# Permutable not implies normal

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., permutable subgroup) neednotsatisfy the second subgroup property (i.e., normal subgroup)

View a complete list of subgroup property non-implications | View a complete list of subgroup property implications

Get more facts about permutable subgroup|Get more facts about normal subgroup

EXPLORE EXAMPLES YOURSELF: View examples of subgroups satisfying property permutable subgroup but not normal subgroup|View examples of subgroups satisfying property permutable subgroup and normal subgroup

## Contents

## Statement

### Property-theoretic statement

The subgroup property of being a permutable subgroup is *not* stronger than the subgroup property of being a normal subgroup.

### Verbal statement

There exist situations where a permutable subgroup of a group is *not* normal.

## Definitions used

### Permutable subgroup

`Further information: Permutable subgroup`

A subgroup of a group is termed permutable if permutes with every cyclic subgroup of .

## Related facts

### Similarities in behavior between permutability and normality

Get more information at permutable versus normal.

- Permutability is strongly join-closed
- Permutability satisfies intermediate subgroup condition
- Permutability satisfies transfer condition
- Permutability satisfies intermediate subgroup condition
- Permutability satisfies image condition

### Differences between permutability and normality

## Facts used

- Omega-1 of odd-order class two p-group has prime exponent: For a -group of nilpotence class two with odd order, the set of elements of order forms a subgroup (along with the identity element).
- Baer norm is hereditarily permutable

## Proof

### Example of a group of prime power order

`Further information: prime-cube order group:p2byp`

Let be an odd prime. Let be the semidirect product of a cyclic group of order and a cyclic group of order acting nontrivially. In other words: is a group generated by , with the relations and :

Thus, has order .

We claim that the subgroup is a permutable subgroup of but is not normal in . The fact that is not normal in is direct from the fact that it is the acting group in a semidirect product corresponding to a nontrivial action. For permutability, we present two (essentially equivalent) forms of the proof.

**First proof**: First, note that (the subgroup generated by elements of order ) is a subgroup of exponent (fact (1)). It cannot equal , since . Also, , so has order at least . Hence, its order is exactly , and it is elementary Abelian on and .

Now, if we pick any element inside , the cyclic subgroup generated by it clearly permutes with , because they're both inside an Abelian group of order . If we pick an element outside , then the cyclic subgroup it generates has order . Hence, that cyclic subgroup is maximal in and hence normal, so it commutes with . Thus, every cyclic subgroup commutes with , and we are done.

**Second proof**: First, note that (the subgroup generated by elements of order ) is a subgroup of exponent (fact (1)). It cannot equal , since . Also, , so has order at least . Hence, its order is exactly , and it is elementary Abelian on and .

By fact (2), it suffices to show that is contained in the Baer norm of : the intersection of normalizers of all subgroups. Note that the normalizer of any subgroup of order or is the whole group. The normalizer of any subgroup of order must contain that subgroup, and also the center, which is generated by . Hence, either the subgroup is normal, or its normalizer is a subgroup of order containing two permuting subgroups of order , forcing it to be (again by fact (1)). Thus, every normalizer is either the whole group or . Moreover, since is not normal in , the normalizer of is , so is the Baer norm. Thus, since , we obtain that is permutable.

## GAP implementation

To do this implementation successfully, first define the GAP function for permutable subgroup, as available at GAP:IsPermutable. This function can be defined inline, in the interactive interface (enter it line by line) or put in a file read by the Read command.

gap> P := CyclicGroup(9); <pc group of size 9 with 2 generators> gap> Q := SylowSubgroup(AutomorphismGroup(P),3); <group> gap> A := SemidirectProduct(Q,P); <pc group with 3 generators> gap> B := Image(Embedding(A,1)); Group([ f1 ]) gap> IsSubgroup(A,B); true gap> IsNormal(A,B); false gap> IsPermutable(A,B); true