Birational invariance of higher Amitsur groups

TL;DR

This paper proves that higher Amitsur groups are stable G-birational invariants for smooth projective G-varieties over fields of characteristic zero, extending known cases and enabling effective computation of obstructions.

Contribution

It generalizes the invariance of Amitsur groups to all n≥2 and links their vanishing to universal torsor obstructions under certain conditions.

Findings

Higher Amitsur groups are stable G-birational invariants for all n≥2.

Vanishing of G-equivariant universal torsor obstruction implies vanishing of all higher Amitsur groups.

Effective methods for computing these obstructions are demonstrated with examples.

Abstract

Let $k$ be a field of characteristic zero and $G$ a finite group. We prove that for all $n\geq 2$, the $n$th Amitsur group is a stable $G$-birational invariant of smooth projective $G$-varieties over $k$. This was previously known for $n=2,3$. For smooth projective $G$-varieties with free and finitely generated Picard group, we also prove that the vanishing of the $G$-equivariant universal torsor obstruction implies the vanishing of the $n$th Amitsur group, for all $n\geq 2$. This was known for $n=2$. Our approach allows for effective computations of these obstructions; we illustrate this with several examples.