Characteristic not implies fully invariant in class three maximal class p-group

Statement

Let $p$ be any prime number. There exists a $p$-group that is of class three that is a Maximal class group (?) with a Characteristic subgroup (?) that is not fully invariant.

Related facts

Similar facts

• Characteristic not implies fully invariant
• Characteristic not implies fully invariant in finite abelian group
• Characteristic not implies fully invariant in odd-order class two p-group

Opposite facts

• Characteristic equals fully invariant in odd-order abelian group

Proof

For odd primes

Suppose $p$ is an odd prime. Let $K$ be the wreath product of groups of order p and $G$ be $K/[[[K,K],K],K]$. $G$ is thus a semidirect product of an elementary abelian group of order $p^{3}$ and a cyclic group of order $p$. Note that for $p>3$, $G$ has exponent $p$ but for $p=3$, $G$ has exponent $9$.

Consider $H=C_{G}([G,G])$, the centralizer of commutator subgroup. $H$ is a characteristic subgroup. Consider now an element $x$ of order $p$ outside $H$ (such an element exists because of the way the group is defined as a semidirect product) and a subgroup $L$ of $G$ of index $p$ other than $H$, that does not contain $x$. Consider the retraction from $G$ to $\langle x\rangle$ with kernel $L$. This retraction doesn't preserve $H$, so $H$ is not fully invariant.

For the prime two

Further information: dihedral group:D16

Consider the dihedral group:

$G:=\langle a,x\mid a^{8}=x^{2}=e,xax=a^{-1}\rangle$.

Let $H=\langle a\rangle$. Then, $H=C_{G}([G,G])$, so $H$ is characteristic in $G$. Consider now the subgroup $L=\langle a^{2},x\rangle$ and the element $ax$. Consider the retraction to the subgroup $\langle ax\rangle$ with kernel $L$. $H$ is not preserved under this retraction (for instance, $a$ gets mapped to $x$), so it is not fully invariant.