# Derived subgroup

This article defines a subgroup-defining function, viz., a rule that takes a group and outputs a unique subgroup
View a complete list of subgroup-defining functions OR View a complete list of quotient-defining functions

## History

The notion of derived subgroup or commutator subgroup naturally arose in the context of finding a natural choice for a good composition series for a solvable group -- solvable groups arise very naturally in the fundamental problems dealt with in Galois theory.

## Definition

### Symbol-free definition

The derived subgroup or commutator subgroup of a group is defined in the following equivalent ways:

1. It is the subgroup generated by all commutators of the whole group.
2. It is the normal closure of the subgroup generated by all commutators of the whole group.
3. It is the intersection of all abelian-quotient subgroups (viz., normal subgroups with abelian quotients). In other words, it is the smallest normal subgroup for which the quotient group is abelian.

### Definition with symbols

The derived subgroup or commutator subgroup of a group $G$, denoted as $[G,G]$ or as $\! G'$, is defined in the following way:

1. It is the subgroup generated by all commutators, or elements of the form $[x,y] = xyx^{-1}y^{-1}$ where $x,y \in G$.
2. It is the normal closure of the subgroup generated by all elements of the form $[x,y]$.
3. it is the intersection of all abelian-quotient subgroups of $G$, viz., subgroups $H \underline{\triangleleft} G$ such that $G/H$ is an abelian group.

### Equivalence of definitions

For full proof, refer: Equivalence of definitions of derived subgroup

## Group properties

It is not true that every group can be realized as the derived subgroup of another group -- for instance, the characteristically metacyclic and commutator-realizable implies abelian tells us that a group whose first two abelianizations are cyclic, but whose second derived subgroup is not trivial, cannot arise as a derived subgroup.

## Associated constructions

### Associated quotient-defining function

The quotient-defining function associated with this subgroup-defining function is: Abelianization

The quotient of a group by its commutator subgroup is termed its Abelianization. This can also be thought of as the largest possible Abelian quotient of the group.

### Associated descending series

The associated descending series to this subgroup-defining function is: Derived series

The series obtained by iterating the commutator subgroup-defining function is termed the derived series. The $n^{th}$ member of this is termed the $n^{th}$ derived subgroup.

A group for which this derived series terminates at the identity in finitely many steps is termed a solvable group and the length of the derived series is termed the derived length.

## Examples

### Examples where the derived subgroup is proper and nontrivial

The quotient part in the examples below is the abelianization of the group.

Group partSubgroup partQuotient part
2-Sylow subgroup of special linear group:SL(2,3)Special linear group:SL(2,3)Quaternion groupCyclic group:Z3
A3 in S3Symmetric group:S3Cyclic group:Z3Cyclic group:Z2
A4 in S4Symmetric group:S4Alternating group:A4Cyclic group:Z2
A5 in S5Symmetric group:S5Alternating group:A5Cyclic group:Z2
Center of dihedral group:D8Dihedral group:D8Cyclic group:Z2Klein four-group
Center of quaternion groupQuaternion groupCyclic group:Z2Klein four-group
Derived subgroup of M16M16Cyclic group:Z2Direct product of Z4 and Z2
Derived subgroup of dihedral group:D16Dihedral group:D16Cyclic group:Z4Klein four-group
Derived subgroup of nontrivial semidirect product of Z4 and Z4Nontrivial semidirect product of Z4 and Z4Cyclic group:Z2Direct product of Z4 and Z2
Klein four-subgroup of alternating group:A4Alternating group:A4Klein four-groupCyclic group:Z3
SL(2,3) in GL(2,3)General linear group:GL(2,3)Special linear group:SL(2,3)Cyclic group:Z2

### Examples where the derived subgroup is trivial

These are precisely the abelian groups (follow through the link for examples).

### Examples where the derived subgroup is the whole group

These are precisely the perfect groups (follow through the link for examples).

## Subgroup properties

### Properties satisfied

Property Meaning Proof of satisfaction
Verbal subgroup generated by a bunch of words with letters freely quantified over the whole group the word here is the commutator
Fully invariant subgroup invariant under all endomorphisms derived subgroup is fully invariant, see also verbal implies fully invariant
Image-closed fully invariant subgroup under any surjective homomorphism, its image is fully invariant in the image of the whole group follows from verbal implies image-closed fully invariant
Characteristic subgroup invariant under all automorphisms derived subgroup is characteristic, also follows from being a verbal subgroup
Upward-closed normal subgroup any subgroup containing it is a normal subgroup derived subgroup is upward-closed normal -- follows from the fact that the quotient is abelian and abelian implies every subgroup is normal
Normal subgroup invariant under all inner automorphisms (via characteristic)
Abelian-quotient subgroup quotient group is abelian

## Effect of operators

### Fixed-point operator

A group which equals its own commutator subgroup is termed a perfect group

### Free operator

A group whose commutator subgroup is trivial is termed an Abelian group

## Subgroup-defining function properties

### Monotonicity

This subgroup-defining function is monotone, viz the image of any subgroup under this function is contained in the image of the whole group

This follows from the fact that any commutator of elements inside a subgroup is also a commutator of elements inside the whole group.

## Computation

### The computation problem

Further information: Commutator subgroup computation problem

The general problem of computing the commutator subgroup given the whole group can be solved, when ther group is described in terms of a generating set. The idea is to take the normal closure of the subgroup generated by all commutators of elements in the generating set.

### GAP command

The command for computing this subgroup-defining function in Groups, Algorithms and Programming (GAP) is:DerivedSubgroup
View other GAP-computable subgroup-defining functions

To compute the commutator subgroup of a group in GAP, the syntax is:

DerivedSubgroup (group);
where
group
could either be an on-the-spot description of the group or a name aluding to a previously defined group.

We can assign this as a value, to a new name, for instance:

dg := DerivedSubgroup (g);
where
g
is the original group and
dg
is the derived subgroup.