C and a naturaltransformation α : 1 −→ Q such that the following three axioms hold:(A1) if f : X −→ Y is a weak equivalence, then so is Qf : QX −→ QY ;(A2) for each object X of C, the maps αQX , QαX : QX −→ QQX are weak equivalences;(A3) for a pullback square
V

k

/X

g


W

h


/Y

f

in C, if f is a fibration between fibrant objects such that α : X −→ QX, α : Y −→ QY and Qh : QW −→ QY
are weak equivalences, then Qk : QV −→ QX is a weak equivalence.
Then the following notions define a proper model structure on C: a morphisms f : X −→ Y is a
Q-cofibration if and only if it is a cofibration, a Q-equivalence if and only if Qf : QX −→ QY is a weakequivalence, and Q-fibration if and only if f is a fibration and the commutative square
X

α

Qf

f


Y

/ QX

α


/ QY

is homotopy cartesian.
The reference is [9, Thm. 9.3].
1.3. Some equivariant homotopy theory.
Proposition 1.9. Let G be a finite groups and f : X −→ Y a morphism of G-simplicial sets. Thenthe following are equivalent.
(i) The morphism f has the right lifting property for all injective morphism of G-simplicial sets which
are weak equivalences of underlying simplicial sets.

144

A

(ii) The morphism f is a G-fibration and the commutative square
X
f


Y

/ map(EG, X)
map(EG,f )/ map(EG, Y )is G-homotopy cartesian. Here map(EG, X) is the simplicial mapping space of all maps from thecontractible free G-simplicial set to X, with G-action by conjugation.
Moreover, the following are equivalent:(a) f is a weak equivalence of underlying simplicial set and has the equivalent properties (i) and (ii)above.
(b) f is a G-acyclic fibration, i.e., induced a weak equivalence and Kan fibration on fixed points forall subgroups of G.
Proof. (i)⇒(ii) If H is a subgroup of G then for all n ≥ 0 and 0 ≤ i ≤ n the map G/H × Λi [n] −→G/H × ∆[n] is an injective G-morphism and weak equivalence on underlying simplicial sets. So f has theright lifting property with respect to it which means by adjunction that the induced morphism on H-fixed
points f H : X H −→ Y H has the right lifting property for all horn inclusions, i.e., is a Kan fibration. So f
is G-fibration.
Now we claim that if f has the right lifting property of (i), then for every injective G-morphism K −→ L
which is an underlying weak equivalence, the induced map(1.10)

map(L, X) −→ map(K, X) ×map(K,Y ) map(L, Y )is a G-fibration and G-equivalence. To prove this, we note that the pushout product morphism
G/H × (L × ∂∆[n] ∪K×∂∆i [n] K × ∆[n]) −→ G/H × L × ∆[n]is injective, equivariant and an underlying weak equivalence for all subgroups H of G and all boundary
inclusions. Since f : X −→ Y has the RLP for such maps, by adjointness the H-fixed points of themap (1.10) have the right lifting property for all boundary inclusions, so they are weak equivalences and
Kan fibrations. Since this holds for all subgroups H of G, the map (1.10) is a G-acyclic fibration.
Now we show that the square of property (ii) is G-homotopy cartesian. Since f is G-fibration, so is themorphism map(EG, f ), and hence it suffices to show that the morphism
X −→ Y ×map(EG,Y ) map(EG, X)
is a G-weak equivalence. The inclusion EG −→ C(EG) of EG into its cone is G-equivariant and an
injective weak equivalence of underlying simplicial sets (but not a G-weak equivalence !). So by the previous
paragraph the induced morphism
map(C(EG), X) −→ map(EG, X) ×map(EG,Y ) map(C(EG), Y )is a G-acyclic fibration. In the commutative square
X = map(∗, X)/ map(EG, X) ×map(EG,Y ) map(∗, Y )map(C(EG), X)/ map(EG, X) ×map(EG,Y ) map(C(EG), Y )the vertical maps are induced by the unique morphism C(EG) −→ ∗ which is a G-equivariant homotopy
equivalence, so induces a homotopy equivalence on mapping spaces. So the top horizontal map is a G-weakequivalence since the other three maps are.

