The fractional Galois ideal for arbitrary order of vanishing

TL;DR

This paper introduces the fractional Galois ideal as a generalization of the Stickelberger ideal, aiming to better understand class-group annihilators through $L$-functions with arbitrary order of vanishing at zero.

Contribution

It proposes a new fractional Galois ideal for arbitrary order of vanishing, extending previous work and relating it to the Fitting ideal of class-groups, with simplified proof techniques.

Findings

The fractional Galois ideal relates closely to the Fitting ideal of class-groups.

Provides evidence of the ideal's effectiveness in class-group annihilation.

Proves an equality involving Stark elements and class-groups under new assumptions.

Abstract

We propose a candidate, which we call the fractional Galois ideal after Snaith's fractional ideal, for replacing the classical Stickelberger ideal associated to an abelian extension of number fields. The Stickelberger ideal can be seen as gathering information about those $L$-functions of the extension which are non-zero at the special point $s = 0$, and was conjectured by Brumer to give annihilators of class-groups viewed as Galois modules. An earlier version of the fractional Galois ideal extended the Stickelberger ideal to include $L$-functions with a simple zero at $s = 0$, and was shown by the present author to provide class-group annihilators not existing in the Stickelberger ideal. The version presented in this article deals with $L$-functions of arbitrary order of vanishing at $s = 0$, and we give evidence using results of Popescu and Rubin that it is closely related to the Fitting ideal of the class-group, a canonical ideal of annihilators. Finally, we prove an equality involving Stark elements and class-groups originally due to B\"uy\"ukboduk, but under a slightly different assumption, the advantage being that we need none of the Kolyvagin system machinery used in the original proof.