# 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.

## 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.