K\"ahler differentials and $\mathbf{Z}_p$-extensions

Abstract

Let $K$ be a $p$-adic field, and let $K_\infty/K$ be a Galois extension that is almost totally ramified, and whose Galois group is a $p$-adic Lie group of dimension $1$. We prove that $K_\infty$ is not dense in $(\mathbf{B}_{\mathrm{dR}}^+ / \operatorname{Fil}^2 \mathbf{B}_{\mathrm{dR}}^+ )^{\operatorname{Gal}(\overline{K}/K_\infty)}$. Moreover, the restriction of $\theta$ to the closure of $K_\infty$ is injective, and its image via $\theta$ is the set of vectors of $\widehat{K}_\infty$ that are $C^1$ with zero derivative for the action of $\operatorname{Gal}(K_\infty/K)$. The main ingredient for proving these results is the construction of an explicit lattice of $\mathcal{O}_{K_\infty}$ that is commensurable with $\mathcal{O}_{K_\infty}^{d=0}$, where $d : \mathcal{O}_{K_\infty} \to \Omega_{\mathcal{O}_{K_\infty} / \mathcal{O}_K}$ is the differential.