Local Kummer theory for Drinfeld modules

Abstract

Let $ϕ$ be a Drinfeld $A$-module of finite residual characteristic $\bar{\mathfrak{p}}$ over a local field $K$. We study the action of the inertia group of $K$ on a modified adelic Tate module $\smash{T^\circ_{\text{ad}}}(ϕ)$ which differs from the usual adelic Tate module only at the $\bar{\mathfrak{p}}$-primary component. After replacing $K$ by a finite extension we can assume that $ϕ$ is the analytic quotient of a Drinfeld module $ψ$ of good reduction by a lattice $M\subset K$. The image of inertia acting on $T^\circ_{\text{ad}}(ϕ)$ is then naturally a subgroup of $\operatorname{Hom}_A(M,T^\circ_\text{ad}(ψ))$. This subgroup is described by a canonical local Kummer pairing that we study extensively in this article. In particular we give an effective formula for the image of inertia up to finite index, and obtain a necessary and sufficient condition for this image to be open. We also determine the image of the ramification filtration