Commutator subgroup

In mathematics, more specifically in abstract algebra, the commutator subgroup or derived subgroup of a group is the subgroup generated by all the commutators of the group.

The commutator subgroup is important because it is the smallest normal subgroup such that the quotient group of the original group by this subgroup is abelian. In other words, G/N is abelian if and only if N contains the commutator subgroup. 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

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. For elements g and h of a group G, the commutator of g and h is $[g,h]:=g^{-1}h^{-1}gh$.  The commutator [g,h] is equal to the identity element e if and only if gh = hg, that is, if and only if g and h commute.  In general, gh = hg[g,h].

An element of G which is of the form [g,h] for some g and h is called a commutator.  The identity element e = [e,e] is always a commutator, and it is the only commutator if and only if G is abelian.

Here are some simple but useful commutator identities, true for any elements s, g, h of a group G:

• $[g,h]^{-1}=[h,g].$
• $[g,h]^{s}=[g^{s},h^{s}]$, where $g^{s}=s^{-1}gs$, the conjugate of g by s.
• For any homomorphism f: G → H, f([g,h]) = [f(g),f(h)].

The first and second identities imply that the set of commutators in G is closed under inversion and under 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 $x\mapsto x^{s}$.

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 [G,G] (also called the derived subgroup, and denoted G′ or G⁽¹⁾) of G: it is the subgroup generated by all the commutators.

It follows from the properties of commutators that any element of [G,G] is of the form

$[g_{1},h_{1}]\cdots [g_{n},h_{n}]$

for some natural number n, where the g_i and h_i are elements of G. Moreover, since $([g_{1},h_{1}]\cdots [g_{n},h_{n}])^{s}=[g_{1}^{s},h_{1}^{s}]\cdots [g_{n}^{s},h_{n}^{s}]$, the commutator subgroup is normal in G. For any homomorphism f: G → H,

$f([g_{1},h_{1}]\cdots [g_{n},h_{n}])=[f(g_{1}),f(h_{1})]\cdots [f(g_{n}),f(h_{n})]$,

so that $f([G,G])\leq [H,H]$.

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 which is considerably stronger than normality.

The commutator subgroup can also be defined as the set of elements g of the group which have an expression as a product g = g₁ g₂ ... g_k that can be rearranged to give the identity.

Derived series

This construction can be iterated:

$G^{(0)}:=G$

$G^{(n)}:=[G^{(n-1)},G^{(n-1)}]\quad n\in \mathbf {N}$

The groups $G^{(2)},G^{(3)},\ldots$ are called the second derived subgroup, third derived subgroup, and so forth, and the descending normal series

$\cdots \triangleleft G^{(2)}\triangleleft G^{(1)}\triangleleft G^{(0)}=G$

is called the derived series. This should not be confused with the lower central series, whose terms are $G_{n}:=[G_{n-1},G]$, not $G^{(n)}:=[G^{(n-1)},G^{(n-1)}]$.

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 G/N is abelian if and only if [G,G] ≤ N.

The quotient G/[G,G] is an abelian group called the abelianization of G or G made abelian. It is usually denoted by G^ab or G_ab.

There is a useful categorical interpretation of the map $\varphi :G\rightarrow G^{\operatorname {ab} }$. Namely $\varphi$ is universal for homomorphisms from G to an abelian group H: for any abelian group H and homomorphism of groups f: G → H there exists a unique homomorphism F: G^ab → H such that $f=F\circ \varphi$. As usual for objects defined by universal mapping properties, this shows the uniqueness of the abelianization G^ab up to canonical isomorphism, whereas the explicit construction G → G/[G,G] shows existence.

The abelianization functor is the left adjoint of the inclusion functor from the category of abelian groups to the category of groups.

Another important interpretation of $G^{\operatorname {ab} }$ is as H₁(G,Z), the first 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] = {e}. 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 $G^{(n)}=\{e\}$ for some n in N is called a solvable group; this is weaker than abelian, which is the case n = 1.

A group with $G^{(n)}\neq \{e\}$ for any n in N is called a non solvable group.

A group with $G^{(\alpha )}=\{e\}$ for some ordinal number, possibly infinite, is called a hypoabelian group; this is weaker than solvable, which is the case α is finite (a natural number).

Examples

• The commutator subgroup of the alternating group A₄ is the Klein four group.
• The commutator subgroup of the symmetric group S_n is the alternating group A_n.
• The commutator subgroup of the quaternion group Q = {1, −1, i, −i, j, −j, k, −k} is [Q,Q]={1, −1}.
• The commutator subgroup of the fundamental group π₁(X) of a path-connected topological space X is the kernel of the natural homomorphism onto the first singular homology group H₁(X).

Map from Out

Since the derived subgroup is 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

${\mbox{Out}}(G)\to {\mbox{Aut}}(G^{\mbox{ab}})$

See also

• solvable group
• nilpotent group

References

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• Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
  
  Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
  
  In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
  
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  Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
  
  15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
  
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