L2-Cohomology and group cohomology

Module cohomology group of inverse semigroup algebras

Let $S$ be an inverse semigroup and let $E$ be its subsemigroup of idempotents. In this paper we define the $n$-th module cohomology group of Banach algebras and show that the first module cohomology group $HH^1_{ell^1(E)}(ell^1(S),ell^1(S)^{(n)})$ is zero, for every odd $ninmathbb{N}$. Next, for a Clifford semigroup $S$ we show that $HH^2_{ell^1(E)}(ell^1(S),ell^1(S)^{(n)})$ is a Banach sp...

Cohomology and Group Representations

L2-cohomology and Complete Hamiltonian Manifolds

A classical theorem of Frankel for compact Kähler manifolds states that a Kähler S-action is Hamiltonian if and only if it has fixed points. We prove a metatheorem which says that when Hodge theory holds on non-compact manifolds, then Frankel’s theorem still holds. Finally, we present several concrete situations in which the assumptions of the metatheorem hold. 1. The Classical Frankel Theorem ...

Rigidity and L2 cohomology of hyperbolic manifolds

— When X = Γ\Hn is a real hyperbolic manifold, it is already known that if the critical exponent is small enough then some cohomology spaces and some spaces of L2 harmonic forms vanish. In this paper, we show rigidity results in the borderline case of these vanishing results. Résumé. — La petitesse de l’exposant critique du groupe fondamental d’une variété hyperbolique implique des résultats d’...

Isotropy in Group Cohomology

The analogue of Lagrangians for symplectic forms over finite groups is studied, motivated by the fact that symplectic G-forms with a normal Lagrangian N⊳G are in one-to-one correspondence, up to inflation, with bijective 1-cocycle data on the quotients G/N . This yields a method to construct groups of central type from such quotients, known as Involutive Yang-Baxter groups. Another motivation f...