Two torsion, the endomorphism field, and the finitude of endomorphism algebras for a class of abelian varieties

Abstract: A conjecture attributed to Robert Coleman states there should only be finitely many isomorphism classes for endomorphism algebras of abelian varieties of fixed dimension over a fixed number field. In this paper, we establish new cases of this conjecture, given conditions on the 2-torsion field. In fact, for the abelian varieties considered, we provide an explicit list of possible endomorphism algebras. This list is then refined by studying the minimal extension of the ground field over which all endomorphisms are defined. In particular, we find conditions which guarantee this extension is contained in the 2-torsion field.