On Galois representations associated to mod $p$ Hilbert modular forms

TL;DR

This paper establishes a link between mod p Hilbert modular forms and Galois representations over totally real fields, generalizing Serre's conjecture and exploring weights in ramified cases.

Contribution

It proves the existence of Galois representations for arbitrary weight mod p Hilbert modular forms and formulates a conjecture on their weights, extending prior results to ramified primes.

Findings

Existence of associated 2D Galois representations for eigenforms.

Formulation of a conjecture predicting weights of eigenforms.

Partial results towards the conjecture for ramified primes in real quadratic fields.

Abstract

We consider mod $p$ Hilbert modular forms for a totally real field $F$, viewed as sections of automorphic line bundles on Hilbert modular varieties in prime characteristic $p$. For a Hecke eigenform of arbitrary weight, we prove the existence of an associated two-dimensional representation of the absolute Galois group of $F$. Furthermore, for any such irreducible Galois representation, we formulate a conjecture predicting the set of weights of eigenforms from which it arises. This generalizes Edixhoven's variant of the weight part of Serre's Conjecture (in the case $F = \mathbb{Q}$), and removes the restriction that $p$ be unramified in $F$ from prior work in this direction. We also establish one direction of a conjectural relation with the algebraic analogue of the weight part of Serre's Conjecture in this context. Finally, we prove results towards our conjecture in the case of partial weight one for real quadratic fields $F$ in which $p$ is ramified.