Sheaf of modules

In mathematics, a sheaf of O-modules or simply an O-module over a ringed space (X, O) is a sheaf F such that, for any open subset U of X, F(U) is an O(U)-module and the restriction maps F(U) →F(V) are compatible with the restriction maps O(U) →O(V): the restriction of fs is the restriction of f times that of s for any f in O(U) and s in F(U).

The standard case is when X is a scheme and O its structure sheaf. If O is the constant sheaf ${\underline {\mathbf {Z} }}$ , then a sheaf of O-modules are the same as a sheaf of abelian groups (i.e., abelian sheaf).

If X is the prime spectrum of a ring R, then any R-module defines an O_X-module (called an associated sheaf) in a natural way. Similarly, if R is a graded ring and X is the Proj of R, then any graded module defines an O_X-module in a natural way. O-modules arising in such a fashion are examples of quasi-coherent sheaves, and in fact, on affine or projective schemes, all quasi-coherent sheaves are obtained this way.

Sheaves of modules over a ringed space form an abelian category.^[1] Moreover, this category has enough injectives,^[2] and consequently one can and does define the sheaf cohomology $\operatorname {H} ^{i}(X,-)$ as the i-th right derived functor of the global section functor $\Gamma (X,-)$ .^[3]

Examples

Given a ringed space (X, O), if F is an O-submodule of O, then it is called the sheaf of ideals or ideal sheaf of O, since for each open subset U of X, F(U) is an ideal of the ring O(U).
Let X be a smooth variety of dimension n. Then the tangent sheaf of X is the dual of the cotangent sheaf $\Omega _{X}$ and the canonical sheaf $\omega _{X}$ is the n-th exterior power (determinant) of $\Omega _{X}$ .

Operations

Let (X, O) be a ringed space. If F and G are O-modules, then their tensor product, denoted by

F\otimes _{O}G

F\otimes G

is the O-module that is the sheaf associated to the presheaf $U\mapsto F(U)\otimes _{O(U)}G(U).$ (To see that sheafification cannot be avoided, compute the global sections of $O(1)\otimes O(-1)=O$ where O(1) is Serre's twisting sheaf on a projective space.)

Similarly, if F and G are O-modules, then

{\mathcal {H}}om_{O}(F,G)

denotes the O-module that is the sheaf $U\mapsto \operatorname {Hom} _{O|_{U}}(F|_{U},G|_{U})$ .^[4] In particular, the O-module

{\mathcal {H}}om_{O}(F,O)

is called the dual module of F and is denoted by ${\check {F}}$ . Note: for any O-modules E, F, there is a canonical homomorphism

{\check {E}}\otimes F\to {\mathcal {H}}om_{O}(E,F)

which is an isomorphism if E is a locally free sheaf of finite rank. In particular, if L is locally free of rank one (such L is called an invertible sheaf or a line bundle),^[5] then this reads:

{\check {L}}\otimes L\simeq O,

implying the isomorphism classes of invertible sheaves form a group. This group is called the Picard group of X and is canonically identified with the first cohomology group $\operatorname {H} ^{1}(X,{\mathcal {O}}^{*})$ (by the standard argument with Čech cohomology).

If E is a locally free sheaf of finite rank, then there is an O-linear map ${\check {E}}\otimes E\simeq \operatorname {End} _{O}(E)\to O$ given by the pairing; it is called the trace map of E.

For any O-module F, the tensor algebra, exterior algebra and symmetric algebra of F are defined in the same way. For example, the k-th exterior power

\wedge ^{k}F

is the sheaf associated to the presheaf $U\mapsto \wedge _{O(U)}^{k}F(U)$ . If F is locally free of rank n, then $\wedge ^{n}F$ is called the determinant line bundle (though technically invertible sheaf) of F, denoted by det(F). There is a natural perfect paring:

\wedge ^{r}F\otimes \wedge ^{n-r}F\to \operatorname {det} (F).

Let f: (X, O) →(X', O') be a morphism of ringed spaces. If F is an O-module, then the direct image sheaf $f_{*}F$ is an O'-module through the natural map O' →f_*O (such a natural map is part of the data of a morphism of ringed spaces.)

If G is an O'-module, then the module inverse image $f^{*}G$ of G is the O-module given as the tensor product of modules:

f^{-1}G\otimes _{f^{-1}O'}O

where $f^{-1}G$ is the inverse image sheaf of G and $f^{-1}O'\to O$ is obtained from $O'\to f_{*}O$ by adjuction.

