## 20.5 First cohomology and extensions

Lemma 20.5.1. Let $(X, \mathcal{O}_ X)$ be a ringed space. Let $\mathcal{F}$ be a sheaf of $\mathcal{O}_ X$-modules. There is a canonical bijection

$\mathop{\mathrm{Ext}}\nolimits ^1_{\textit{Mod}(\mathcal{O}_ X)}(\mathcal{O}_ X, \mathcal{F}) \longrightarrow H^1(X, \mathcal{F})$

which associates to the extension

$0 \to \mathcal{F} \to \mathcal{E} \to \mathcal{O}_ X \to 0$

the image of $1 \in \Gamma (X, \mathcal{O}_ X)$ in $H^1(X, \mathcal{F})$.

Proof. Let us construct the inverse of the map given in the lemma. Let $\xi \in H^1(X, \mathcal{F})$. Choose an injection $\mathcal{F} \subset \mathcal{I}$ with $\mathcal{I}$ injective in $\textit{Mod}(\mathcal{O}_ X)$. Set $\mathcal{Q} = \mathcal{I}/\mathcal{F}$. By the long exact sequence of cohomology, we see that $\xi$ is the image of a section $\tilde\xi \in \Gamma (X, \mathcal{Q}) = \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_ X}(\mathcal{O}_ X, \mathcal{Q})$. Now, we just form the pullback

$\xymatrix{ 0 \ar[r] & \mathcal{F} \ar[r] \ar@{=}[d] & \mathcal{E} \ar[r] \ar[d] & \mathcal{O}_ X \ar[r] \ar[d]^{\tilde\xi } & 0 \\ 0 \ar[r] & \mathcal{F} \ar[r] & \mathcal{I} \ar[r] & \mathcal{Q} \ar[r] & 0 }$

see Homology, Section 12.6. $\square$

Comment #1606 by Keenan Kidwell on

The cohomology group at the end of the statement of the lemma should be $H^1(X,\mathcal{F})$.

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