Farkas’ identities with quartic characters

Abstract

Farkas in (On an Arithmetical Function II. Complex Analysis and Dynamical Systems II. Contemporary Mathematics, American Mathematical Society, Providence, 2005) introduced an arithmetic function $\delta$ and found an identity involving $\delta$ and a sum of divisor function $\sigma '$. The first-named author and Raji in (Ramanujan J 19(1):19–27, 2009) discussed a natural generalization of the identity by introducing a quadratic character $\chi$ modulo a prime $p \equiv 3 \ (\mathrm {mod}\ 4)$. In particular, it turns out that, besides the original case $p=3$ considered by Farkas, an exact analog (in a certain precise sense) of Farkas’ identity happens only for $p=7$. Recently, for quadratic characters of small composite moduli, Williams in (Ramanujan J 43(1):197–213, 2017) found a finite list of identities of similar flavor using different methods. Clearly, if $p \not \equiv 3 \ (\mathrm {mod}\ 4)$, the character $\chi$ is either not quadratic or even. In this paper, we prove that, under certain conditions, no analogs of Farkas’ identity exist for even characters. Assuming $\chi$ to be odd quartic, we produce something surprisingly similar to the results from Guerzhoy and Raji (Ramanujan J 19(1):19–27, 2009): exact analogs of Farkas’ identity happen exactly for $p=5$ and 13.abstract