Unramifiedness of Galois representations attached to weight one Hilbert modular eigenforms mod p
Abstract
The main result of this article states that the Galois representation attached to a Hilbert modular eigenform defined over Fpbar of parallel weight 1 and level prime to p is unramified above p. This includes the important case of eigenforms that do not lift to Hilbert modular forms in characteristic 0 of parallel weight 1. The proof is based on the observation that parallel weight 1 forms in characteristic p embed into the ordinary part of parallel weight p forms in two different ways per prime dividing p, namely via `partial' Frobenius operators.
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