The image of the generalized Dedekind sum

Abstract

The newform Dedekind sum $S_{\chi_1, \chi_2}$ associated to a pair of primitive Dirichlet characters $\chi_1$, $\chi_2$ of respective conductors $q_1$, $q_2$, is a group homomorphism from $\Gamma_1(q_1 q_2)$ into the number field $F_{\chi_1, \chi_2}$ generated by the values of the characters. It is a basic question to identify the image of this map, which is known to be a lattice $L_{\chi_1, \chi_2}$ in $F_{\chi_1, \chi_2}$. It has recently been conjectured that when $\chi_1$ and $\chi_2$ are quadratic, then $ L_{\chi_1, \chi_2} = 2 \mathbb{Z}$. In this paper, we make some progress towards this conjecture by exhibiting an explicit lattice in which $L_{\chi_1, \chi_2}$ is contained; in particular, when the characters are quadratic, the $q_i$ are coprime, odd, and sufficiently large, then $L_{\chi_1, \chi_2} \subseteq \mathbb{Z}$.