## Non-zero sheaf cohomology

Let R denote the real line with its usual topology. Does there exist a sheaf F of abelian groups on R whose second cohomology group H^2(R,F) is non-zero? What about H^j(R,F) for integers j>=2 ?

(Here cohomology means derived functor cohomology as in, say, Hartshorne or EGA. Anyway this cohomology coincides with Cech cohomology since R is paracompact.)

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 What is R here? – Ben Webster♦ Oct 11 2009 at 13:28 @Ben- My guess: R is a commutative ring and the sheaf is on Spec(R). – Anton Geraschenko♦ Oct 11 2009 at 14:23 Your question would certainly benefit from more info – what is R and what type of sheaf cohomology you are considering? – Ilya Nikokoshev Oct 11 2009 at 14:38 Now I agree with ilya, R is probably the real line. – Anton Geraschenko♦ Oct 11 2009 at 14:43 Yes, R is the real line with its usual topology. Cohomology is derived functor cohomology for the functor "Global Sections" (the cohomology used in Hartshorne or EGA, defined via injective resolutions) . It coincides with Cech cohomology since R is paracompact. @ilya n. Could you please explain how the vanishing of cohomology in dimensions 2 and larger is a simple consequence of the definition of Cech cohomology ? Of course I do not assume that my sheaf is constant (in which case even the first cohomology group would vanish). Thanks to all for your interest. – Georges Elencwajg Oct 11 2009 at 18:01

The sheaf cohomology Hi(X,F) of a (topological) manifold X of dimension n vanishes for i > n. This is a topological version of Grothendieck's vanishing theorem above. You can find this result in Kashiwara-Schapira's "Sheaves on manifolds" proposition III.3.2.2.

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 Thank you,Abdo', you have given my question a definitive answer. Congratulations on your erudition: I have the pleasant feeling I will read more from you on this site! – Georges Elencwajg Oct 16 2009 at 19:01

The result you're after is in Hartshorne:

Theorem 2.7 (Grothendieck)
Let X be a noetherian topological space of dimension n.
Then Hi(X,F) = 0 for all i > n and all sheaves of abelian groups F.

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But as a topological space, R is non-noetherian and infinite-dimensional. – Anton Geraschenko Oct 16 2009 at 5:03
Good point, silly me. – Sam Derbyshire Oct 16 2009 at 14:22

Since now we know that R in your question refers to real line equipped with standard topology, sheaf cohomology will always have H^i(F) = 0 for i>1 — depending on how you define sheaf cohomology this is a theorem of different difficulty.

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