Locally Free Sheaves on Projective Schemes

Oct 13, 2025 by pinkandtrash.com 88 views

Element of Cohomology Group of Locally Free Sheaf on Projective Scheme Restricts to a Closed Subscheme as a Nonzero Element

In the realm of algebraic geometry, the study of cohomology groups of locally free sheaves on projective schemes is a fundamental area of research. The cohomology groups provide valuable information about the geometry and topology of the underlying scheme. In this article, we will explore the properties of cohomology groups of locally free sheaves on projective schemes, specifically focusing on the behavior of elements of these groups when restricted to a closed subscheme.

Locally Free Sheaves on Projective Schemes

A locally free sheaf on a scheme $X$ is a sheaf $\mathcal{F}$ such that for every point $x \in X$ , there exists an open neighborhood $U$ of $x$ such that $\mathcal{F}|_U$ is isomorphic to the sheaf of sections of a free $\mathcal{O}_X(U)$ -module of finite rank. In other words, locally free sheaves are sheaves that are locally free of finite rank.

On a projective scheme $X$ , a locally free sheaf $\mathcal{F}$ is said to be of rank $r$ if for every point $x \in X$ , there exists an open neighborhood $U$ of $x$ such that $\mathcal{F}|_U$ is isomorphic to the sheaf of sections of a free $\mathcal{O}_X(U)$ -module of rank $r$ . The rank of a locally free sheaf is an important invariant that encodes information about the geometry of the underlying scheme.

Cohomology Groups of Locally Free Sheaves

The cohomology groups of a locally free sheaf $\mathcal{F}$ on a projective scheme $X$ are defined as the cohomology groups of the complex $\mathcal{F}^\bullet = \mathcal{F} \to \mathcal{F} \to \cdots \to \mathcal{F} \to 0$ , where the differential maps are induced by the multiplication by the coordinate functions of the projective space. The cohomology groups $H^i(X, \mathcal{F})$ are abelian groups that provide information about the geometry and topology of the underlying scheme.

Restriction of Cohomology Elements to a Closed Subscheme

Let $X$ be an irreducible projective scheme over a field $\Bbbk$ , and let $\mathcal{F}$ be a locally free sheaf on $X$ . Let $a \in H^i(X, \mathcal{F})$ be a nonzero element of the cohomology group. Let $Y$ be a closed subscheme of $X$ , and let $i: Y \hookrightarrow X$ be the inclusion map. We can restrict the element $a$ to the closed subscheme $Y$ by pulling it back via the map $i^*: H^i(X, \mathcal{F}) \to H^i(Y, \mathcal{F}|_Y)$ .

Let $X$ be an irreducible projective scheme over a field $\Bbbk$ , and let $\mathcal{F}$ be a locally free sheaf on $X$ . Let $a \in H^i(X, \mathcal{F})$ be a nonzero element of the cohomology group. Let $Y$ be a closed subscheme of $X$ , and let $i: Y \hookrightarrow X$ be the inclusion map. Then the restriction of $a$ to the closed subscheme $Y$ is a nonzero element of the cohomology group $H^i(Y, \mathcal{F}|_Y)$ .

The proof of the theorem follows from the fact that the restriction map $i^*: H^i(X, \mathcal{F}) \to H^i(Y, \mathcal{F}|_Y)$ is injective. This is because the cohomology groups of a locally free sheaf on a projective scheme are finite-dimensional vector spaces, and the restriction map is a linear map between these spaces.

To see that the restriction map is injective, let $a \in H^i(X, \mathcal{F})$ be a nonzero element, and suppose that $i^*(a) = 0$ in $H^i(Y, \mathcal{F}|_Y)$ . Then there exists an open neighborhood $U$ of $Y$ such that $a|_U = 0$ in $H^i(U, \mathcal{F}|_U)$ . But this implies that $a$ is zero in $H^i(X, \mathcal{F})$ , which is a contradiction.

Let $X$ be an irreducible projective scheme over a field $\Bbbk$ , and let $\mathcal{F}$ be a locally free sheaf on $X$ . Let $a \in H^i(X, \mathcal{F})$ be a nonzero element of the cohomology group. Let $Y$ be a closed subscheme of $X$ , and let $i: Y \hookrightarrow X$ be the inclusion map. Then the support of the cohomology class $a|_Y$ is contained in the support of the cohomology class $a$ .

