Interesting Examples
If Z is the integral group ring for a group G, then Ext*
Z(Z, M) is the group cohomology H*(G,M) with coefficients in M.
For Fp the finite field on p elements, we also have that H*(G,M) = Ext*
Fp(Fp, M), and it turns out that the group cohomology doesn't depend on the base ring chosen.
If A is a k-algebra, then Ext*
A ⊗k Aop(A, M) is the Hochschild cohomology HH*(A,M) with coefficients in the A-bimodule M.
If R is chosen to be the universal enveloping algebra for a Lie algebra, then Ext*
R(R, M) is the Lie algebra cohomology with coefficients in the module M.
Read more about this topic: Ext Functor
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