On the Brumer-Stark Conjecture

TL;DR

This paper proves the Brumer-Stark conjecture for abelian extensions of number fields away from p=2, establishing a formula for class group annihilation using advanced modular forms and congruences.

Contribution

It extends Ribet's method with group ring valued Hilbert modular forms to prove a stronger version of the Brumer-Stark conjecture, including implications for Rubin's conjecture.

Findings

Proves Brumer-Stark conjecture away from p=2.

Establishes a formula for the Fitting ideal of class groups.

Shows the stronger result implies Rubin's higher rank conjecture.

Abstract

Let $H/F$ be a finite abelian extension of number fields with $F$ totally real and $H$ a CM field. Let $S$ and $T$ be disjoint finite sets of places of $F$ satisfying the standard conditions. The Brumer-Stark conjecture states that the Stickelberger element $\Theta^{H/F}_{S, T}$ annihilates the $T$-smoothed class group $\text{Cl}^T(H)$. We prove this conjecture away from $p=2$, that is, after tensoring with $\mathbf{Z}[1/2]$. We prove a stronger version of this result conjectured by Kurihara that gives a formula for the 0th Fitting ideal of the minus part of the Pontryagin dual of $\text{Cl}^T(H) \otimes \mathbf{Z}[1/2]$ in terms of Stickelberger elements. We also show that this stronger result implies Rubin's higher rank version of the Brumer-Stark conjecture, again away from 2. Our technique is a generalization of Ribet's method, building upon on our earlier work on the Gross-Stark conjecture. Here we work with group ring valued Hilbert modular forms as introduced by Wiles. A key aspect of our approach is the construction of congruences between cusp forms and Eisenstein series that are stronger than usually expected, arising as shadows of the trivial zeroes of $p$-adic $L$-functions. These stronger congruences are essential to proving that the cohomology classes we construct are unramified at $p$.