CM Elliptic Curves: Volcanoes, Reality and Applications, Part II

Abstract: Let $M \mid N$ be positive integers, and let $\Delta$ be the discriminant of an order in an imaginary quadratic field $K$. When $\Delta_K < -4$, the first author determined the fiber of the morphism $X_0(M,N) \rightarrow X(1)$ over the closed point $J_{\Delta}$ corresponding to $\Delta$ and showed that all fibers of the map $X_1(M,N) \rightarrow X_0(M,N)$ over $J_{\Delta}$ were connected. Here we complement this prior work by addressing the most difficult cases $\Delta_K \in {-3,-4}$. These works provide all the information needed to compute, for each positive integer $d$, all subgroups of $E(F)[\operatorname{tors}]$, where $F$ is a number field of degree $d$ and $E_{/F}$ is an elliptic curve with complex multiplication.