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The Stacks project

Lemma 20.33.1. Let (X, \mathcal{O}_ X) be a ringed space. Let X = U \cup V be the union of two open subspaces. For any object E of D(\mathcal{O}_ X) we have a distinguished triangle

j_{U \cap V!}E|_{U \cap V} \to j_{U!}E|_ U \oplus j_{V!}E|_ V \to E \to j_{U \cap V!}E|_{U \cap V}[1]

in D(\mathcal{O}_ X).

Proof. We have seen in Section 20.32 that the restriction functors and the extension by zero functors are computed by just applying the functors to any complex. Let \mathcal{E}^\bullet be a complex of \mathcal{O}_ X-modules representing E. The distinguished triangle of the lemma is the distinguished triangle associated (by Derived Categories, Section 13.12 and especially Lemma 13.12.1) to the short exact sequence of complexes of \mathcal{O}_ X-modules

0 \to j_{U \cap V!}\mathcal{E}^\bullet |_{U \cap V} \to j_{U!}\mathcal{E}^\bullet |_ U \oplus j_{V!}\mathcal{E}^\bullet |_ V \to \mathcal{E}^\bullet \to 0

To see this sequence is exact one checks on stalks using Sheaves, Lemma 6.31.8 (computation omitted). \square


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