$G$-torsors on perfectoid spaces

TL;DR

This paper proves the equivalence of G-torsors in the étale and v-topologies on perfectoid spaces over non-archimedean fields, extending known cases and applying to p-adic Simpson correspondence.

Contribution

It generalizes the equivalence of G-torsors to all perfectoid spaces and explores reductions of structure groups on general adic spaces, with applications in p-adic Hodge theory.

Findings

G-torsors in étale and v-topologies are equivalent on perfectoid spaces.

Any G-torsor admits a reduction of structure group to an open subgroup étale-locally.

Generalized Q_p-representations are equivalent to v-vector bundles on adic spaces.

Abstract

For any rigid analytic group variety $G$ over a non-archimedean field $K$ over $\mathbb Q_p$, we study $G$-torsors on adic spaces over $K$ in the $v$-topology. Our main result is that on perfectoid spaces, $G$-torsors in the \'etale and $v$-topology are equivalent. This generalises the known cases of $G=\mathbb G_a$ and $G=\mathrm{GL}_n$ due to Scholze and Kedlaya--Liu. On a general adic space $X$ over $K$, where there can be more $v$-topological $G$-torsors than \'etale ones, we show that for any open subgroup $U\subseteq G$, any $G$-torsor on $X_v$ admits a reduction of structure group to $U$ \'etale-locally on $X$. This has applications in the context of the $p$-adic Simpson correspondence: For example, we use it to show that on any adic space, generalised $\mathbb Q_p$-representations are equivalent to $v$-vector bundles.