On the number of Galois orbits of newforms

Abstract

Counting the number of Galois orbits of newforms in Sk(Γ0(N) and giving some arithmetic sense to this number is an interesting open problem. The case N D 1 corresponds to Maeda's conjecture (still an open problem) and the expected number of orbits in this case is 1, for any k ≥ 16. In this article we give local invariants of Galois orbits of newforms for general N and count their number. Using an existence result of newforms with prescribed local invariants we prove a lower bound for the number of non-CM Galois orbits of newforms for Γ00(N) for large enough weight k (under some technical assumptions on N). Numerical evidence suggests that in most cases this lower bound is indeed an equality, thus we leave as a question the possibility that a generalization of Maeda's conjecture could follow from our work. We finish the paper with some natural generalizations of the problem and show some of the implications that a generalization of Maeda's conjecture has.