Hauptmoduln and even-order mock theta functions modulo 2

Soon-Yi Kang, Seonkyung Kim, Toshiki Matsusaka +1

The Fourier coefficients $c_1(n)$ of the elliptic modular $j$-function are always even for $n\not\equiv 7(\mathrm{mod}8)$. In contrast, for $n\equiv 7(\mathrm{mod}8)$, it is conjectured that…