Abstract

We construct a natural basis for the space of weak harmonic Maass forms of weight 5/2 on the full modular group. The nonholomorphic part of the first element of this basis encodes the values of the ordinary partition function p(n). We obtain a formula for the coefficients of the mock modular forms of weight 5/2 in terms of regularized inner products of weakly holomorphic modular forms of weight −1/2, and we obtain Hecke-type relations among these mock modular forms.

Highlights

  • A number of recent works have considered bases for spaces of weak harmonic Maass forms of small weight

  • Borcherds [4] and Zagier [25] made use of the basis {fd}d>0 defined by f−d = q−d+O(q) for the space of weakly holomorphic modular forms of weight 1/2 in the Kohnen plus space of level 4

  • Imamoglu and Tóth [14] extended this to a basis {fd}d∈Z for the space of weak harmonic Maass forms of the same weight and level and interpreted the coefficients in terms of cycle integrals of the modular j-function

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Summary

Introduction

A number of recent works have considered bases for spaces of weak harmonic Maass forms of small weight. In subsequent work, [16] they constructed a similar basis in the case of weight 2 for the full modular group, and related the coefficients of these forms to regularized inner products of an infinite family of modular functions. To construct these bases requires various types of Maass-Poincaré series. We will construct a natural basis for the space of weak harmonic Maass forms of weight 5/2 on SL2(Z) with a certain multiplier. The last three sections contain the proofs of the remaining results

Weak harmonic Maass forms
Poincaré series
Whittaker functions and nonholomorphic Maass-Poincaré series
Proof of Theorem 5
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