Hardy-Ramanujan Journal |

The Eichler-Selberg trace formulas express the traces of Hecke operators on a spaces of cusp forms in terms of weighted sums of Hurwitz-Kronecker class numbers. For cusp forms on $\text {\rm SL}_2(\mathbb{Z}),$ Zagier proved these formulas by cleverly making use of the weight 3/2 nonholomorphic Eisenstein series he discovered in the 1970s. The holomorphic part of this form, its so-called {\it mock modular form}, is the generating function for these class numbers. In this expository note we revisit Zagier's method, and we show how to obtain such formulas for congruence subgroups, working out the details for $\Gamma_0(2)$ and $\Gamma_0(4).$ The trace formulas fall out naturally from the computation of the Rankin-Cohen brackets of Zagier's mock modular form with specific theta functions.

Source: HAL:hal-03914242v1

Volume: Volume 45 - 2022

Published on: February 6, 2023

Imported on: February 6, 2023

Keywords: Eichler-Selberg trace formula,Harmonic Maass forms,mock modular forms,11F11, 11F37,[MATH]Mathematics [math]

Funding:

- Source : OpenAIRE Graph
*Harmonic Maass Forms, "Moonshine," and Arithmetic Statistics*; Funder: National Science Foundation; Code: 2055118*REU Site: Algebra and Number Theory at Emory University*; Funder: National Science Foundation; Code: 2002265

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