Dihedral Galois representations and Katz modular forms

Abstract

We show that any two-dimensional odd dihedral representation ρover a finite field of characteristic p>0 of the absolute Galois group of the rational numbers can be obtained from a Katz modular form of level N, character εand weight k, where N is the conductor, εis the prime-to-p part of the determinant and k is the so-called minimal weight of ρ. In particular, k=1 if and only if ρis unramified at p. Direct arguments are used in the exceptional cases, where general results on weight and level lowering are not available.