2. TRIANGULATED CATEGORIES

145(ii)⇒(i) Let i : K −→ L be a monomorphism of pointed G-simplicial sets which is a weak equivalenceof underlying simplicial sets. Then i is a cofibration in the strong G-model structure. Since that model
structure is monoidal, the induced map
map(L, X) −→ map(K, X) ×map(K,Y ) map(L, Y )is a G-fibration. We show that it is also a G-weak equivalence, thus a G-acyclic fibration. Since the squareis G-homotopy cartesian, we can replace the G-fibration f : X −→ Y be the G-fibration map(EG, f ) andshow that the G-fibration.
map(L, map(EG, X)) −→ map(K, map(EG, X)) ×map(K,map(EG,Y )) map(L, map(EG, Y ))is a G-weak equivalence. This map is isomorphic tomap(L × EG, X) −→ map(K × EG, X) ×map(K×EG,Y ) map(L × EG, Y ) .
What we have gained now is that the morphism i × Id : K × EG −→ L × EG is a G-equivariant weakequivalence between free G-simplicial sets, thus a G-weak equivalence. So the latter morphism is a G-acyclic
fibration by the adjoint of the pushout product property.
By taking G-fixed points we then get an acyclic fibration of simplicial sets
mapG (L, X) −→ mapG (K, X) ×mapG (K,Y ) mapG (L, Y )which is in particular surjective on vertices. This exactly means that f : X −→ Y has the right lifting
property with respect to i : K −→ L.
(a)⇒(b) The map f is a G-fibration by assumption (ii), so we need to show that it is also a G-weakequivalence. We show that for every free G-simplicial set L the induced map map(L, f ) : map(L, X) −→map(L, Y ) is a G-acyclic fibration. We first show this for finite-dimensional L by induction over thedimension of L. We start the induction with L = ∅ being empty, when there is nothing to show. If L has
dimension n ≥ 0 then there exists a pushout square of G-simplicial sets
X × ∂∆[n]/ X × ∆[n]


L0


/L

where L0 has strictly smaller dimension than L and X is a free G-set. [...] In the general case of an arbitrary
free G-simplicial set L we use that L is the union of its simplicial skeleta, for which we have proved theclaim. So the morphism map(L, f ) : map(L, X) −→ map(L, Y ) is the inverse limit of a tower of G-acyclic
fibrations, thus itself a G-acyclic fibration.
Now we specialize the above to the free G-simplicial set L = EG. We deduce that map(EG, f ) :map(EG, X) −→ map(EG, Y ) is a G-acyclic fibration. Since the square of condition (ii) is G-homotopy
cartesian, the map f : X −→ Y is then also a G-weak equivalence.
(b)⇒(a) If f is a G-acyclic fibration, then so is map(EG, X). Thus the square of condition (ii) is
G-homotopy cartesian.
[remark about Shipley’s mixed model structure]
2. Triangulated categories
Let T be a category equipped with an endofunctor [1] : T −→ T . A triangle in T (with respect tothe functor [1]) is a triple (f, g, h) of composable morphisms in T such that the target of h is equal to [1]applied to the source of f . We will often display a triangle in the form
f

g

h

A−
→B−
→C−
→ A[1] .

146

A

A morphism from a triangle (f, g, h) to a triangle (f 0 , g 0 , h0 ) is a triple of morphisms α : A −→ A0 , β : B −→B 0 and γ : C −→ C 0 in T such that the diagram
A

