Vector-valued Hirzebruch-Zagier series and class number sums

Abstract

For any number $m\equiv 0,1(4)$ we correct the generating function of Hurwitz class number sums ${\sum }_{r}H(4n-m{r}^2)$ to a modular form (or quasimodular form if $m$ is a square) of weight two for the Weil representation attached to a binary quadratic form of discriminant $m$ and determine its behavior in the Petersson scalar product. This modular form arises through holomorphic projection of the zero-value of a nonholomorphic Jacobi Eisenstein series of index $1/m$. When $m$ is prime, we recover the classical Hirzebruch-Zagier series whose coefficients are intersection numbers of curves on a Hilbert modular surface. Finally we calculate certain sums over class numbers by comparing coefficients with an Eisenstein series.

12 pages

References in corpus (1)

• Elliptic curves, modular forms, and sums of Hurwitz class numbers