There is an adjoint relation between $f_{*}$ and $f^{*}$ : for any O-module F and O'-module G,

\operatorname {Hom} _{O}(f^{*}G,F)\simeq \operatorname {Hom} _{O'}(G,f_{*}F)

as abelian group. There is also the projection formula: for an O-module F and a locally free O'-module E of finite rank,

f_{*}(F\otimes f^{*}E)\simeq f_{*}F\otimes E.

Properties

Let (X, O) be a ringed space. An O-module F is said to be generated by global sections if there is a surjection of O-modules:

\oplus _{i\in I}O\to F\to 0

Explicitly, this means that there are global sections s_i of F such that the images of s_i in each stalk F_x generates F_x as O_x-module.

An example of such a sheaf is that associated in algebraic geometry to an R-module M, R being any commutative ring, on the spectrum of a ring Spec(R). Another example: according to Cartan's theorem A, any coherent sheaf on a Stein manifold is spanned by global sections. (cf. Serre's theorem A below.) In the theory of schemes, a related notion is ample line bundle. (For example, if L is an ample line bundle, some power of it is generated by global sections.)

An injective O-module is flasque (i.e., all restrictions maps F(U) → F(V) are surjective.)^[6] Since a flasque sheaf is acyclic in the category of abelian sheaves, this implies that the i-th right derived functor of the global section functor $\Gamma (X,-)$ in the category of O-modules coincides with the usual i-th sheaf cohomology in the category of abelian sheaves.^[7]

Sheaf associated to a module

Let M be a module over a ring A. Put X = Spec A. For each pair $D(f)\subset D(g)$ , by the universal property of localization, there is a natural map

\rho _{g,f}:M[g^{-1}]\to M[f^{-1}]

which has the property that $\rho _{g,f}=\rho _{g,h}\circ \rho _{h,f}$ . Then

D(f)\mapsto M[f^{-1}]

is a contravariant functor from the category whose objects are the sets D(f) and morphisms the inclusions of sets to the category of abelian groups. One can show^[8] it is in fact a B-sheaf (i.e., it satisfies the gluing axiom) and thus defines the sheaf ${\widetilde {M}}$ on X called the sheaf associated to M.

The most basic example is the structure sheaf on X; i.e., ${\mathcal {O}}_{X}={\widetilde {A}}$ . Moreover, ${\widetilde {M}}$ has the structure of ${\mathcal {O}}_{X}={\widetilde {A}}$ and thus one gets the exact functor $M\mapsto {\widetilde {M}}$ from Mod_A, the category of modules over A to the category of modules over ${\mathcal {O}}_{X}$ . It defines an equivalence from Mod_A to the category of quasi-coherent sheaves on X, with the inverse $\Gamma (X,-)$ , the global section functor. When X is Noetherian, the functor is an equivalence from the category of finitely generated A-modules to the category of coherent sheaves on X.

The construction has the following properties: for any A-modules M, N,

$M[f^{-1}]^{\sim }={\widetilde {M}}|_{D(f)}$ .^[9]
For any prime ideal p of A, ${\widetilde {M}}_{p}\simeq M_{p}$ as O_p = A_p-module.
$(M\otimes _{A}N)^{\sim }\simeq {\widetilde {M}}\otimes _{\widetilde {A}}{\widetilde {N}}$ .^[10]
If M is finitely presented, $\operatorname {Hom} _{A}(M,N)^{\sim }\simeq {\mathcal {H}}om_{\widetilde {A}}({\widetilde {M}},{\widetilde {N}})$ .^[10]
$\operatorname {Hom} _{A}(M,N)\simeq \Gamma (X,{\mathcal {H}}om_{\widetilde {A}}({\widetilde {M}},{\widetilde {N}}))$ , since the equivalence between Mod_A and the category of quasi-coherent sheaves on X.
$(\varinjlim M_{i})^{\sim }\simeq \varinjlim {\widetilde {M_{i}}}$ ;^[11] in particular, taking a direct sum and ~ commute.

Sheaf associated to a graded module

There is a graded analog of the construction and equivalence in the preceding section. Let R be a graded ring generated by degree-one elements as R₀-algebra (R₀ means the degree-zero piece) and M a graded R-module. Let X be the Proj of R (so X is a projective scheme). Then there is an O-module ${\widetilde {M}}$ such that for any homogeneous element f of positive degree of R, there is a natural isomorphism

{\widetilde {M}}|_{\{f\neq 0\}}\simeq (M[f^{-1}]_{0})^{\sim }

as sheaves of modules on the affine scheme $\{f\neq 0\}=\operatorname {Spec} (R[f^{-1}]_{0})$ ;^[12] in fact, this defines ${\widetilde {M}}$ by gluing.

Example: Let R(1) be the graded R-module given by R(1)_n = R_n+1. Then $O(1)={\widetilde {R(1)}}$ is called Serre's twisting sheaf (the dual of the tautological line bundle.)

