Non-existence of integral Hopf orders for twists of several simple groups of Lie type

TL;DR

This paper proves that certain twisted complex group algebras of specific simple groups of Lie type cannot be realized over any number ring as Hopf orders, revealing new examples of complex semisimple Hopf algebras without such structures.

Contribution

It establishes the non-existence of Hopf orders over number rings for a broad class of twisted group algebras of simple groups of Lie type, extending previous knowledge.

Findings

Twisted group algebras of PSL_2(q) do not admit Hopf orders over number rings.

Similar non-existence results hold for Suzuki groups and SL_3(p) with specific twists.

The results apply broadly to all finite non-abelian simple groups via known theorems.

Abstract

Let $p$ be a prime number and $q=p^m$, with $m \geq 1$ if $p \neq 2,3$ and $m>1$ otherwise. Let $\Omega$ be any non-trivial twist for the complex group algebra of $\mathbf{PSL}_2(q)$ arising from a $2$-cocycle on an abelian subgroup of $\mathbf{PSL}_2(q)$. We show that the twisted Hopf algebra $(\mathbb{C} \mathbf{PSL}_2(q))_{\Omega}$ does not admit a Hopf order over any number ring. The same conclusion is proved for the Suzuki groups, and for $\mathbf{SL}_3(p)$ when the twist stems from an abelian $p$-subgroup. This supplies new families of complex semisimple (and simple) Hopf algebras that do not admit a Hopf order over any number ring. The strategy of the proof is formulated in a general framework that includes the finite simple groups of Lie type. As an application, we combine our results with two theorems of Thompson and Barry and Ward on minimal simple groups to establish that for any finite non-abelian simple group $G$ there is a twist $\Omega$ for $\mathbb{C} G$, arising from a $2$-cocycle on an abelian subgroup of $G$, such that $(\mathbb{C} G)_{\Omega}$ does not admit a Hopf order over any number ring. This partially answers in the negative a question posed by Meir and the second author.