On Divisors of Modular Forms

Abstract

The denominator formula for the Monster Lie algebra is the product expansion for the modular function $j(z)-j(\tau )$ given in terms of the Hecke system of ${\mathrm{SL}}_2(\mathbb{Z})$-modular functions $j_n(\tau )$. It is prominent in Zagier's seminal paper on traces of singular moduli, and in the Duncan-Frenkel work on Moonshine. The formula is equivalent to the description of the generating function for the $j_n(z)$ as a weight 2 modular form with a pole at $z$. Although these results rely on the fact that $X_0(1)$ has genus 0, here we obtain a generalization, framed in terms of polar harmonic Maass forms, for all of the $X_0(N)$ modular curves. We use these functions to study divisors of modular forms.