The Stacks project

Lemma 62.4.5. Let $f : X \to Y$ be a locally quasi-finite morphism of schemes. Then

  1. for $\mathcal{F}$ in $\textit{Ab}(X_{\acute{e}tale})$ and a geometric point $\overline{y} : \mathop{\mathrm{Spec}}(k) \to Y$ we have

    \[ (f_!\mathcal{F})_{\overline{y}} = \bigoplus \nolimits _{f(\overline{x}) = \overline{y}} \mathcal{F}_{\overline{x}} \]

    functorially in $\mathcal{F}$, and

  2. the functor $f_! : \textit{Ab}(X_{\acute{e}tale}) \to \textit{Ab}(Y_{\acute{e}tale})$ is exact and commutes with direct sums.

Proof. The formula for the stalks is immediate (and in fact equivalent) to Lemma 62.4.2. The exactness of the functor follows immediately from this and the fact that exactness may be checked on stalks, see √Čtale Cohomology, Theorem 59.29.10. $\square$


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