Elliptic curves with a rational 2-torsion point ordered by conductor and the boundedness of average rank

Abstract

In this paper we refine recent work due to A. Shankar, A. N. Shankar, and X. Wang on counting elliptic curves by conductor to the case of elliptic curves with a rational 2-torsion point. This family is a small family, as opposed to the large families considered by the aforementioned authors. We prove the analogous counting theorem for elliptic curves with so-called square-free index as well as for curves with suitably bounded Szpiro ratios. We note that our assumptions on the size of the Szpiro ratios is less stringent than would be expected by the naive generalization of their approach.