Image-closed characteristic not implies fully invariant
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 and an image-closed characteristic subgroup of such that is a fully invariant subgroup of .
Proof
Further information: direct product of S3 and Z3
Suppose is the direct product of the symmetric group of degree three and the cyclic group of order three. Suppose is the second direct factor of , so is the cyclic group of order three. We have:
- For any surjective homomorphism , is a characteristic subgroup of : Note first that itself equals the center of , hence is characteristic in . For any surjective homomorphism, the kernel is either or the cyclic subgroup of order three inside the first direct factor. In either case, is characteristic in .
- is not a fully invariant subgroup of : is not invariant under the endomorphism of that has kernel the first direct factor and sends to the cyclic subgroup of order three inside the first direct factor.