A Note on Congruences for Weakly Holomorphic Modular Forms

TL;DR

This paper proves new congruence properties of coefficients of weakly holomorphic modular forms of low weight, extending previous results and addressing special cases for primes 2 and 3.

Contribution

It establishes congruences for Fourier coefficients of weakly holomorphic modular forms for primes 2 and 3, completing the known cases of a theorem by Jin, Ma, and Ono.

Findings

Coefficients vanish modulo p for primes p ≥ 5 under specified conditions.

Similar vanishing results hold for p=2,3 with additional parity conditions.

Completes the proof of a conjecture by Jin, Ma, and Ono for all primes.

Abstract

Let $O_L$ be the ring of integers of a number field $L$. Write $q = e^{2 \pi i z}$, and suppose that $$f(z) = \sum_{n \gg - \infty}^{\infty} a_f(n) q^n \in M_{k}^{!}(\operatorname{SL}_2(\mathbb{Z})) \cap O_L[[q]]$$ is a weakly holomorphic modular form of even weight $k \leq 2$. We answer a question of Ono by showing that if $p \geq 5$ is prime and $ 2-k = r(p-1) + 2 p^t$ for some $r \geq 0$ and $t > 0$, then $a_f(p^t) \equiv 0 \pmod p$. For $p = 2,3,$ we show the same result, under the condition that $2 - k - 2 p^t$ is even and at least $4$. This represents the "missing case" of a theorem proved by Jin, Ma, and Ono.