Supports unipotents de faisceaux caracteres

Abstract

Soit G un groupe algebrique reductif sur la cloture algebrique d'un corps fini F_q et defini sur ce dernier. L'existence du support unipotent d'un caractere irreductible du groupe fini G(F_q), ou d'un faisceau caractere de G, a ete etablie dans differents cas par Lusztig, Geck et Malle, et le second auteur. Dans cet article, nous demontrons que toute classe unipotente sur laquelle la restriction du faisceau caractere ou du caractere donne est non nulle est contenue dans l'adherence de Zariski de son support unipotent. Pour etablir ce resultat, nous etudions certaines representations des groupes de Weyl, dites "bien supportees". Let G be a reductive algebraic group over the algebraic closure of a finite field F_q that is defined over F_q. Under various conditions, Lusztig, Geck and Malle, and the second author have shown that an irreducible character of the finite group G(F_q), or a character sheaf on G, has a unipotent support. In this paper, we show that every unipotent class on which a given character or character sheaf has nonzero restriction is contained in the Zariski closure of its unipotent support. This result is obtained by studying the class of so-called "well-supported" representations of Weyl groups.