Points of finite order on elliptic curves with complex multiplication

Abstract

Let E be an elliptic curve defined overQ. The group ofQ- rational points of finite order on E is a finite group T(E). In this article T(E) is computed for all elliptic curves defined overQ admitting complex multiplication. The only possible values for the order t of T(E) are 1, 2, 3, 4, or 6 in these cases. A standard form for an affine equation describing an elliptic curve with a given j-invariant is obtained. This is used to show that if j ≠ 0, 26 33, then the number ofQ- rational points of order 2 on E depends only on j. The results are summarized in the accompanying table.