# Proper morphism

In algebraic geometry, a proper morphism between schemes is an analog of a proper map between complex analytic spaces.
Some authors call a proper variety over a field $k$ a complete variety. For example, every projective variety over a field $k$ is proper over $k$. A scheme $X$ of finite type over the complex numbers (for example, a variety) is proper over C if and only if the space $X$(C) of complex points with the classical (Euclidean) topology is compact and Hausdorff.
A closed immersion is proper. A morphism is finite if and only if it is proper and quasi-finite.

## Definition

A morphism $f:X\to Y$ of schemes is called universally closed if for every scheme $Z$ with a morphism $Z\to Y$, the projection from the fiber product
    $X\times _{Y}Z\to Z$
is a closed map of the underlying topological spaces. A morphism of schemes is called proper if it is separated, of finite type, and universally closed ([EGA] II, 5.4.1 ). One also says that $X$ is proper over $Y$. In particular, a variety $X$ over a field $k$ is said to be proper over $k$ if the morphism $X\to \operatorname {Spec} (k)$ is proper.

## Examples

For any natural number n, projective space P$^n$ over a commutative ring R is proper over R. Projective morphisms are proper, but not all proper morphisms are projective. For example, there is a smooth proper complex variety of dimension 3 which is not projective over C. Affine varieties of positive dimension over a field k are never proper over k. More generally, a proper affine morphism of schemes must be finite. For example, it is not hard to see that the affine line A$^1$ over a field k is not proper over k, because the morphism A$^1$ → Spec(k) is not universally closed. Indeed, the pulled-back morphism
    $\mathbb {A} ^{1}\times _{k}\mathbb {A} ^{1}\to \mathbb {A} ^{1}$
(given by (x,y) ↦ y) is not closed, because the image of the closed subset xy = 1 in A$^1$ × A$^1$ = A$^2$ is A$^1$ − 0, which is not closed in A$^1$.

## Properties and characterizations of proper morphisms

In the following, let f: X → Y be a morphism of schemes.
* The composition of two proper morphisms is proper.
* Any base change of a proper morphism f: X → Y is proper. That is, if g: Z → Y is any morphism of schemes, then the resulting morphism X ×$_Y$ Z → Z is proper.
* Properness is a local property on the base (in the Zariski topology). That is, if Y is covered by some open subschemes Y$_i$ and the restriction of f to all f$^{−1}$(Y$_i$) is proper, then so is f.
* More strongly, properness is local on the base in the fpqc topology. For example, if X is a scheme over a field k and E is a field extension of k, then X is proper over k if and only if the base change X$_E$ is proper over E.
* Closed immersions are proper.
* More generally, finite morphisms are proper. This is a consequence of the going up theorem.
* By Deligne, a morphism of schemes is finite if and only if it is proper and quasi-finite. This had been shown by Grothendieck if the morphism f: X → Y is locally of finite presentation, which follows from the other assumptions if Y is noetherian.
* For X proper over a scheme S, and Y separated over S, the image of any morphism X → Y over S is a closed subset of Y. This is analogous to the theorem in topology that the image of a continuous map from a compact space to a Hausdorff space is a closed subset.
* The Stein factorization theorem states that any proper morphism to a locally noetherian scheme can be factored as X → Z → Y, where X → Z is proper, surjective, and has geometrically connected fibers, and Z → Y is finite.
* Chow's lemma says that proper morphisms are closely related to projective morphisms. One version is: if X is proper over a quasi-compact scheme Y and X has only finitely many irreducible components (which is automatic for Y noetherian), then there is a projective surjective morphism g: W → X such that W is projective over Y. Moreover, one can arrange that g is an isomorphism over a dense open subset U of X, and that g$^{−1}$(U) is dense in W. One can also arrange that W is integral if X is integral.
* Nagata's compactification theorem, as generalized by Deligne, says that a separated morphism of finite type between quasi-compact and quasi-separated schemes factors as an open immersion followed by a proper morphism.
* Proper morphisms between locally noetherian schemes preserve coherent sheaves, in the sense that the higher direct images R$^i$f$_∗$(F) (in particular the direct image f$_∗$(F)) of a coherent sheaf F are coherent (EGA III, 3.2.1). (Analogously, for a proper map between complex analytic spaces, Grauert and Remmert showed that the higher direct images preserve coherent analytic sheaves.) As a very special case: the ring of regular functions on a proper scheme X over a field k has finite dimension as a k-vector space. By contrast, the ring of regular functions on the affine line over k is the polynomial ring k[x], which does not have finite dimension as a k-vector space.
* There is also a slightly stronger statement of this:(EGA III, 3.2.4) let $f\colon X\to S$ be a morphism of finite type, S locally noetherian and $F$ a ${\mathcal {O}}_{X}$-module. If the support of F is proper over S, then for each $i\geq 0$ the higher direct image $R^{i}f_{*}F$ is coherent.
* For a scheme X of finite type over the complex numbers, the set X(C) of complex points is a complex analytic space, using the classical (Euclidean) topology. For X and Y separated and of finite type over C, a morphism f: X → Y over C is proper if and only if the continuous map f: X(C) → Y(C) is proper in the sense that the inverse image of every compact set is compact.
* If f: X→Y and g: Y→Z are such that gf is proper and g is separated, then f is proper. This can for example be easily proven using the following criterion.

