The First Cohomology Group H^1(G,M)

Abstract

This paper characterizes the first cohomology group H^1(G,M) where M is a Banach space (with norm ||.||) that is also a left CG-module such that the elements of G act on M as continuous complex-linear transformations. Of particular interest is the topology on this group induced by the norm topology on M. The first result is that H^1(G,CG) imbeds in H^1(G,M) whenever CG is contained in M which is in turn contained in L^p(G) for some p. This shows immediately that if H^1(G,M)=0, then G has exactly 1 end. Secondly, it is shown that H^1(G,M) is not Hausdorff if and only if there exist f_i in M with norm 1 (||f_i||=1) for all i with the property that ||gf_i-f_i||->0 as i goes to infinity for every g in G. This is then used to show that if ||.|| and M satisfy certain properties and if G satisfies a "strong Folner condition," then H^1(G,M) is not Hausdorff. The second half of the paper gives several applications of these theorems focusing on the free abelian group on n generators. Of particular interest is the case that M is the reduced group C^* algebra of G.