Thursday, 17 May 2012

homological algebra - Is tensor product exact in abelian tensor categories with duals?

Suppose we are in an abelian tensor category with duals, where all objects have finite length. Let 0toAtoBtoCto0 be a short exact sequence and Z an object of the category. Is
0toZotimesAtoZotimesBtoZotimesCto0


exact?



Motivation: I am reading the proof of Proposition 5.7 in this paper of Deligne and trying to figure out why the lower sequence at the bottom of page 23 is exact. I believe mathcalHom(X,Y) here is XveeotimesY, although I have not actually found the point in the paper where he defines it. What he is trying to prove is that the corresponding sequence of external Hom's is exact, so he can't be using that fact.



There are, of course, tons of abelian tensor categories where otimes is not exact. For example, modules for any commutative ring A which is not a field, with tensor product otimesA. But these don't usually have duals for all of their objects.

No comments:

Post a Comment