Fourier coefficients of half-integral weight modular forms modulo ell

Abstract

For each prime $\ell$, let $|\cdot|_\ell$ be an extension to $\bar \Q$ of the usual $\ell$-adic absolute value on $\Q$. Suppose $g(z) = \sum_{n=0}^\infty c(n)q^n \in M_{k+\half}(N)$ is an eigenform whose Fourier coefficients are algebraic integers. Under a mild condition, for all but finitely many primes $\ell$ there are infinitely many square-free integers $m$ for which $|c(m)|_\ell = 1$. Consequently we obtain indivisibility results for ``algebraic parts'' of central critical values of modular $L$-functions and class numbers of imaginary quadratic fields. These results partially answer a conjecture of Kolyvagin regarding Tate-Shafarevich groups of modular elliptic curves. Similar results were obtained earlier by Jochnowitz by a completely different method. Our method uses standard facts about Galois representations attached to modular forms, and pleasantly uncovers surprising Kronecker-style congruences for $L$-function values. For example if $\Delta(z)$ is Ramanujan's cusp form and $g(z)=\sum_{n=1}^{\infty}c(n)q^n$ is the cusp form for which $$L(\Delta_D,6)=\fracwithdelims(){\pi}{D}^6\frac{\sqrt{D}}{5!}\frac{< \Delta(z),\Delta(z)>} {< g(z),g(z)>}\cdot c(D)^2,$$ for fundamental discriminants $D>0,$ then for $N\geq 1$ $$\sum_{k=-\infty}^\infty c(N-k^2) \equiv \half \sum_{d|N}(\chi_{-1}(d)+\chi_{-1}(N/d))d^6 \pmod {61}. \tag{0}$$