The Stacks project

Lemma 20.21.2. Let $i : Z \to X$ be the inclusion of a closed subset. Let $\mathcal{G}$ be an injective abelian sheaf on $Z$. Then $\mathcal{H}^ p_ Z(i_*\mathcal{G}) = 0$ for $p > 0$.

Proof. This is true because the functor $i_*$ is exact and transforms injective abelian sheaves into injective abelian sheaves by Lemma 20.11.11. $\square$


Comments (2)

Comment #1801 by Keenan Kidwell on

Currently the proof cites 20.11.11, which says that for a flat map of ringed spaces, preserves injectives (the reason being that is exact in this case). This seems awkward to me since the current lemma is in the context of abelian sheaves on a topological space. Wouldn't it make more sense to cite the fact that is (always) exact? Although upon doing some searching, this is only stated for maps of ringed spaces as 17.3.3 (3)...but this seems a bit awkward for the same reason. Should there be a separate tag, perhaps in the section 6.22, deducing exactness of for all maps of spaces from the description on stalks?

Comment #1826 by on

OK, yes we could and should state separate lemmas for abelian sheaves. The general idea is that once sheaves of modules have been defined, then most results should be stated for cohomology and derived categories of sheaves of modules with results for abelian sheaves as special cases. However, in this section we only do the material for abelian sheaves, because a good notion of closed immersions for ringed spaces is a bit tricky (for a bunch of reasons).

There are also:

  • 1 comment(s) on Section 20.21: Cohomology with support in a closed

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