Indices isotypiques des éléments cyclotomiques

Abstract

Given $F$ a real abelian field, $p$ an odd prime and $\chi$ any Dirichlet character of $F$ we give a method for computing the $\chi$-index $(H^1(G_S,{\mathbb{Z}}_p(r){)}^{\chi }:C^F(r{)}^{\chi })$ where the Tate twist $r$ is an odd integer $r\ge 3$, the group $C^F(r)$ is the group of higher circular units, $G_S$ is the Galois group over $F$ of the maximal $S$ ramified algebraic extension of $F$, and $S$ is the set of places of $F$ dividing $p$. This $\chi$-index can now be computed in terms only of elementary arithmetic of finite fields $\FM_\ell$. Our work generalizes previous results by Kurihara who used the assumption that the order of $\chi$ divides $p-1$.

25 pages, revised version, accepted for publication by Tokyo J. Maths