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The Stacks project

Lemma 70.14.7. Let S be a scheme. Let f : X \to Y be a quasi-compact and quasi-separated morphism of algebraic spaces over S. Let T \subset |Y| be a closed subset. Assume |Y| \setminus T corresponds to an open subspace V \subset Y such that V \to Y is quasi-compact. Let \mathcal{F} be a quasi-coherent \mathcal{O}_ X-module. Let \mathcal{F}' \subset \mathcal{F} be the subsheaf of sections supported on |f|^{-1}T. Then f_*\mathcal{F}' \subset f_*\mathcal{F} is the subsheaf of sections supported on T.

Proof. Omitted. Hints: |X| \setminus |f|^{-1}T is the support of the open subspace U = f^{-1}V \subset X. Since V \to Y is quasi-compact, so is U \to X (by base change). The assumption that f is quasi-compact and quasi-separated implies that f_*\mathcal{F} is quasi-coherent. Hence Lemma 70.14.5 applies to T and f_*\mathcal{F} as well as to |f|^{-1}T and \mathcal{F}. The equality of the given quasi-coherent modules is immediate from the definitions. \square


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