/B

f

α

β


A0


/ B0

f0

g

/C

h

γ

g0


/ C0

/ A[1]α[1]h0
/ A0 [1]commutes. A morphism of triangles is an isomorphism (i.e., has an inverse morphism) if and only all threecomponents are isomorphisms in T .
Definition 2.1. A triangulated category is an additive category T together with a self-equivalence[1] : T −→ T and a collection of triangles, called distinguished triangles which satisfy the following axioms(T1), (T2) and (T3).
(T1)
Id(a) For every object X and every zero object 0 the triangle X −→ X −→ 0 −→ X[1] is distinguished.
(b) Every morphism f is part of a distinguished triangle (f, g, h).
(c) Any triangle which is isomorphic to a distinguished triangle is itself distinguished.
(T2) Distinguished triangles can be rotated: if a triangle (f, g, h) is distinguished, then so is the triangle(g, h, −f [1]).
(TR3) [Octahedral axiom] Consider distinguished triangles (f1 , g1 , h1 ), (f2 , g2 , h2 ) and (f3 , g3 , h3 ) suchthat f1 and f2 are composable and f3 = f2 f1 . Then there exist morphisms x and y such that (x, y, g1 [1]◦h2 )is a distinguished triangle and the following diagram commutes
A

f1

/B

g1

/C

A

f3

g3


/E

g2

h3

y


F


F
h2


B[1]

/ A[1]

x

f2


/D

h1

/ A[1]
f1 [1]h2/ B[1]

g1 [1]◦h2

g1 [1]

/ C[1]

This formulation of the axioms is due to May [44] who noticed a redundancy in Verdier’s originalaxioms [63]. So the axioms are seemingly weaker, but in fact equivalent to the ones of Verdier [63]. There
are two differences: Verdier’s formulation (TR2) asks that distinguished triangles can be rotated in both
directions, where (T2) above only asks for one direction; the extra implication is part (v) of the following
proposition. Verdier has another axiom (TR3) which is the content of part (i) of the following proposition.
For the convenience of the reader we recall various useful standard facts about distinguished triangles,
including Verdier’s stronger forms of the axioms.
Proposition 2.2. Let T be a triangulated category in the sense of Definition 2.1. Then the following
two properties hold.

2. TRIANGULATED CATEGORIES

147(i) Consider two distinguished triangles (f, g, h) and (f 0 , g 0 , h0 ). Any pair (α, β) of morphisms satisfying βf = f 0 α can be extended to a morphism of triangles, i.e., there exists a morphism γ making
the following diagram commute
A

/B

f

α

β


A0


/ B0

f0

g

/C

/ A[1]

h

γ

g0


/ C0

α[1]

h0


/ A0 [1](ii) For every distinguished triangle (f, g, h) and every object X of T , the two sequences of abeliangroupsT (X,f )T (X,g)T (X,h)T (X, A) −−−−−→ T (X, B) −−−−→ T (X, C) −−−−−→ T (X, A[1])andT (h,X)

T (g,X)

T (f,X)

T (A[1], X) −−−−−→ T (C, X) −−−−−→ T (B, X) −−−−−→ T (A, X)
are exact.
(iii) Let (α, β, γ) be a morphism of distinguished triangles. If two out of the three morphisms are
isomorphisms, then so is the third.
(iv) A triangle (f, g, h) is distinguished if and only if the triangle (−f [1], −g[1], −h[1]) is distinguished.
(v) Distinguished triangles can be rotated to the left: given any triangle (f, g, h) such that (g, h, −f [1])is distinguished, then so is the original triangle (f, g, h).
Proof. (i) [...](ii) We start by showing exactness of the first sequence at T (X, B). By part (i) applied to the pair(Id, f ) there is a (necessarily unique) morphism from any zero object to C such that the diagram
Id

A

/A

/0

/ A[1]

f

A

f


/B

g


/C

h

/ A[1]

commutes (the top row is distinguished by (T1 a)). So gf = 0 and thus the image of T (X, f ) is containedin the kernel of T (X, g) for every object X.
Conversely, let ψ : X −→ B be a morphism in the kernel of T (X, g), i.e., such that gψ = 0. Applying
part (i) to the pair (ψ, 0) gives a morphism ϕ̄ : X[1] −→ A[1] such that the diagramX/0/ X[1]/C/ A[1]

ϕ̄

ψ


B

− Id

g

h

/ X[1]
ψ[1]−f [1]

/ B[1]

commutes (both rows are distinguished by (T1 a) and (T2)). Since shifting is full, there exists a morphism
ϕ : X −→ A such that ϕ̄ = ϕ[1], and since shifting is faithful we have f ϕ = ψ, so ψ is in the image ofT (X, f ). Altogether, the first sequence is exact at T (X, B). If we apply this to the triangle (g, h, −f [1])(which is distinguished by (T2)), we deduce that the first sequence is also exact at T (X, C). Exactness ofthe second sequence is similar, using part (i) for suitable maps out of the distinguished triangle (f, g, h)and its rotations.

