Torsion groups of elliptic curves over quadratic fields

Abstract

We describe methods to determine all the possible torsion groups of an elliptic curve that actually appear over a xed quadratic field. We use these methods to find, for each group that can appear over a quadratic field, the field with the smallest absolute value of its discriminant such that there exists an elliptic curve with that torsion. We also examine the interplay of the torsion and rank over a fixed quadratic field and see that what happens is very different than over Q. Finally we give some results concerning the number and density of fields with an elliptic curve with given torsion over them.