# An application of cohomological invariants

## TL;DR

This paper uses cohomological invariants to generalize results on the non-rationality of fixed fields under group actions, extending known theorems to broader classes of finite groups and fields.

## Contribution

It introduces a cohomological invariant method to prove non-rationality of fixed fields for groups with certain quotient structures, generalizing previous results.

## Key findings

- Proves non-stable rationality for fixed fields when G/N is cyclic of order 2^n with n≥3.
- Extends non-rationality results to fields where certain cyclotomic extensions are non-cyclic.
- Utilizes cohomological invariants to analyze rationality problems in Noether's problem.

## Abstract

Let $G$ be a finite group, $k$ be a field and $G\to GL(V_{\rm reg})$ be the regular representation of $G$ over $k$. Then $G$ acts naturally on the rational function field $k(V_{\rm reg})$ by $k$-automorphisms. Define $k(G)$ to be the fixed field $k(V_{\rm reg})^G$. Noether's problem asks whether $k(G)$ is rational (resp. stably rational) over $k$. When $k=\bQ$ and $G$ contains a normal subgroup $N$ with $G/H\simeq C_8$ (the cyclic group of order $8$), Jack Sonn proves that $\bQ(G)$ is not stably rational over $\bQ$, which is a non-abelian extension of a theorem of Endo-Miyata, Voskresenskii, Lenstra and Saltman for the abelian Noether's problem $\bQ(C_8)$. Using the method of cohomological invariants, we are able to generalize Sonn's theorem as follows. Theorem. Let $G$ be a finite group and $N$ $\lhd$ $G$ such that $G/N\simeq C_{2^n}$ with $n\geq 3$. If $k$ is a field satisfying that ${\rm char}\,k=0$ and $k(\zeta_{2^n})/k$ is not a cyclic extension where $\zeta_{2^n}$ is a primitive $2^n$-th root of unity, then $k(G)$ is not stably rational (resp. not retract rational) over $k$. \end{abstract}