On the Pronormality of Second Maximal Subgroups in Finite Groups with Socle $L_2(q)$
V. I. Zenkovab a Institute of Mathematics and Mechanics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg
b Ural Federal University named after the First President of Russia B. N. Yeltsin, Ekaterinburg
Abstract:
According to P. Hall, a subgroup
$H$ of a finite group
$G$ is called pronormal in
$G$ if, for any element
$g$ of
$G$, the subgroups
$H$ and
$H^g$ are conjugate in
$\langle H,H^g\rangle$. The simplest examples of pronormal subgroups of finite groups are normal subgroups, maximal subgroups, and Sylow subgroups. Pronormal subgroups of finite groups were studied by a number of authors. For example, Legovini (1981) studied finite groups in which every subgroup is subnormal or pronormal. Later, Li and Zhang (2013) described the structure of a finite group
$G$ in which, for a second maximal subgroup
$H$, its index in
$\langle H,H^g\rangle$ does not contain squares for any
$g$ from
$G$. A number of papers by Kondrat'ev, Maslova, Revin, and Vdovin (2012–2019) are devoted to studying the pronormality of subgroups in a finite simple nonabelian group and, in particular, the existence of a nonpronormal subgroup of odd index in a finite simple nonabelian group. In {The Kourovka Notebook}, the author formulated Question 19.109 on the equivalence in a finite simple nonabelian group of the condition of pronormality of its second maximal subgroups and the condition of Hallness of its maximal subgroups. Tyutyanov gave a counterexample
$L_2(2^{11})$ to this question. In the present paper, we provide necessary and sufficient conditions for the pronormality of second maximal subgroups in the group
$L_2(q)$. In addition, for
$q\le 11$, we find the finite almost simple groups with socle
$L_2(q)$ in which all second maximal subgroups are pronormal.
Keywords:
finite group, simple group, maximal subgroup, pronormal subgroup.
UDC:
512.542
MSC: 20D06,
20D30,
20E28 Received: 29.10.2019
Revised: 11.07.2020
Accepted: 03.08.2020
DOI:
10.21538/0134-4889-2020-26-3-32-43