Elliptic curves over the rational numbers with semi-abelian reduction and two-division points

Abstract

We classify elliptic curves over the rationals whose Néron model over the integers is semi-abelian, with good reduction at p=2, and whose Mordell--Weil group contains an element of order two that stays non-trivial at p=2. Furthermore, we describe those curves where the element of order two is narrow, or where another element of order two exists, and also express our findings in terms of Deligne--Mumford stacks of pointed curves of genus one.

19 pages, minor changes, to appear in J. Number Theory