NON-OPTIMAL LEVELS OF A REDUCIBLE MOD l MODULAR REPRESENTATION

Abstract

Let l >= 5 be a prime and let N be a square-free integer prime to l. For each prime p dividing N, let ap be either 1 or -1. We give sufficient criteria for the existence of a newform f of weight 2 for G0( N) such that the mod l Galois representation attached to f is reducible and Upf = apf for primes p dividing N. The main techniques used are level raising methods based on an exact sequence due to Ribet. c.2018 American Mathematical Society