Locally analytic vectors and decompletion in mixed characteristic

Abstract: In $p$-adic Hodge theory and the $p$-adic Langlands program, Banach spaces with $\mathbb{Q}p$-coefficients and $p$-adic Lie group actions are central. Studying the subrepresentation of $\Gamma$-locally analytic vectors, $W^{\mathrm{la}}$, is useful because $W^{\mathrm{la}}$ can be analyzed via the Lie algebra $\mathrm{Lie}(\Gamma)$, which simplifies the action of $\Gamma$. Additionally, $W^{\mathrm{la}}$ often behaves as a decompletion of $W$, making it closer to an algebraic or geometric object. This article introduces a notion of locally analytic vectors for $W$ in a mixed characteristic setting, specifically for $\mathbb{Z}_p$-Tate algebras. This generalization encompasses the classical definition and also specializes to super-H\"older vectors in characteristic $p$. Using binomial expansions instead of Taylor series, this new definition bridges locally analytic vectors in characteristic $0$ and $p$. Our main theorem shows that under certain conditions, the map $W \mapsto W^{\mathrm{la}}$ acts as a descent, and the derived locally analytic vectors $\mathrm{R}{\mathrm{la}}^i(W)$ vanish for $i \geq 1$. This result extends Theorem C of \cite{Po24}, providing new tools for propagating information about locally analytic vectors from characteristic $0$ to characteristic $p$. We provide three applications: a new proof of Berger-Rozensztajn's main result using characteristic $0$ methods, the introduction of an integral multivariable ring $\widetilde{\mathbf{A}}{\mathrm{LT}}^{\dagger,\mathrm{la}}$ in the Lubin-Tate setting, and a novel interpretation of the classical Cohen ring ${\mathbf{A}}{\mathbb{Q}_p}$ from the theory of $(\varphi,\Gamma)$-modules in terms of locally analytic vectors.