The purpose of this note is to introduce a trick which relates the (non)-vanishing of the top-dimensional ℓ2-Betti numbers of actions with that of sub-actions. We provide three different types of applications: we prove that the ℓ2-Betti numbers of Aut(Fn) and Out(Fn) (and of their Torelli subgroups) do not vanish in degree equal to their virtual cohomological dimension, we prove that the subgroups of the 3-manifold groups have vanishing ℓ2-Betti numbers in degree 3 and 2 and we prove for instance that F2d × Z has ergodic dimension d + 1.
What
is the meaning of Camille Noûs ? (Curriculum Vitæ of
Camille Noûs)