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Hi everybody!! I am studing cohomology of sheaves on schemes. I have a question for you. Let $X$ be a noetherian scheme. Do you know if the cathegory QCoh(X) has enough injectives? I know that it has "enough flasque" and that Mod(X) has enough injective but what about QCoh(X)? Thank you

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Yes. If $X$ is a Noetherian scheme, then the category of quasi-coherent sheaves on $X$ has enough injectives. See Hartshorne's Algebraic Geometry, exercise III.3.6(a).

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First of all, let me quote the famous paper by Grothendieck "Sur quelques points d'algebre homologique" (Tohoku Math part 1 & part 2). According to Theorem 1.10.1 there, every abelian category satisfying (AB5) (equivalent to exactness of filtered direct limits) and with a generator has enough injectives, For every scheme $X$ it is well known that $Qco(X)$ is abelian and satisfies (AB5), see, for instance, EGA I, new edition, Corollaire (2.2.2)(iv) where it is proved that such a limit preserves quasi-coherence.

So, the only issue is the existence of a generator. For some time it was known that over a quasi-compact quasi-separated scheme, $Qco(X)$ has a generator. For a nice geometric argument, consult the proof of Theorem (4) in Kleiman's "Relative duality for quasi-coherent sheaves". Note that any noetherian scheme $X$ is quasi-compact and quasi-separated, EGA I, (6.1.1) and (6.1.13).

Surprisingly, using techniques from relative homological algebra, Enochs and Estrada proved in 2005 the existence of a generator for any scheme. See their paper "Relative homological algebra in the category of quasi-coherent sheaves". (There was a previous unpublished proof by O. Gabber).

Summing up, for any scheme $X$, the category $Qco(X)$ has enough injectives.

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For an english translation of Grothendieck's Tohoku paper by Michael Barr, look at – Leo Alonso Sep 22 '10 at 15:19
Is it possible to give a better link to the english translation of Tohoku? When I click on your link, it doesn't work. (Thanks to you and to Michael Barr!) – Ravi Vakil May 31 '12 at 16:14
@Ravil Vakil Here is a working link: – Leo Alonso Oct 15 '15 at 9:51

Yes, it has enough injectives. As explained in Hartshorne's "Residues and Duality", the injective hull of a quasi-coherent sheaf in Mod(X) is again quasi-coherent. This is even more than you asked for.

Less explicit, you can show that Qcoh(X) is a Grothendieck category (see Daniel Murfet's Notes) provided X is concentrated, and as such has enough injectives.

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