Some identities for Appell–Lerch sums and a universal mock theta function

Abstract

In their paper “A survey of classical mock theta functions”, Gordon and McIntosh observed that the classical mock \(\theta \)-functions, including those found by Ramanujan, can be expressed in terms of two ‘universal’ mock \(\theta \)-functions denoted by \(g_{_{2}}\) and \(g_{_{3}}\). These functions are normalized level 2 and level 3 Appell–Lerch functions. In the survey paper, the authors list several identities for certain Appell–Lerch functions and refer the proofs to a future paper with this title, listed in their references as [GM3]. The purpose of this paper is to prove these identities. One of the identities removes the \( \theta \) -quotients from Kang’s formulas, which express \(g_{_{2}}\) and \({g}_{{_{3}}}\) in terms of Zwegers’ \(\mu \)-function and \( \theta \)-quotients.