Cyclic subgroup is characteristic in dihedral group

This article gives the statement, and possibly proof, of a particular subgroup or type of subgroup satisfying a particular subgroup property (namely, Characteristic subgroup (?)) in a particular group or type of group (namely, Dihedral group (?)).

Statement

Let $G$ be a dihedral group defined as follows:

$G = \langle a,x \mid a^n = x^2 = e, xax^{-1} = a^{-1} \rangle$

or the infinite dihedral group:

$G = \langle a,x \mid x^2 = e, xax^{-1} = a^{-1} \rangle$.

Then, for $n \ge 3$ or for the infinite dihedral group, the cyclic subgroup $\langle a \rangle$ is a characteristic subgroup of $G$.

Proof

Let $H = \langle a \rangle$. Then, $H$ has index two in $G$. Any element of $G$ outside $H$ is of the form $a^kx$. Further, $(a^kx)^2 = a^kxa^kx^{-1} = a^ka^{-k} = e$. Thus, every element of $G$ outside of $H$ has order two.

Thus, if $n \ge 3$, $H$ can be defined as the unique cyclic subgroup generated by an element of order $n$.