Soit un groupe muni d’une fonction-longueur , et soit un sous-espace vectoriel de . On dira que satisfait à l’inégalité de Haagerup s’il existe des constantes telles que, pour tout , la norme de convolution de sur soit dominée par fois la norme de . Nous montrons que, pour , l’inégalité de Haagerup s’exprime en termes de décroissance des marches aléatoires associées à des mesures de probabilité symétriques à support fini sur . Si est la longueur des mots sur un groupe de type fini, nous montrons que, si l’espace des fonctions radiales par rapport à satisfait à l’inégalité de Haagerup, alors est non moyennable si et seulement si est à croissance superexponentielle. Nous montrons aussi que l’inégalité de Haagerup pour a une interprétation purement combinatoire; en utilisant le résultat principal de l’article de J. Swiatkowski dans le même fascicule, nous déduisons que, pour un groupe opérant simplement transitivement sur les sommets d’un immeuble euclidien épais de type , l’espace satisfait à l’inégalité de Haagerup, et est non moyennable.
Let be a group endowed with a length function , and let be a linear subspace of . We say that satisfies the Haagerup inequality 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 with respect to satisfies the Haagerup inequality, then is non-amenable if and only if has superpolynomial growth. We also show that the Haagerup inequality for has a purely combinatorial interpretation; thus, using the main result of the companion paper by J. Swiatkowski, we deduce that, for a group acting simply transitively on the vertices of a thick euclidean building of type , the space satisfies the Haagerup inequality, and is non-amenable.
@article{AIF_1997__47_4_1195_0, author = {Valette, Alain}, title = {On the Haagerup inequality and groups acting on $\tilde{A}\_n$-buildings}, journal = {Annales de l'Institut Fourier}, volume = {47}, year = {1997}, pages = {1195-1208}, doi = {10.5802/aif.1596}, mrnumber = {99f:43001}, zbl = {0886.51003}, language = {en}, url = {http://dml.mathdoc.fr/item/AIF_1997__47_4_1195_0} }
Valette, Alain. On the Haagerup inequality and groups acting on $\tilde{A}_n$-buildings. Annales de l'Institut Fourier, Tome 47 (1997) pp. 1195-1208. doi : 10.5802/aif.1596. http://gdmltest.u-ga.fr/item/AIF_1997__47_4_1195_0/
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