Fields Generated by a Single Point on Elliptic Curves

Abstract

We study the finite extensions of a number field generated by a single point on an elliptic curve. We construct geometrically non-isomorphic elliptic curves with complex multiplication that generate the same collection of extensions, giving a negative answer to a question of Mazur and Rubin. For torsion-generated fields, we prove that inclusion of the collections up to finitely many exceptions implies isogeny over the ground field, and in the non-CM case even isomorphism. When all geometric endomorphisms are defined over the ground field, we characterize equality of the torsion-generated collections by the existence, for every prime $p$, of an isogeny over the ground field of degree prime to $p$. This condition also implies equality of the full collections of fields generated by a single point.