Kummer-faithfulness for function fields

Abstract

A perfect field $K$ is said to be Kummer-faithful if the Mordell-Weil group of every semi-abelian variety over every finite extension of $K$ has no nonzero divisible element. The class of Kummer-faithful fields contains that of sub-$p$-adic fields and is thought to be suitable for developing anabelian geometry. In this paper, we investigate a function field analogue of the notion of Kummer-faithful fields. We introduce a notion of Drinfeld-Kummer-faithful (DKF) fields using Drinfeld modules. A sufficient condition for a Galois extension of a function field to be DKF is provided in terms of ramification theory. More precisely, a Galois extension with finite maximal ramification break outside the infinite prime $(1 / t)$ over a finite extension of the rational function field $\mathbb{F}_q(t)$ over the finite field $\mathbb{F}_q$ of $q$ elements is DKF. Some examples of DKF fields are also given. The construction of these examples is inspired by Ozeki and Taguchi's examples of highly Kummer-faithful fields.