If F is an O-module on X, then, writing $F(n)=F\otimes O(n)$ , there is a canonical homomorphism:

(\oplus _{n\geq 0}\Gamma (X,F(n)))^{\sim }\to F

which is an isomorphism if and only if F is quasi-coherent.

Computing sheaf cohomology

Main article: sheaf cohomology

Sheaf cohomology has a reputation for being difficult to calculate. Because of this, the next general fact is fundamental for any practical computation:

Theorem — Let X be a topological space, F an abelian sheaf on it and ${\mathfrak {U}}$ an open cover of X such that $\operatorname {H} ^{i}(U_{i_{0}}\cap \cdots \cap U_{i_{p}},F)=0$ for any i, p and $U_{i_{j}}$ 's in ${\mathfrak {U}}$ . Then for any i,

\operatorname {H} ^{i}(X,F)=\operatorname {H} ^{i}(C^{\bullet }({\mathfrak {U}},F))

where the right-hand side is the i-th Čech cohomology.

Serre's theorem A states that if X is a projective variety and F a coherent sheaf on it, then, for sufficiently large n, F(n) is generated by finitely many global sections. Moreover,

(a) For each i, Hⁱ(X, F) is finitely generated over R₀, and

(b) (Serre's theorem B) There is an integer n₀, depending on F, such that

\operatorname {H} ^{i}(X,F(n))=0,\,i\geq 1,n\geq n_{0}

Sheaf extension

Let (X, O) be a ringed space, and let F, H be sheaves of O-modules on X. An extension of H by F is a short exact sequence of O-modules

0\rightarrow F\rightarrow G\rightarrow H\rightarrow 0.

As with group extensions, if we fix F and H, then all equivalence classes of extensions of H by F form an abelian group (cf. Baer sum), which is isomorphic to the Ext group $\mathrm {Ext} _{O}^{1}(H,F)$ , where the identity element in $\mathrm {Ext} _{O}^{1}(H,F)$ corresponds to the trivial extension.

In the case where H is O, we have: for any i ≥ 0,

\operatorname {H} ^{i}(X,F)=\mathrm {Ext} _{O}^{i}(O,F),

since both the sides are the right derived functors of the same functor $\Gamma (X,-)=\operatorname {Hom} _{O}(O,-).$

Note: Some authors, notably Hartshorne, drop the subscript O.

Assume X is a projective scheme over a Noetherian ring. Let F, G be coherent sheaves on X and i an integer. Then there exists n₀ such that

\operatorname {Ext} _{O}^{i}(F,G(n))=\Gamma (X,{\mathcal {E}}xt_{O}^{i}(F,G(n))),\,n\geq n_{0}

.^[13]

Notes

↑ Vakil, Math 216: Foundations of algebraic geometry, 2.5.
↑ Hartshorne, Ch. III, Proposition 2.2.
↑ This cohomology functor coincides with the right derived functor of the global section functor in the category of abelian sheaves; cf. Hartshorne, Ch. III, Proposition 2.6.
↑ There is a canonical homomorphism:
${\mathcal {H}}om_{O}(F,O)_{x}\to \operatorname {Hom} _{O_{x}}(F_{x},O_{x}),$
which is an isomorphism if F is of finite presentation (EGA, Ch. 0, 5.2.6.)
↑ For coherent sheaves, having a tensor inverse is the same as being locally free of rank one; in fact, there is the following fact: if $F\otimes G\simeq O$ and if F is coherent, then F, G are locally free of rank one. (cf. EGA, Ch 0, 5.4.3.)
↑ Hartshorne, Ch III, Lemma 2.4.
↑ see also: http://math.stackexchange.com/questions/447220/hartshornes-weird-definition-of-right-derived-functors-and-prop-iii-2-6/447234#447234
↑ Hartshorne, Ch. II, Proposition 5.1.
↑ EGA I, Ch. I, Proposition 1.3.6.
1 2 EGA I, Ch. I, Corollaire 1.3.12.
↑ EGA I, Ch. I, Corollaire 1.3.9.
↑ Hartshorne, Ch. II, Proposition 5.11.
↑ Hartshorne, Ch. III, Proposition 6.9.

References

Grothendieck, Alexandre; Dieudonné, Jean (1960). "Éléments de géométrie algébrique: I. Le langage des schémas". Publications Mathématiques de l'IHÉS. 4. doi:10.1007/bf02684778. MR 0217083.
Hartshorne, Robin (1977), Algebraic Geometry, Graduate Texts in Mathematics, 52, New York: Springer-Verlag, ISBN 978-0-387-90244-9, MR 0463157

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