L^2-Betti numbers of locally compact groups and their cross section equivalence relations

2011

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We prove that the L-Betti numbers of a unimodular locally compact group G coincide, up to a natural scaling constant, with the L-Betti numbers of the countable equivalence relation induced on a cross section of any essentially free ergodic probability measure preserving action of G. As a consequence, we obtain that the reduced and un-reduced L-Betti numbers of G agree and that the L-Betti numbers of a lattice Γ in G equal those of G up to scaling by the covolume of Γ in G. We also deduce several vanishing results, including the vanishing of the reduced L-cohomology for amenable locally compact groups.

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