Image-closed characteristic not implies fully invariant

From Groupprops

This article gives the statement and possibly, proof, of a non-implication relation between two subgroup properties. That is, it states that every subgroup satisfying the first subgroup property (i.e., image-closed characteristic subgroup) need not satisfy the second subgroup property (i.e., fully invariant subgroup)
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Statement

It is possible to have a group G and an image-closed characteristic subgroup H of G such that H is a fully invariant subgroup of G.

Proof

Further information: direct product of S3 and Z3

Suppose G is the direct product of the symmetric group of degree three and the cyclic group of order three. Suppose H is the second direct factor of G, so H is the cyclic group of order three. We have:

  • For any surjective homomorphism ρ:GK, ρ(H) is a characteristic subgroup of K: Note first that H itself equals the center of G, hence is characteristic in G. For any surjective homomorphism, the kernel is either H or the cyclic subgroup of order three inside the first direct factor. In either case, ρ(H) is characteristic in K.
  • H is not a fully invariant subgroup of G: H is not invariant under the endomorphism of G that has kernel the first direct factor and sends H to the cyclic subgroup of order three inside the first direct factor.