Cremona groups over finite fields, Neretin groups, and non-positively curved cube complexes

TL;DR

This paper establishes embeddings of plane Cremona groups over finite fields into Neretin groups, explores permutation properties in even characteristic, and constructs CAT(0) cube complexes to analyze Neretin groups' geometric and algebraic properties.

Contribution

It introduces explicit embeddings of Cremona groups into Neretin groups and constructs CAT(0) cube complexes to study their geometric properties.

Findings

Cremona groups embed densely into Neretin groups.

Permutations induced by birational transformations are even in certain cases.

Neretin groups act properly on constructed CAT(0) cube complexes.

Abstract

We show that plane Cremona groups over finite fields embed as dense subgroups into Neretin groups, i.e. groups of almost automorphisms of rooted trees. We also show that if the finite base field has even characteristic and contains at least 4 elements, then the permutations induced by birational transformations on rational points of regular projective surfaces are even. In a second part, we construct explicit locally compact CAT(0) cube complexes, on which Neretin groups act properly. This allows us to recover in a unified way various results on Neretin groups such as that they are of type $F_{\infty}$. We also prove a new fixed-point theorem for CAT(0) cube complexes without infinite cubes and use it to deduce a regularisation theorem for plane Cremona groups over finite fields.