148

A

(iii) We first treat the case where α and β are isomorphisms. If X is any object of T we have acommutative diagram
T (X, A)f∗α∗
T (X, A0 )/ T (X, B)/ T (X, C)

g∗

γ∗

β∗

f∗0


/ T (X, B 0 )/ T (X, A[1]) (−f [1])∗ / T (X, B[1])h∗/ T (X, C 0 )

g∗0



α[1]∗

/ T (X, A0 [1])

h0∗

β[1]∗

(−f 0 [1])∗/ T (X, B 0 [1])where we write f∗ for T (X, f ), etc. The top row is exact by part (ii) applied to the triangles (f, g, h) and(g, h, −f [1]), which are distinguished by hypothesis, respectively axiom (T2). Similarly, the bottom row isexact. Since α and β (and hence α[1] and β[1]) are isomorphisms, all vertical maps except possibly the
middle one are isomorphisms of abelian groups. So the five lemma says that γ∗ is an isomorphisms. Since
this holds for all objects X, the morphism γ : C −→ C 0 is an isomorphism.
If β and γ are isomorphisms, we apply the same argument to the triple (β, γ, α[1]). This is a morphismfrom the distinguished (by (T2)) triangle (g, h, −f [1]) to the distinguished triangle (g 0 , h0 , −f 0 [1]). By theabove, α[1] is an isomorphism, hence so is α since shifting is an equivalence of categories. The third case issimilar.
(iv) One direction is a direct consequence of the axioms: if (f, g, h) is distinguished, then so is(−f [1], −g[1], −h[1]) by three applications of (T2). Now suppose that (−f [1], −g[1], −h[1]) is distinguished.
Axiom (T1 b) let’s us choose a distinguished triangle
f

ḡ

h̄

A−
→B−
→ C̄ −→ A[1]
and by the first sentence, the triangle (−f [1], −ḡ[1], −h̄[1]) is distinguished. By (i) there is a morphismγ̄ : C[1] −→ C̄[1] such that the diagramA[1]−f [1]/ B[1] −g[1] / C[1] −h[1] / A[2]
γ̄

A[1]

−f [1]/ B[1]−ḡ[1]

/ C̄[1]−h̄[1]/ A[2]

commutes. By part (iii), γ is an isomorphism. Since shifting is an equivalence of categories, we have γ̄ = γ[1]for a unique isomorphism γ : C −→ C̄. Thus (f, g, h) is isomorphic to the distinguished triangle (f, ḡ, h̄),so it is itself distinguished by axiom (T1 c).
(v) If (g, h, −f [1]) is distinguished, then so is (−f [1], −g[1], −h[1]) by two applications of (T2). So(f, g, h) is distinguished by part (v).
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Index

B(n), 78
BO, 33
BP , 16, 78
BP hni, 78
BSO, 33
BSU , 33
BSp, 33
BSpin, 33
BU , 33
CA, 51
E(n), 78
HA, 37I, 31
I-cell complex, 140
I-functor, 55
I-space, 31
K-theory
algebraic, 21
Morava, 78
topological, 20
K(n), 78
KO, 20
KU , 20
L .m X, 26, 46, 61
Ln A, 115
M O, 18
M SO, 19
M SU , 19
M Sp, 19
M Spin, 19
M U , 19, 40
M U P , 40
M ∧R N , 133
P (n), 78
P X, 49RX, 66
R[1/x], 34, 75
R[M ], 32
R∞ X, 66
Rop , 71
S-modules, 40
S· -construction, 21
T X, 49
W (1), 63
X[1], 89Γ, 23, 37

