Hereditarily characteristic not implies cyclic in finite
This article gives the statement and possibly, proof, of a non-implication relation between two subgroup properties, when the big group is a finite group. That is, it states that in a finite group, every subgroup satisfying the first subgroup property (i.e., hereditarily characteristic subgroup) need not satisfy the second subgroup property (i.e., cyclic characteristic subgroup)
View all subgroup property non-implications | View all subgroup property implications
Statement
It is possible to have a finite group , and a subgroup of such that:
- is a Hereditarily characteristic subgroup (?) of : For every subgroup of , is a characteristic subgroup of .
- is not a cyclic group.
Related facts
- Finite group implies cyclic iff every subgroup is characteristic: This just states that we cannot get a counterexample when .
- Upward-closed characteristic not implies cyclic-quotient in finite
Proof
Further information: nontrivial semidirect product of Z4 and Z4, subgroup structure of nontrivial semidirect product of Z4 and Z4
Consider the semidirect product of cyclic group:Z4 and cyclic group:Z4 where the generator of the acting group acts via the inverse map on the other group. In other words, it has the presentation:
.
Let be the subgroup . Then, is a Klein four-group but every subgroup of is characteristic in . We explain here why each of the subgroups of order two is characteristic:
- The subgroup is the derived subgroup.
- The subgroup is the subgroup generated by the unique element that is a square but not a commutator.
- The subgroup is the subgroup generated by the unique element that is a product of squares but not a square.
itself is and is characteristic in , and the trivial subgroup is characteristic in . Thus, all subgroups of are characteristic in .