Linearization of finite subgroups of Cremona groups over non-closed fields

TL;DR

This paper investigates the linearizability of finite subgroups of Cremona groups over global fields, revealing local-global discrepancies and introducing a new birational invariant to analyze involutions.

Contribution

It introduces a new birational invariant that generalizes previous invariants, enabling the study of linearizability issues over non-closed fields and providing counterexamples to local-global principles.

Findings

Existence of involutions not globally linearizable but locally linearizable at all places.

Development of a new birational invariant for analyzing Cremona group actions.

Application of the invariant to real plane involutions.

Abstract

We study linearizability properties of finite subgroups of the Cremona group ${\mathrm{Cr}}_n(k)$ in the case where $k$ is a global field, with the focus on the local-global principle. For every global field $k$ of characteristic different from 2 and every $n \ge 3$ we give an example of a birational involution of $\mathbb P^n_k$ (=an element $g$ of order $2$ in ${\mathrm{Cr}}_n(k)$) such that $g$ is not $k$-linearizable but $g$ is $k_v$-linearizable in ${\mathrm{Cr}}_n(k_v)$ for all places $v$ of $k$. The main tool is a new birational invariant generalizing those introduced by Manin and Voskresenski\u\i\ in the arithmetic case and by Bogomolov--Prokhorov in the geometric case. We also apply it to the study of birational involutions in real plane.