Γ-space, 37very special, 38
Hom(X, Y ), 29
HomR (M, N ), 133
M, 55, 56, 58
M-module, 55Ω-spectrum, 11
positive, 11
Pn , 60
S(X), 10T, 4
S̄, 115
χn,m , 8diagi X i , 30η, 15γ, 103λX , 26
S[M ], 37
S, 14, 85
S[1/m], 34S[k] , 116map(X, Y ), 28
νn (f ), 126
ρx , 72
SHC, 86
telN X i , 30
f ∗ , 135f∗ , 135
f! , 135k(n), 78
assembly map, 36, 120
bimorphism, 42
Bott periodicity theorem, 21
Brown-Peterson spectrum, 16, 78
category with cofibrations and weak equivalences, 21
coextension of scalars, 135
cofibrantly generated, 140
cofibration
flat, 126level, 126
projective, 126
colimitof symmetric spectra, 24
cone, 51151

152

connecting homomorphism, 51
continuous functor, 36
diagonal, 30
distinguished triangle
elementary, 94
in the stable homotopy category, 94
Eilenberg-Mac Lane spectrum, 18, 37, 121, 136of an M-module, 61
endomorphism ring spectrum, 29
excisionfor stable homotopy groups, 50
extension of scalars, 135
fibrationflat, 128
injective, 93, 129
projective, 128
stable flat, 131
stable injective, 132
stable projective, 131
function spectrum, 27, 29
homotopyof spectrum morphisms, 85
homotopy fibre, 51
homotopy group, 8of a shift, 26
of a spectrum, 85
of a suspension, 25
of loops, 28of spheres, 15
homotopy groups
of a symmetric ring spectrum, 69
true, 107
homotopy ring spectrum, 114
Hopf map, 15
indeterminacy
of a Toda bracket, 79
injection monoid, 55
action on F (ω), 56
Johnson-Wilson spectrum, 78
juggling formula
for Toda brackets, 79
killhomotopy class, 72
regular sequence, 74
latching space, 115
levelof a symmetric spectrum, 7
limit
of symmetric ring spectra, 25of symmetric spectra, 24
localizationof a category, 104
long exact sequenceof homotopy groups, 52loop, 63

INDEX

loop spectrum, 28
map, 29
mapping cone, 50
mapping space, 28
mapping telescope, 30
matrix ring spectrum, 33
model category
cofibrantly generated, 140
model structure
creating, 140
for R-modules, 134
level, 128, 131
stable, 131
moduleover a symmetric ring spectrum, 8monoid ring spectrum, 32
Moore spectrum
mod-p, 114
Morava K-theory, 78morphismof module spectra, 8of symmetric ring spectra, 8of symmetric spectra, 7of triangles, 146
multiplication
in homotopy of symmetric ring spectrum, 69
in stable stems, 16
orthogonal ring spectrum, 38
orthogonal spectrum, 38
coordinate free, 39
pushout product, 130
regular ideal, 74
regular sequence, 74
restriction of scalars, 135
ring spectrum
up to homotopy, 114
roots of unity, 35
shift, 26, 63, 88, 89
simplicial functor, 36
small object argument, 140smash productderived, 112
of R-modules, 133of a space and symmetric spectrum, 25
of an I-space and symmetric spectrum, 31
of symmetric ring spectra, 49of symmetric spectra, 41
twisted, 26
with semifree symmetric spectrum, 46
sphere spectrum, 14, 85
stable equivalence, 98, 131
stable homotopy category, 86
universal property, 105stable homotopy groupof a space, 18of spheres, 15
stable stem, 15

INDEX

suspension, 25, 63
suspension spectrum, 18
symmetric algebraof a symmetric spectrum, 49
symmetric ring spectrum, 7commutative, 8, 72
implicit, 48
opposite, 71
symmetric spectrum, 7
co-free, 86
co-semifree, 86
coordinate free, 11flat, 114
free, 24, 62
injective, 86of simplicial sets, 9
projective, 126
semifree, 24, 62, 114
semistable, 66
tensor algebraof a symmetric spectrum, 49
Thom spectrum, 18, 19
Toda bracket, 74, 79
transfinite composition, 139
triangle, 145
triangulated category, 146unit maps, 7, 9
unitary spectrum, 39
universal propertyof smash product, 42

153