Quadratic Congruences for half-integral weight cusp forms with the eta multiplier

Abstract

Let $\ell \geq 5$ be a prime, and let $ν_η$ denote the Dedekind eta multiplier. For an odd integer $r$, and a real Dirichlet character $ψ$, recent work of Ahlgren, Andersen, and the author showed that quadratic congruences modulo $\ell$ hold for a wide range of half-integral weight cusp forms with multiplier $ψν_η^r$, vastly generalizing certain congruences discovered by Atkin for the partition function. In this paper, we show that such congruences hold when $ψ$ is an arbitrary character. Our methods rely on the theory of modular Galois representations. For primes $\ell \geq 5$, the core of our work is the study of modular Galois representations modulo $\ell$ attached to integer-weight eigenforms with arbitrary Nebentypus whose images are large in a precise sense. Our key new result is that, given a finite set of such representations and $γ\in \SL_2(\F_\ell)$, there exists $σ\in \Gal(\bar{\Q}/\Q(ζ_\ell))$ whose images under the representations are in the conjugacy class of $γ^2$.