Anatomy of torsion in the CM case

Abstract

Let $T_{\mathrm{CM}}(d)$ denote the maximum size of a torsion subgroup of a CM elliptic curve over a degree $d$ number field. We initiate a systematic study of the asymptotic behavior of $T_{\mathrm{CM}}(d)$ as an "arithmetic function". Whereas a recent result of the last two authors computes the upper order of $T_{\mathrm{CM}}(d)$, here we determine the lower order, the typical order and the average order of $T_{\mathrm{CM}}(d)$ as well as study the number of isomorphism classes of groups $G$ of order $T_{\mathrm{CM}}(d)$ which arise as the torsion subgroup of a CM elliptic curve over a degree $d$ number field. To establish these analytic results we need to extend some prior algebraic results. Especially, if $E_{/F}$ is a CM elliptic curve over a degree $d$ number field, we show that $d$ is divisible by a certain function of $\# E(F)[\mathrm{tors}]$, and we give a complete characterization of all degrees $d$ such that every torsion subgroup of a CM elliptic curve defined over a degree $d$ number field already occurs over $\mathbb{Q}$.