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
in D(\mathcal{O}_ X).
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
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
To see this sequence is exact one checks on stalks using Sheaves, Lemma 6.31.8 (computation omitted). \square
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