## Valuative criterion of properness

There is a very intuitive criterion for properness which goes back to Chevalley. It is commonly called the valuative criterion of properness. Let f: X → Y be a morphism of finite type of Noetherian schemes. Then f is proper if and only if for all discrete valuation rings R with fraction field K and for any K-valued point x ∈ X(K) that maps to a point f(x) that is defined over R, there is a unique lift of x to ${\overline {x}}\in X(R)$. (EGA II, 7.3.8). More generally, a quasi-separated morphism f: X → Y of finite type (note: finite type includes quasi-compact) of 'any' schemes X, Y is proper if and only if for all valuation rings R with fraction field K and for any K-valued point x ∈ X(K) that maps to a point f(x) that is defined over R, there is a unique lift of x to ${\overline {x}}\in X(R)$. (Stacks project Tags 01KF and 01KY). Noting that Spec K is the generic point of Spec R and discrete valuation rings are precisely the regular local one-dimensional rings, one may rephrase the criterion: given a regular curve on Y (corresponding to the morphism s: Spec R → Y) and given a lift of the generic point of this curve to X, f is proper if and only if there is exactly one way to complete the curve.
Similarly, f is separated if and only if in every such diagram, there is at most one lift ${\overline {x}}\in X(R)$.
For example, given the valuative criterion, it becomes easy to check that projective space P$^n$ is proper over a field (or even over Z). One simply observes that for a discrete valuation ring R with fraction field K, every K-point [x$_0$,...,x$_n$] of projective space comes from an R-point, by scaling the coordinates so that all lie in R and at least one is a unit in R.

### Geometric interpretation with disks

One of the motivating examples for the valuative criterion of properness is the interpretation of ${\text{Spec}}(\mathbb {C} [[t]])$ as an infinitesimal disk, or complex-analytically, as the disk $\Delta =\{x\in \mathbb {C} :|x|<1\}$. This comes from the fact that every power series
   $f(t)=\sum _{n=0}^{\infty }a_{n}t^{n}$
converges in some disk of radius $r$ around the origin. Then, using a change of coordinates, this can be expressed as a power series on the unit disk. Then, if we invert $t$, this is the ring $\mathbb {C} [[t]][t^{-1}]=\mathbb {C} ((t))$ which are the power series which may have a pole at the origin. This is represented topologically as the open disk $\Delta ^{*}=\{x\in \mathbb {C} :0<|x|<1\}$ with the origin removed. For a morphism of schemes over ${\text{Spec}}(\mathbb {C} )$, this is given by the commutative diagram
   ${\begin{matrix}\Delta ^{*}&\to &X\\\downarrow &&\downarrow \\\Delta &\to &Y\end{matrix}}$
Then, the valuative criterion for properness would be a filling in of the point $0\in \Delta$ in the image of $\Delta ^{*}$.

