Groupes de type mixte

Décompositions de Groupes par Produit Direct et Groupes de Coxeter

We provide examples of groups which are indecomposable by direct product, and more generally which are uniquely decomposable as direct products of indecomposable groups. Examples include Coxeter groups, for which we give an alternative approach to recent results of L. Paris. For a finitely generated linear group Γ, we establish an upper bound on the number of factors of which Γ can be the direc...

Normalisateurs et groupes d’Artin-Tits de type sphérique

Let (AS , S) be an Artin-Tits and X a subset of S ; denote by AX the subgroup of AS generated by X. When AS is of spherical type, we prove that the normalizer and the commensurator of AX in AS are equal and are the product of AX by the quasi-centralizer of AX in AS . Looking to the associated monoids A + S and A + X , we describe the quasi-centralizer of A+X in A + S thanks to results in Coxete...

Groupes de Galois et nombres automatiques

Description explicite d’une famille de groupes de Kac-Moody affines

For each standard affine data satisfying some condition on the coroots, we define a group functor on the category of all rings which is a subgroup of some group functor R 7→ G(R[t, t−1])o T (R), where G is a Chevalley scheme and T is a torus. This functor is then shown to satisfy the conditions of the five axioms required by [Ti4] to be a ‘minimal’ Kac-Moody group.

Treillis et bases des groupes de Coxeter

A Dominique Foata, pionnier de la combinatoirè a qui elle doit tant 1. Introduction. — Le présent travail est une contributionà l'´ etude de l'ordre fort (dit aussi de Bruhat) dans un groupe de Coxeter fini W. Les définitions et propriétés relatives aux treillis, aux ensembles or-donnés, aux bases et aux treillis enveloppants, &c. sont rassemblées de manì ere minimale dans la section 2, les pre...