commutator subgroup

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In
mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no general consensus abo ...
, more specifically in
abstract algebra In algebra, which is a broad division of mathematics, abstract algebra (occasionally called modern algebra) is the study of algebraic structures. Algebraic structures include group (mathematics), groups, ring (mathematics), rings, field (mathema ...
, the commutator subgroup or derived subgroup of a
group A group is a number A number is a mathematical object used to counting, count, measurement, measure, and nominal number, label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with ...
is the
subgroup In group theory, a branch of mathematics, given a group (mathematics), group ''G'' under a binary operation ∗, a subset ''H'' of ''G'' is called a subgroup of ''G'' if ''H'' also forms a group under the operation ∗. More precisely ...
generated by all the
commutator In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
s of the group. The commutator subgroup is important because it is the smallest
normal subgroup In abstract algebra In algebra, which is a broad division of mathematics, abstract algebra (occasionally called modern algebra) is the study of algebraic structures. Algebraic structures include group (mathematics), groups, ring (mathematics), ...
such that the
quotient group A quotient group or factor group is a math Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geome ...
of the original group by this subgroup is abelian. In other words, $G/N$ is abelian
if and only if In logic Logic is an interdisciplinary field which studies truth and reasoning Reason is the capacity of consciously making sense of things, applying logic Logic (from Ancient Greek, Greek: grc, wikt:λογική, λογική, l ...
$N$ contains the commutator subgroup of $G$. So in some sense it provides a measure of how far the group is from being abelian; the larger the commutator subgroup is, the "less abelian" the group is.

# Commutators

For elements $g$ and $h$ of a group ''G'', the
commutator In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
of $g$ and $h$ is . The commutator
identity element In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
''e'' if and only if $gh = hg$ , that is, if and only if $g$ and $h$ commute. In general,
conjugate Conjugation or conjugate may refer to: Linguistics * Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics * Complex conjugation, the change ...
of $g$ by $s,$ * for any
homomorphism In algebra Algebra (from ar, الجبر, lit=reunion of broken parts, bonesetting, translit=al-jabr) is one of the areas of mathematics, broad areas of mathematics, together with number theory, geometry and mathematical analysis, analysis. I ...

$f: G \to H$, The first and second identities imply that the Set (mathematics), set of commutators in ''G'' is closed under inversion and conjugation. If in the third identity we take ''H'' = ''G'', we get that the set of commutators is stable under any endomorphism of ''G''. This is in fact a generalization of the second identity, since we can take ''f'' to be the conjugation automorphism on ''G'', $x \mapsto x^s$, to get the second identity. However, the product of two or more commutators need not be a commutator. A generic example is [''a'',''b''][''c'',''d''] in the free group on ''a'',''b'',''c'',''d''. It is known that the least order of a finite group for which there exists two commutators whose product is not a commutator is 96; in fact there are two nonisomorphic groups of order 96 with this property.

# Definition

This motivates the definition of the commutator subgroup $\left[G, G\right]$ (also called the derived subgroup, and denoted $G\text{'}$ or $G^$) of ''G'': it is the subgroup generated by all the commutators. It follows from the properties of commutators that any element of $\left[G, G\right]$ is of the form :$\left[g_1,h_1\right] \cdots \left[g_n,h_n\right]$ for some natural number $n$, where the ''g''''i'' and ''h''''i'' are elements of ''G''. Moreover, since $\left(\left[g_1,h_1\right] \cdots \left[g_n,h_n\right]\right)^s = \left[g_1^s,h_1^s\right] \cdots \left[g_n^s,h_n^s\right]$, the commutator subgroup is normal in ''G''. For any homomorphism ''f'': ''G'' → ''H'', :$f\left(\left[g_1,h_1\right] \cdots \left[g_n,h_n\right]\right) = \left[f\left(g_1\right),f\left(h_1\right)\right] \cdots \left[f\left(g_n\right),f\left(h_n\right)\right]$, so that $f\left(\left[G,G\right]\right) \leq \left[H,H\right]$. This shows that the commutator subgroup can be viewed as a functor on the category of groups, some implications of which are explored below. Moreover, taking ''G'' = ''H'' it shows that the commutator subgroup is stable under every endomorphism of ''G'': that is, [''G'',''G''] is a fully characteristic subgroup of ''G'', a property considerably stronger than normality. The commutator subgroup can also be defined as the set of elements ''g'' of the group that have an expression as a product ''g'' = ''g''1 ''g''2 ... ''g''''k'' that can be rearranged to give the identity.

