Companion forms and weight one forms

Abstract

In this paper we prove the following theorem. Let L/\Q_p be a finite extension with ring of integers O_L and maximal ideal lambda. Theorem 1. Suppose that p >= 5. Suppose also that \rho:G_\Q -> GL_2(O_L) is a continuous representation satisfying the following conditions. 1. \rho ramifies at only finitely many primes. 2. \rho mod \lambda is modular and absolutely irreducible. 3. \rho is unramified at p and \rho(Frob_p) has eigenvalues \alpha and \beta with distinct reductions modulo \lambda. Then there exists a classical weight one eigenform f = \sum_{n=1}^\infty a_m(f) q^m and an embedding of \Q(a_m(f)) into L such that for almost all primes q, a_q(f)=tr(\rho(\Frob_q)). In particular \rho has finite image and for any embedding i of L in \C, the Artin L-function L(i o \rho, s) is entire.