Local types of $(\Gamma,G)$-bundles and parahoric group schemes

TL;DR

This paper classifies local types of $( ext{Gamma}, G)$-bundles on $ ext{Gamma}$-curves using non-abelian cohomology, and shows how parahoric Bruhat--Tits group schemes relate to these bundles, especially in characteristic zero.

Contribution

It provides a classification of $( ext{Gamma}, G)$-bundles on $ ext{Gamma}$-curves and connects parahoric group schemes to these bundles in characteristic zero.

Findings

Classification of local types via non-abelian cohomology

Any generically simply-connected parahoric Bruhat--Tits group scheme arises from a $( ext{Gamma}, G_{ ext{ad}})$-bundle

Parahoric group schemes over the formal disc come from constant group schemes via tamely ramified coverings

Abstract

Let $G$ be a simple algebraic group over an algebraically closed field $k$. Let $\Gamma$ be a finite group acting on $G$. We classify and compute the local types of $(\Gamma, G)$-bundles on a smooth projective $\Gamma$-curve in terms of the first non-abelian group cohomology of the stabilizer groups at the tamely ramified points with coefficients in $G$. When $\text{char}(k)=0$, we prove that any generically simply-connected parahoric Bruhat--Tits group scheme can arise from a $(\Gamma,G_{\text{ad}})$-bundle. We also prove a local version of this theorem, i.e. parahoric group schemes over the formal disc arise from constant group schemes via tamely ramified coverings.