## 21.2 Cohomology of sheaves

Let $\mathcal{C}$ be a site, see Sites, Definition 7.6.2. Let $\mathcal{F}$ be an abelian sheaf on $\mathcal{C}$. We know that the category of abelian sheaves on $\mathcal{C}$ has enough injectives, see Injectives, Theorem 19.7.4. Hence we can choose an injective resolution $\mathcal{F}[0] \to \mathcal{I}^\bullet $. For any object $U$ of the site $\mathcal{C}$ we define

to be the *$i$th cohomology group of the abelian sheaf $\mathcal{F}$ over the object $U$*. In other words, these are the right derived functors of the functor $\mathcal{F} \mapsto \mathcal{F}(U)$. The family of functors $H^ i(U, -)$ forms a universal $\delta $-functor $\textit{Ab}(\mathcal{C}) \to \textit{Ab}$.

It sometimes happens that the site $\mathcal{C}$ does not have a final object. In this case we define the *global sections* of a presheaf of sets $\mathcal{F}$ over $\mathcal{C}$ to be the set

where $e$ is a final object in the category of presheaves on $\mathcal{C}$. In this case, given an abelian sheaf $\mathcal{F}$ on $\mathcal{C}$, we define the *$i$th cohomology group of $\mathcal{F}$ on $\mathcal{C}$* as follows

in other words, it is the $i$th right derived functor of the global sections functor. The family of functors $H^ i(\mathcal{C}, -)$ forms a universal $\delta $-functor $\textit{Ab}(\mathcal{C}) \to \textit{Ab}$.

Let $f : \mathop{\mathit{Sh}}\nolimits (\mathcal{C}) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{D})$ be a morphism of topoi, see Sites, Definition 7.15.1. With $\mathcal{F}[0] \to \mathcal{I}^\bullet $ as above we define

to be the *$i$th higher direct image of $\mathcal{F}$*. These are the right derived functors of $f_*$. The family of functors $R^ if_*$ forms a universal $\delta $-functor from $\textit{Ab}(\mathcal{C}) \to \textit{Ab}(\mathcal{D})$.

Let $(\mathcal{C}, \mathcal{O})$ be a ringed site, see Modules on Sites, Definition 18.6.1. Let $\mathcal{F}$ be an $\mathcal{O}$-module. We know that the category of $\mathcal{O}$-modules has enough injectives, see Injectives, Theorem 19.8.4. Hence we can choose an injective resolution $\mathcal{F}[0] \to \mathcal{I}^\bullet $. For any object $U$ of the site $\mathcal{C}$ we define

to be the *the $i$th cohomology group of $\mathcal{F}$ over $U$*. The family of functors $H^ i(U, -)$ forms a universal $\delta $-functor $\textit{Mod}(\mathcal{O}) \to \text{Mod}_{\mathcal{O}(U)}$. Similarly

it the *$i$th cohomology group of $\mathcal{F}$ on $\mathcal{C}$*. The family of functors $H^ i(\mathcal{C}, -)$ forms a universal $\delta $-functor $\textit{Mod}(\mathcal{C}) \to \text{Mod}_{\Gamma (\mathcal{C}, \mathcal{O})}$.

Let $f : (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}), \mathcal{O}) \to (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}), \mathcal{O}')$ be a morphism of ringed topoi, see Modules on Sites, Definition 18.7.1. With $\mathcal{F}[0] \to \mathcal{I}^\bullet $ as above we define

to be the *$i$th higher direct image of $\mathcal{F}$*. These are the right derived functors of $f_*$. The family of functors $R^ if_*$ forms a universal $\delta $-functor from $\textit{Mod}(\mathcal{O}) \to \textit{Mod}(\mathcal{O}')$.

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