Skip to main content
3 of 3
deleted 10 characters in body
Bobby Grizzard
  • 1.5k
  • 1
  • 10
  • 21

Are elliptic Kummer extensions big?

Loosely speaking, are elliptic Kummer extensions big? More concretely:

Let $E$ be an elliptic curve over $\mathbb{Q}$, let $p$ be a prime, and let $F$ be a subfield of $\overline{\mathbb{Q}}$ containing the coordinates of all the $p$-power torsion of $E$. Given $c > 0$, does there exist $N > 0$ (depending only on $E$, $F$, and $p$) such that: If P is a point of $E(\overline{\mathbb{Q}})$ whose image in $(E(\overline{\mathbb{Q}})/E(F))$ has order $p^n$ (in other words, $[p^n]P \in E(F)$, but this does not hold for any smaller power of $p$) with $n \geq N$, then $[F(P):F] \geq c$?

Remarks:

  • I am aware of some Theorems of Basmakov, Ribet, and Bertrand that are roughly in this direction, and in fact more general cases than elliptic curves are considered (e.g. Ribet's paper "Kummer theory on extensions of abelian varieties by tori" -- Duke Math J. 1979). One shortcoming of these excellent papers is that the results are only for almost every prime. I am very interested in the prime 2 (unfortunately!).

  • $\operatorname{Gal}(F(P)/F)$ is naturally a subgroup of $E[p^n]$.

  • The G_m analogue is easy -- the degree of a Kummer extension is as big as it could be.

  • If it's easier, the most important case for me is when $E$ has CM and $F$ is a finite extension of $\mathbb{Q}(E_{tors})$.

  • Does the following make sense: Given a point $Q \in E(F)$, there is a representation $G_F \to (\mathbb{Z}_p)^2$ and more generally $G_\mathbb{Q} \to (\mathbb{Z}_p)^2 \rtimes \operatorname{GL}_2(\mathbb{Z_p})$ coming from the Galois action on points $P$ satisfying $[p^n]P = Q, n \in \mathbb{N}$, and in this context what I'm asking is in the spirit of Serre's Theorem on the large image of the $p$-adic representation, but I'm asking the image to be large uniformly in $Q$?

Bobby Grizzard
  • 1.5k
  • 1
  • 10
  • 21