The Stacks project

Lemma 66.14.1. Let $S$ be a scheme. Let $i : Z \to X$ be a closed immersion of algebraic spaces over $S$. Let $\mathcal{I} \subset \mathcal{O}_ X$ be the quasi-coherent sheaf of ideals cutting out $Z$.

  1. For any $\mathcal{O}_ X$-module $\mathcal{F}$ the adjunction map $\mathcal{F} \to i_*i^*\mathcal{F}$ induces an isomorphism $\mathcal{F}/\mathcal{I}\mathcal{F} \cong i_*i^*\mathcal{F}$.

  2. The functor $i^*$ is a left inverse to $i_*$, i.e., for any $\mathcal{O}_ Z$-module $\mathcal{G}$ the adjunction map $i^*i_*\mathcal{G} \to \mathcal{G}$ is an isomorphism.

  3. The functor

    \[ i_* : \mathit{QCoh}(\mathcal{O}_ Z) \longrightarrow \mathit{QCoh}(\mathcal{O}_ X) \]

    is exact, fully faithful, with essential image those quasi-coherent $\mathcal{O}_ X$-modules $\mathcal{F}$ such that $\mathcal{I}\mathcal{F} = 0$.

Proof. During this proof we work exclusively with sheaves on the small ├ętale sites, and we use $i_*, i^{-1}, \ldots $ to denote pushforward and pullback of sheaves of abelian groups instead of $i_{small, *}, i_{small}^{-1}$.

Let $\mathcal{F}$ be an $\mathcal{O}_ X$-module. By Lemma 66.13.7 applied with $\mathcal{A} = \mathcal{O}_ X$ and $\mathcal{G} = \mathcal{B} = \mathcal{O}_ Z$ we see that $i_*i^*\mathcal{F} = \mathcal{F} \otimes _{\mathcal{O}_ X} \mathcal{O}_ Z$. By Lemma 66.13.1 we see that we have a short exact sequence

\[ 0 \to \mathcal{I} \to \mathcal{O}_ X \to i_*\mathcal{O}_ Z \to 0 \]

It follows from properties of the tensor product that $\mathcal{F} \otimes _{\mathcal{O}_ X} i_*\mathcal{O}_ Z = \mathcal{F}/\mathcal{I}\mathcal{F}$. This proves (1) (except that we omit the verification that the map is induced by the adjunction mapping).

Let $\mathcal{G}$ be any $\mathcal{O}_ Z$-module. By Lemma 66.13.5 we see that $i^{-1}i_*\mathcal{G} = \mathcal{G}$. Hence to prove (2) we have to show that the canonical map $\mathcal{G} \otimes _{i^{-1}\mathcal{O}_ X} \mathcal{O}_ Z \to \mathcal{G}$ is an isomorphism. This follows from general properties of tensor products if we can show that $i^{-1}\mathcal{O}_ X \to \mathcal{O}_ Z$ is surjective. By Lemma 66.13.5 it suffices to prove that $i_*i^{-1}\mathcal{O}_ X \to i_*\mathcal{O}_ Z$ is surjective. Since the surjective map $\mathcal{O}_ X \to i_*\mathcal{O}_ Z$ factors through this map we see that (2) holds.

Finally we prove the most interesting part of the lemma, namely part (3). A closed immersion is quasi-compact and separated, see Lemmas 66.13.3 and 66.13.4. Hence Lemma 66.11.2 applies and the pushforward of a quasi-coherent sheaf on $Z$ is indeed a quasi-coherent sheaf on $X$. Thus we obtain our functor $i^{QCoh}_* : \mathit{QCoh}(\mathcal{O}_ Z) \to \mathit{QCoh}(\mathcal{O}_ X)$. It is clear from part (2) that $i^{QCoh}_*$ is fully faithful since it has a left inverse, namely $i^*$.

Now we turn to the description of the essential image of the functor $i_*$. It is clear that $\mathcal{I}(i_*\mathcal{G}) = 0$ for any $\mathcal{O}_ Z$-module, since $\mathcal{I}$ is the kernel of the map $\mathcal{O}_ X \to i_*\mathcal{O}_ Z$ which is the map we use to put an $\mathcal{O}_ X$-module structure on $i_*\mathcal{G}$. Next, suppose that $\mathcal{F}$ is any quasi-coherent $\mathcal{O}_ X$-module such that $\mathcal{I}\mathcal{F} = 0$. Then we see that $\mathcal{F}$ is an $i_*\mathcal{O}_ Z$-module because $i_*\mathcal{O}_ Z = \mathcal{O}_ X/\mathcal{I}$. Hence in particular its support is contained in $|Z|$. We apply Lemma 66.13.5 to see that $\mathcal{F} \cong i_*\mathcal{G}$ for some $\mathcal{O}_ Z$-module $\mathcal{G}$. The only small detail left over is to see why $\mathcal{G}$ is quasi-coherent. This is true because $\mathcal{G} \cong i^*\mathcal{F}$ by part (2) and Properties of Spaces, Lemma 65.29.2. $\square$

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