Constants in Titchmarsh divisor problems for elliptic curves

Abstract

Inspired by the analogy between the group of units $$\mathbb {F}_p^{\times }$$ of the finite field with p elements and the group of points $$E(\mathbb {F}_p)$$ of an elliptic curve $$E/\mathbb {F}_p$$, E. Kowalski, A. Akbary & D. Ghioca, and T. Freiberg & P. Kurlberg investigated the asymptotic behaviour of elliptic curve sums analogous to the Titchmarsh divisor sum $$\sum \nolimits _{p \le x} \tau (p + a) \sim C x$$. In this paper, we present a comprehensive study of the constants C(E) emerging in the asymptotic study of these elliptic curve divisor sums in place of the constant C above. Specifically, by analyzing the division fields of an elliptic curve $$E/\mathbb {Q}$$, we prove bounds for the constants C(E) and, in the generic case of a Serre curve, we prove explicit closed formulae for C(E) amenable to concrete computations. Moreover, we compute the moments of the constants C(E) over two-parameter families of elliptic curves $$E/\mathbb {Q}$$. Our methods and results complement recent studies of average constants occurring in other conjectures about reductions of elliptic curves by addressing not only the average behaviour, but also the individual behaviour of these constants, and by providing explicit tools towards the computational verifications of the expected asymptotics.