#### Example

It's instructive to look at a counter-example to see why the valuative criterion of properness should hold on spaces analogous to closed compact manifolds. If we take $X=\mathbb {P} ^{1}-\{x\}$ and $Y={\text{Spec}}(\mathbb {C} )$, then a morphism ${\text{Spec}}(\mathbb {C} ((t)))\to X$ factors through an affine chart of $X$, reducing the diagram to
   ${\begin{matrix}{\text{Spec}}(\mathbb {C} ((t)))&\to &{\text{Spec}}(\mathbb {C} [t,t^{-1}])\\\downarrow &&\downarrow \\{\text{Spec}}(\mathbb {C} [[t]])&\to &{\text{Spec}}(\mathbb {C} )\end{matrix}}$
where ${\text{Spec}}(\mathbb {C} [t,t^{-1}])=\mathbb {A} ^{1}-\{0\}$ is the chart centered around $\{x\}$ on $X$. This gives the commutative diagram of commutative algebras
   ${\begin{matrix}\mathbb {C} ((t))&\leftarrow &\mathbb {C} [t,t^{-1}]\\\uparrow &&\uparrow \\\mathbb {C} [[t]]&\leftarrow &\mathbb {C} \end{matrix}}$
Then, a lifting of the diagram of schemes, ${\text{Spec}}(\mathbb {C} [[t]])\to {\text{Spec}}(\mathbb {C} [t,t^{-1}])$, would imply there is a morphism $\mathbb {C} [t,t^{-1}]\to \mathbb {C} [[t]]$ sending $t\mapsto t$ from the commutative diagram of algebras. This, of course, cannot happen. Therefore $X$ is not proper over $Y$.

### Geometric interpretation with curves

There is another similar example of the valuative criterion of properness which captures some of the intuition for why this theorem should hold. Consider a curve $C$ and the complement of a point $C-\{p\}$. Then the valuative criterion for properness would read as a diagram
   ${\begin{matrix}C-\{p\}&\rightarrow &X\\\downarrow &&\downarrow \\C&\rightarrow &Y\end{matrix}}$
with a lifting of $C\to X$. Geometrically this means every curve in the scheme $X$ can be completed to a compact curve. This bit of intuition aligns with what the scheme-theoretic interpretation of a morphism of topological spaces with compact fibers, that a sequence in one of the fibers must converge. Because this geometric situation is a problem locally, the diagram is replaced by looking at the local ring ${\mathcal {O}}_{C,{\mathfrak {p}}}$, which is a DVR, and its fraction field ${\text{Frac}}({\mathcal {O}}_{C,{\mathfrak {p}}})$. Then, the lifting problem then gives the commutative diagram
   ${\begin{matrix}{\text{Spec}}({\text{Frac}}({\mathcal {O}}_{C,{\mathfrak {p}}}))&\rightarrow &X\\\downarrow &&\downarrow \\{\text{Spec}}({\mathcal {O}}_{C,{\mathfrak {p}}})&\rightarrow &Y\end{matrix}}$
where the scheme ${\text{Spec}}({\text{Frac}}({\mathcal {O}}_{C,{\mathfrak {p}}}))$ represents a local disk around ${\mathfrak {p}}$ with the closed point ${\mathfrak {p}}$ removed.

## Proper morphism of formal schemes

Let $f\colon {\mathfrak {X}}\to {\mathfrak {S}}$ be a morphism between locally noetherian formal schemes. We say f is proper or ${\mathfrak {X}}$ is proper over ${\mathfrak {S}}$ if (i) f is an adic morphism (i.e., maps the ideal of definition to the ideal of definition) and (ii) the induced map $f_{0}\colon X_{0}\to S_{0}$ is proper, where $X_{0}=({\mathfrak {X}},{\mathcal {O}}_{\mathfrak {X}}/I),S_{0}=({\mathfrak {S}},{\mathcal {O}}_{\mathfrak {S}}/K),I=f^{*}(K){\mathcal {O}}_{\mathfrak {X}}$ and K is the ideal of definition of ${\mathfrak {S}}$.(EGA III, 3.4.1) The definition is independent of the choice of K.
For example, if g: Y → Z is a proper morphism of locally noetherian schemes, Z$_0$ is a closed subset of Z, and Y$_0$ is a closed subset of Y such that g(Y$_0$) ⊂ Z$_0$, then the morphism ${\widehat {g}}\colon Y_{/Y_{0}}\to Z_{/Z_{0}}$ on formal completions is a proper morphism of formal schemes.
Grothendieck proved the coherence theorem in this setting. Namely, let $f\colon {\mathfrak {X}}\to {\mathfrak {S}}$ be a proper morphism of locally noetherian formal schemes. If F is a coherent sheaf on ${\mathfrak {X}}$, then the higher direct images $R^{i}f_{*}F$ are coherent.