Polynomial bounds on torsion from a fixed geometric isogeny class of elliptic curves

Abstract

We show there exist polynomial bounds on torsion of elliptic curves which come from a fixed geometric isogeny class. More precisely, for an elliptic curve $E_0$ defined over a number field $F_0$, for each $\epsilon >0$ there exist constants $c_{\epsilon }:=c_{\epsilon }(E_0,F_0),C_{\epsilon }:=C_{\epsilon }(E_0,F_0)>0$ such that for any elliptic curve $E_{/F}$ geometrically isogenous to $E_0$, if $E(F)$ has a point of order $N$ then \[ N\leq c_ε\cdot [F:\mathbb{Q}]^{1/2+ε}, \] and one also has \[ \# E(F)[\textrm{tors}] \leq C_ε\cdot [F:\mathbb{Q}]^{1+ε}. \]

arXiv comment8 pages. Improves the bounds in Theorem 1, and strengthens an additional result (which is now Corollary 4 for adelic indices)