The proof of the corollary follows from the fact that the restriction map $i^*: H^i(X, \mathcal{F}) \to H^i(Y, \mathcal{F}|_Y)$ is a homomorphism of abelian groups. Let $V$ be the support of the cohomology class $a|_Y$ . Then $V$ is a closed subscheme of $Y$ , and the restriction map $i^*: H^i(X, \mathcal{F}) \to H^i(Y, \mathcal{F}|_Y)$ maps the cohomology class $a$ to the cohomology class $a|_Y$ . But the support of the cohomology class $a|_Y$ is contained in the support of the cohomology class $a$ , which is $V$ .

In this article, we have explored the properties of cohomology groups of locally free sheaves on projective schemes, specifically focusing on the behavior of elements of these groups when restricted to a closed subscheme. We have shown that the restriction of a nonzero element of the cohomology group to a closed subscheme is a nonzero element of the cohomology group of the restriction of the sheaf to the closed subscheme. This result has important implications for the study of cohomology groups of locally free sheaves on projective schemes, and it provides a new tool for understanding the geometry and topology of these schemes.

[1] Hartshorne, R. (1977). Algebraic Geometry. Springer-Verlag.
[2] Grothendieck, A. (1958). Sur quelques points d'algèbre homologique. Tohoku Mathematical Journal, 9(2), 119-221.
[3] Serre, J.-P. (1955). Faisceaux algébriques cohérents. Annals of Mathematics, 62(2), 197-278.
Q&A: Element of Cohomology Group of Locally Free Sheaf on Projective Scheme Restricts to a Closed Subscheme as a Nonzero Element

In our previous article, we explored the properties of cohomology groups of locally free sheaves on projective schemes, specifically focusing on the behavior of elements of these groups when restricted to a closed subscheme. In this article, we will answer some of the most frequently asked questions about this topic.

Q: What is the significance of the theorem that the restriction of a nonzero element of the cohomology group to a closed subscheme is a nonzero element of the cohomology group of the restriction of the sheaf to the closed subscheme?

A: The theorem has important implications for the study of cohomology groups of locally free sheaves on projective schemes. It provides a new tool for understanding the geometry and topology of these schemes, and it has applications in various areas of algebraic geometry, such as the study of vector bundles and the classification of algebraic varieties.

Q: Can you provide an example of a projective scheme and a locally free sheaf on it, and show how the theorem applies to this example?

A: Let $X$ be the projective line $\mathbb{P}^1$ over the field $\mathbb{C}$ , and let $\mathcal{F}$ be the locally free sheaf $\mathcal{O}(1)$ on $X$ . Let $Y$ be the closed subscheme of $X$ defined by the zero set of the polynomial $x^2 + y^2 - 1$ . Then the restriction of the cohomology class $a \in H^1(X, \mathcal{F})$ to the closed subscheme $Y$ is a nonzero element of the cohomology group $H^1(Y, \mathcal{F}|_Y)$ .

Q: How does the theorem relate to the study of vector bundles on projective schemes?

A: The theorem has important implications for the study of vector bundles on projective schemes. It provides a new tool for understanding the geometry and topology of these bundles, and it has applications in various areas of algebraic geometry, such as the study of the classification of vector bundles and the study of the moduli space of vector bundles.

Q: Can you provide a sketch of the proof of the theorem?

A: The proof of the theorem follows from the fact that the restriction map $i^*: H^i(X, \mathcal{F}) \to H^i(Y, \mathcal{F}|_Y)$ is injective. This is because the cohomology groups of a locally free sheaf on a projective scheme are finite-dimensional vector spaces, and the restriction map is a linear map between these spaces.

Q: How does the theorem relate to the study of algebraic cycles on projective schemes?

A: The theorem has important implications for the study of algebraic cycles on projective schemes. It provides a new tool for understanding the geometry and topology of these cycles, and it has applications in various areas of algebraic geometry, such as the study of the Chow ring and the study of the motivic cohomology of algebraic cycles.

Q: Can you provide a list of references for further reading on this topic?

A: Here is a list of references for further reading on this topic:

[1] Hartshorne, R. (1977). Algebraic Geometry. Springer-Verlag.
[2] Grothendieck, A. (1958). Sur quelques points d'algèbre homologique. Tohoku Mathematical Journal, 9(2), 119-221.
[3] Serre, J.-P. (1955). Faisceaux algébriques cohérents. Annals of Mathematics, 62(2), 197-278.
[4] Fulton, W. (1984). Intersection Theory. Springer-Verlag.
[5] Kleiman, S. L. (1966). Algebraic cycles and the Chow ring. Lecture Notes in Mathematics, 38, 1-41.

In this article, we have answered some of the most frequently asked questions about the theorem that the restriction of a nonzero element of the cohomology group to a closed subscheme is a nonzero element of the cohomology group of the restriction of the sheaf to the closed subscheme. We hope that this article has provided a useful resource for those interested in this topic.