# sheaf cohomology

Let $X$ be a topological space. The category of sheaves of abelian groups on $X$ has enough injectives. So we can define the sheaf cohomology $H^{i}(X,\mathcal{F})$ of a sheaf $\mathcal{F}$ to be the right derived functors of the global sections functor $\mathcal{F}\to\Gamma(X,\mathcal{F})$.

Usually we are interested in the case where $X$ is a scheme, and $\mathcal{F}$ is a coherent sheaf. In this case, it does not matter if we take the derived functors in the category of sheaves of abelian groups or coherent sheaves.

Sheaf cohomology can be explicitly calculated using Čech cohomology (http://planetmath.org/CechCohomologyGroup2). Choose an open cover $\{U_{i}\}$ of $X$. We define

 $C^{i}(\mathcal{F})=\prod\mathcal{F}(U_{j_{0}\cdots j_{i}})$

where the product is over $i+1$ element subsets of $\{1,\ldots,n\}$ and $U_{j_{0}\cdots j_{i}}=U_{j_{0}}\cap\cdots\cap U_{j_{i}}$. If $s\in\mathcal{F}(U_{j_{0}\cdots j_{i}})$ is thought of as an element of $C^{i}(\mathcal{F})$, then the differential

 $\partial(s)=\prod_{\ell}\left(\prod_{k=j_{\ell}+1}^{j_{\ell+1}-1}(-1)^{\ell}s|% _{U_{j_{0}\cdots j_{\ell}kj_{\ell+1}\cdots j_{i}}}\right)$

makes $C^{*}(\mathcal{F})$ into a chain complex. The cohomology of this complex is denoted $\check{H}^{i}(X,\mathcal{F})$ and called the Čech cohomology of $\mathcal{F}$ with respect to the cover $\{U_{i}\}$. There is a natural map $H^{i}(X,\mathcal{F})\to\check{H}^{i}(X,\mathcal{F})$ which is an isomorphism for sufficiently fine covers. (A cover is sufficiently fine if $H^{i}(U_{j},\mathcal{F})=0$ for all $i>0$, for every $j$ and for every sheaf $\mathcal{F}$). In the category of schemes, for example, any cover by open affine schemes has this property. What this means is that if one can find a finite fine enough cover of $X$, sheaf cohomology becomes computable by a finite process. In fact in [2], this is how the cohomology of projective space is explicitly calculated.

## References

• 1 Grothendieck, A. Sur quelques points d’algèbre homologique, Tôhoku Math. J., Second Series, 9 (1957), 119–221.
• 2 Hartshorne, R. Algebraic Geometry, Springer-Verlag Graduate Texts in Mathematics 52, 1977
Title sheaf cohomology SheafCohomology 2013-03-22 13:50:59 2013-03-22 13:50:59 mathcam (2727) mathcam (2727) 14 mathcam (2727) Definition msc 14F25 EtaleCohomology LeraysTheorem AcyclicSheaf DeRhamWeilTheorem sufficiently fine