Eichler-Selberg relations for the third-order mock theta functions

Abstract

Ramanujan's third-order mock theta function $f(q)$ admits the well-known Appell-Lerch series representation: \[ \sum_{n=0}^{\infty}\frac{q^{n^2}}{(-q;q)_n^2}=\frac{2}{(q;q)_{\infty}}\sum_{n=-\infty}^{\infty}\frac{(-1)^nq^{\frac{3}{2}n^2+\frac{1}{2}n}}{1+q^n}. \] In this paper, we establish a natural generalization of this classical identity by utilizing the theory of harmonic Maass forms. Furthermore, we prove analogous Eichler-Selberg type relations for another third-order mock theta function $ω(q)$. The method presented in this paper can be extended to study other classes of mock theta functions.