Lemma 59.70.8 (095L)—The Stacks project

Lemma 59.70.8. Let $X$ be a scheme. Let $Z \subset X$ be a closed subscheme and let $U \subset X$ be the complement. Denote $i : Z \to X$ and $j : U \to X$ the inclusion morphisms. For every abelian sheaf $\mathcal{F}$ on $X_{\acute{e}tale}$ there is a canonical short exact sequence

\[ 0 \to j_!j^{-1}\mathcal{F} \to \mathcal{F} \to i_*i^{-1}\mathcal{F} \to 0 \]

on $X_{\acute{e}tale}$.

Proof. We obtain the maps by the adjointness properties of the functors involved. For a geometric point $\overline{x}$ in $X$ we have either $\overline{x} \in U$ in which case the map on the left hand side is an isomorphism on stalks and the stalk of $i_*i^{-1}\mathcal{F}$ is zero or $\overline{x} \in Z$ in which case the map on the right hand side is an isomorphism on stalks and the stalk of $j_!j^{-1}\mathcal{F}$ is zero. Here we have used the description of stalks of Lemma 59.46.3 and Proposition 59.70.3. $\square$

Comments (2)

Comment #3249 by William Chen on April 09, 2018 at 17:35

To make it slightly easier to read maybe F should be included in the statement of the lemma before the displaymath? Ie, "For every abelian sheaf F on X_etale..."

Comment #3345 by Johan on May 19, 2018 at 15:48

Thanks, fixed here.

Comments (2)

Post a comment