A Characterization of Alternating Groups. II
V. D. Mazurov Sobolev Institute of Mathematics, Siberian Branch of the Russian Academy of Sciences
Abstract:
Let
$G$ be a group. A subset
$X$ of
$G$ is called an
$A$-subset if
$X$ consists of elements of order 3,
$X$ is invariant in
$G$, and every two non-commuting members of
$X$ generate a subgroup isomorphic to
$A_4$ or to
$A_5$. Let
$X$ be the
$A$-subset of
$G$. Define a non-oriented graph
$\Gamma(X)$ with vertex set
$X$ in which two vertices are adjacent iff they generate a subgroup isomorphic to
$A_4$.
Theorem 1 states the following. Let
$X$ be a non-empty
$A$-subset of
$G$. (1) Suppose that
$C$ is a connected component of
$\Gamma(X)$ and
$H=\langle C\rangle$. If
$H\cap X$ does not contain a pair of elements generating a subgroup isomorphic to
$A_5$ then
$H$ contains a normal elementary Abelian 2-subgroup of index 3 and a subgroup of order 3 which coincides with its centralizer in
$H$. In the opposite case,
$H$ is isomorphic to the alternating group
$A(I)$ for some
$($possibly infinite
$)$ set
$I$,
$|I|\geqslant 5$. (2) The subgroup
$\langle X^G\rangle$ is a direct product of subgroups
$\langle C_\alpha\rangle$ generated by some connected components
$C_\alpha$ of
$\Gamma (X)$.
Theorem 2 asserts the following. Let
$G$ be a group and
$X\subseteq G$ be a non-empty
$G$-invariant set of elements of order 5 such that every two non-commuting members of
$X$ generate a subgroup isomorphic to
$A_5$. Then
$\langle X^G\rangle$ is a direct product of groups each of which either is isomorphic to
$A_5$ or is cyclic of order 5.
Keywords:
alternating group, non-oriented graph.
UDC:
512.5
Received: 12.09.2005