# Exercise in Galois cohomology

Let $K$ be a local field, and $\bar{K}$ its algebraic closure. Let $\mathcal{C}$ be the category of (continuous) $G=\text{Gal}(\bar{K}/K)$-modules. Let $M$ be a finite $G$-module. For any injective module $I$ of $\mathcal{C}$,

is $H^r(G, \text{Hom}_{\mathbb{Z}}(M,~I))=0$ for all $r>0$?

(Here on $\text{Hom}_{\mathbb{Z}}(M,~A)$, $g \in G$ acts as $(g\phi)(x)=g(\phi(g^{-1}(x)))$.)

• Just to clarify, are you really considering all $G$-modules? No topology involved, just some abelian group with a $G$-action? – user94041 Feb 24 '17 at 16:41
• Sorry, I changed a bit. Thank you very much. – user1225 Feb 24 '17 at 16:49
• With the new formulation, I would be very surprised if $\mathcal{C}$ had any non-zero injectives. – David Loeffler Feb 24 '17 at 17:04
• Thanks David, I changed again, which seems better than before. – user1225 Feb 24 '17 at 17:30
• If this "exercise" (for which no motivation has been given) is now rendered moot due to the answer to your other question at mathoverflow.net/questions/263088/… (which seems almost surely to be what the motivation was), perhaps the above question should be edited to reflect that the exercise is no longer of interest to you (if I understand correctly the motivation). – nfdc23 Feb 25 '17 at 23:14