Variants of some of the Brauer-Fowler Theorems

TL;DR

This paper explores variants of Brauer-Fowler theorems, establishing bounds on group order related to involutions and their centralizers, with some results depending on the classification of finite simple groups and others not.

Contribution

It introduces new bounds on finite groups involving involutions, including results that do not rely on the classification of finite simple groups.

Findings

For any finite group of even order, |G| is less than the number of conjugacy classes of its Fitting subgroup times the fourth power of an involution's centralizer.

Every finite group of even order contains an involution u with [G:F(G)] less than the cube of the centralizer's order.

A general result on fixed point spaces of involutions in finite irreducible linear groups is established without using the classification of finite simple groups.

Abstract

Brauer and Fowler noted restrictions on the structure of a finite group G in terms of the order of the centralizer of an involution t in G. We consider variants of these themes. We first note that for an arbitrary finite group G of even order, we have |G| is less than the number of conjugacy classes of the Fitting subgroup times the order of the centralizer to the fourth power of any involution in G. This result does require the classification of the finite simple groups. The groups SL(2,q) with q even shows that the exponent 4 cannot be replaced by any exponent less than 3. We do not know at present whether the exponent 4 can be improved in general, though we note that the exponent 3 suffices for almost simple groups G. We are however able to prove that every finite group $G$ of even order contains an involution u such that [G:F(G)] is less than the cube of the order of the centralizer of u. There is a dichotomy in the proof of this fact, as it reduces to proving two residual cases: one in which G is almost simple (where the classification of the finite simple groups is needed) and one when G has a Sylow 2-subgroup of order 2. For the latter result, the classification of finite simple groups is not needed (though the Feit-Thompson odd order theorem is). We also prove a very general result on fixed point spaces of involutions in finite irreducible linear groups which does not make use of the classification of the finite simple groups, and some other results on the existence of non-central elements (not necessarily involutions) with large centralizers in general finite groups. We also show (without the classification of finite simple groups) that if t is an involution in G and p is a prime divisor of [G:F(G)], then p is at most 1 plus the order of the centralizer of t (and this is best possible).