## Derived series

This construction can be iterated: :$G^ := G$ :$G^ := \left[G^,G^\right] \quad n \in \mathbf$ The groups $G^, G^, \ldots$ are called the second derived subgroup, third derived subgroup, and so forth, and the descending normal series :$\cdots \triangleleft G^ \triangleleft G^ \triangleleft G^ = G$ is called the derived series. This should not be confused with the lower central series, whose terms are $G_n := \left[G_,G\right]$. For a finite group, the derived series terminates in a perfect group, which may or may not be trivial. For an infinite group, the derived series need not terminate at a finite stage, and one can continue it to infinite ordinal numbers via transfinite recursion, thereby obtaining the transfinite derived series, which eventually terminates at the perfect core of the group.

## Abelianization

Given a group $G$, a
quotient group A quotient group or factor group is a math Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geome ...
$G/N$ is abelian if and only if $\left[G, G\right]\leq N$. The quotient $G/\left[G, G\right]$ is an abelian group called the abelianization of $G$ or $G$ made abelian. It is usually denoted by $G^$ or $G_$. There is a useful categorical interpretation of the map $\varphi: G \rightarrow G^$. Namely $\varphi$ is universal for homomorphisms from $G$ to an abelian group $H$: for any abelian group $H$ and homomorphism of groups $f: G \to H$ there exists a unique homomorphism $F: G^\to H$ such that $f = F \circ \varphi$. As usual for objects defined by universal mapping properties, this shows the uniqueness of the abelianization $G^$ up to canonical isomorphism, whereas the explicit construction $G\to G/\left[G, G\right]$ shows existence. The abelianization functor is the adjoint functors, left adjoint of the inclusion functor from the category of abelian groups to the category of groups. The existence of the abelianization functor Grp → Ab makes the category Ab a reflective subcategory of the category of groups, defined as a full subcategory whose inclusion functor has a left adjoint. Another important interpretation of $G^$ is as $H_1\left(G, \mathbb\right)$, the first group homology, homology group of $G$ with integral coefficients.

## Classes of groups

A group $G$ is an abelian group if and only if the derived group is trivial: [''G'',''G''] = . Equivalently, if and only if the group equals its abelianization. See above for the definition of a group's abelianization. A group $G$ is a perfect group if and only if the derived group equals the group itself: [''G'',''G''] = ''G''. Equivalently, if and only if the abelianization of the group is trivial. This is "opposite" to abelian. A group with $G^=\$ for some ''n'' in N is called a solvable group; this is weaker than abelian, which is the case ''n'' = 1. A group with $G^ \neq \$ for all ''n'' in N is called a non-solvable group. A group with $G^=\$ for some ordinal number, possibly infinite, is called a perfect radical, hypoabelian group; this is weaker than solvable, which is the case ''α'' is finite (a natural number).

## Perfect group

Whenever a group $G$ has derived subgroup equal to itself, $G^ =G$, it is called a perfect group. This includes non-abelian Simple group, simple groups and the Special linear group, special linear groups $\operatorname_n\left(k\right)$ for a fixed field $k$.

# Examples

* The commutator subgroup of any abelian group is Trivial group, trivial. * The commutator subgroup of the general linear group $\operatorname_n\left(k\right)$ over a Field (mathematics), field or a division ring ''k'' equals the special linear group $\operatorname_n\left(k\right)$ provided that $n \ne 2$ or ''k'' is not the finite field, field with two elements., Theorem II.9.4 * The commutator subgroup of the alternating group ''A''4 is the Klein four group. * The commutator subgroup of the symmetric group ''Sn'' is the alternating group ''An''. * The commutator subgroup of the quaternion group ''Q'' = is [''Q'',''Q''] = . * The commutator subgroup of the fundamental group π1(''X'') of a path-connected topological space ''X'' is the Kernel (algebra), kernel of the natural homomorphism onto the first singular homology group ''H''1(''X'').

## Map from Out

Since the derived subgroup is Characteristic subgroup, characteristic, any automorphism of ''G'' induces an automorphism of the abelianization. Since the abelianization is abelian, inner automorphisms act trivially, hence this yields a map :$\operatorname\left(G\right) \to \operatorname\left(G^\right)$