Un groupe localement compact a la propriété (T) de Kazhdan si la -cohomologie de tout -module hilbertien est nulle. Cette propriété de rigidité de la théorie des représentations de a trouvé des applications qui vont de la théorie ergodique à la théorie des graphes. Pendant près de 30 ans, les seuls exemples connus de groupes avec la propriété (T), provenaient des groupes algébriques simples sur les corps locaux, ou de leurs réseaux. La situation a radicalement changé ces dernières années :...
Let be a group endowed with a length function , and let be a linear subspace of . We say that if there exists constants such that, for any , the convolutor norm of on is dominated by times the norm of . We show that, for , the Haagerup inequality can be expressed in terms of decay of random walks associated with finitely supported symmetric probability measures on . If is a word length function on a finitely generated group , we show that, if the space of radial functions...
For a normalized positive definite function on a locally compact abelian group , let be the unitary representation associated to by the GNS construction. We give necessary and sufficient conditions for the vanishing of 1-cohomology and reduced 1-cohomology . For example, if and only if either or , where is the trivial character of and is the probability measure on the Pontryagin dual associated to by Bochner’s Theorem. This streamlines an argument of Guichardet (see Theorem...
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