Finite dimensional Hopf actions on central division algebras

TL;DR

This paper characterizes finite group gradings on division algebras and bounds the PI degree of Hopf algebras coacting on them, also constructing specific Hopf-Galois actions on twisted group algebra division algebras.

Contribution

It provides a complete characterization of finite group gradings on division algebras and establishes an upper bound on the PI degree for Hopf algebra coactions, introducing new constructions.

Findings

Finite group G faithfully grades D iff G has a normal abelian subgroup of index dividing d.

PI degree of Hopf algebra coacting on D is at most d^2.

Constructs Hopf-Galois actions on division algebras from twisted group algebras with bijective cocycles.

Abstract

Let $\mathbb{k}$ be an algebraically closed field of characteristic zero. Let $D$ be a division algebra of degree $d$ over its center $Z(D)$. Assume that $\mathbb{k}\subset Z(D)$. We show that a finite group $G$ faithfully grades $D$ if and only if $G$ contains a normal abelian subgroup of index dividing $d$. We also prove that if a finite dimensional Hopf algebra coacts on $D$ defining a Hopf-Galois extension, then its PI degree is at most $d^2$. Finally, we construct Hopf-Galois actions on division algebras of twisted group algebras attached to bijective cocycles.