Explicit Kummer Theory for Elliptic Curves

Abstract

Let $E$ be an elliptic curve defined over a number field $K$, let $\alpha \in E(K)$ be a point of infinite order, and let $N^{-1}\alpha$ be the set of $N$-division points of $\alpha$ in $E(\bar{K})$. We prove strong effective and uniform results for the degrees of the Kummer extensions $[K(E[N],N^{-1}\alpha) : K(E[N])]$. When $K=\mathbb{Q}$, and under a minimal assumption on $\alpha$, we show that the inequality $[\mathbb{Q}(E[N],N^{-1}\alpha) : \mathbb{Q}(E[N])] \geq cN^2$ holds with a constant $c$ independent of both $E$ and $\alpha$.