A modular framework for generalized Hurwitz class numbers II

In a recent preprint, we constructed a sesquiharmonic Maass form $\mathcal{G}$ of weight $\frac{1}{2}$ and level $4N$ with $N$ odd and squarefree. Extending seminal work by Duke, Imamoglu, and Tóth, $\mathcal{G}$ maps to Zagier’s non-holomorphic Eisenstein series and a linear combination of Pei and Wang’s generalized Cohen–Eisenstein series under the Bruinier–Funke operator ${\xi }_{\frac{1}{2}}$. In this paper, we realize $\mathcal{G}$ as the output of a regularized Siegel theta lift of $1$ whenever $N=p$ is an odd prime building on more general work by Bruinier, Funke and Imamoglu. In addition, we supply the computation of the square-indexed Fourier coefficients of $\mathcal{G}$. This yields explicit identities between the Fourier coefficients of $\mathcal{G}$ and all quadratic traces of $1$. Furthermore, we evaluate the Millson theta lift of $1$ and consider spectral deformations of $1$.