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For a discrete group G, if $M$ is a direct/inverse limit of $M_i$, is $H^i(G, M)$ the direct/inverse limit of the $H^i(G, M_i)$? Of course, cohomology commutes with finite direct sums, but how about general limits/colimits over finite categories? Thank you!

flag	asked Feb 18 at 19:12 Igor Minevich 73●6

Related: mathoverflow.net/questions/120808 – Martin Brandenburg Feb 18 at 21:02

Benjamin Steinberg · Accepted Answer · 2013-02-18 19:34:04Z UTC

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Ken Brown shows in http://www.math.cornell.edu/~kbrown/scan/1975.0050.pdf that group cohomology for a group G commutes with direct limits iff G is of type $FP_\infty$. That is the trivial module $\mathbb Z$ has a projective $\mathbb ZG$ resolution which is finitely generated in each degree.

answered Feb 18 at 19:34

Benjamin Steinberg
10.6k●1●14●49

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I believe all limits are preserved because Hom preserved the and group cohomology is a derived functor of Hom. – Benjamin Steinberg Feb 18 at 19:39

Don't you have problems since inverse limits are not exact? – Lennart Meier Feb 18 at 20:05

By "all limits" in the comment you mean "all inverse limits" ? – Demin Hu Feb 18 at 20:12

By limit I mean in the category sense as opposed to colimits like the misnamed direct limit. – Benjamin Steinberg Feb 18 at 20:17

My first comment about inverse limits is probably wrong because inverse limits are not exact – Benjamin Steinberg Feb 18 at 20:30

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Which limits does group cohomology commute with?

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