Eisenstein series modulo $p^2$

Abstract

We study congruences for Eisenstein series on ${\mathrm{SL}}_2(\mathbb{Z})$ modulo $p^2$, where $p\ge 5$ is prime. It is classically known that all Eisenstein series of weight at least $4$ are determined modulo $p^2$ by those of weight at most $p^2-p+2$. We prove that up to powers of $E_{p-1}$, each such Eisenstein series is in fact determined modulo $p^2$ by a modular form of weight at most $2p-4$. We also determine $E_2$ modulo $p^2$ in terms of a modular form of weight $p+1$.