A modular framework of generalized Hurwitz class numbers

Abstract

We discover a non-trivial rather simple relation between the mock modular generating functions of the level $1$ and level $N$ Hurwitz class numbers. This relation yields a holomorphic modular form of weight $\frac{3}{2}$ and level $4N$, where $N > 1$ is stipulated to be odd and square-free. We extend this observation to a non-holomorphic framework and obtain a higher level non-holomorphic Zagier Eisenstein series as well as a higher level preimage of it under the Bruinier--Funke operator $ξ_{\frac{1}{2}}$. All of these observations are deduced from a more general inspection of a certain weight $\frac{1}{2}$ Maass--Eisenstein series of level $4N$ at its spectral point $s=\frac{3}{4}$. This idea goes back to Duke, Imamoglu and Tóth in level $4$ and relies on the theory of so-called sesquiharmonic Maass forms. We calculate the Fourier expansion of our sesquiharmonic preimage and of its shadow. We conclude by offering an example if $N=5$ or $N=7$ and we provide the SAGE code to compute the Fourier coefficients involved.

Theorems & Definitions (43)

• Theorem 1.1
• Remark
• Theorem 1.2
• Theorem 1.3
• Remark
• Example
• Definition 2.1
• Lemma 2.2
• proof
• Lemma 2.3: koh85*p. 257