# Solvable group

In mathematics, more specifically in the field of group theory, a **solvable group** or **soluble group** is a group that can be constructed from abelian groups using extensions. Equivalently, a solvable group is a group whose derived series terminates in the trivial subgroup.

A group *G* is called **solvable** if it has a subnormal series whose factor groups (quotient groups) are all abelian, that is, if there are subgroups 1 = *G*_{0} < *G*_{1} < ⋅⋅⋅ < *G _{k}* =

*G*such that

*G*

_{j−1}is normal in

*G*, and

_{j}*G*/

_{j}*G*

_{j−1}is an abelian group, for

*j*= 1, 2, …,

*k*.

where every subgroup is the commutator subgroup of the previous one, eventually reaches the trivial subgroup of *G*. These two definitions are equivalent, since for every group *H* and every normal subgroup *N* of *H*, the quotient *H*/*N* is abelian if and only if *N* includes the commutator subgroup of *H*. The least *n* such that *G*^{(n)} = 1 is called the **derived length** of the solvable group *G*.

The basic example of solvable groups are abelian groups. They are trivially solvable since a subnormal series being given by just the group itself and the trivial group. But non-abelian groups may or may not be solvable.

More generally, all nilpotent groups are solvable. In particular, finite *p*-groups are solvable, as all finite *p*-groups are nilpotent.

In particular, the quaternion group is a solvable group given by the group extension

A small example of a solvable, non-nilpotent group is the symmetric group *S*_{3}. In fact, as the smallest simple non-abelian group is *A*_{5}, (the alternating group of degree 5) it follows that *every* group with order less than 60 is solvable.

The celebrated Feit–Thompson theorem states that every finite group of odd order is solvable. In particular this implies that if a finite group is simple, it is either a prime cyclic or of even order.

Any finite group whose *p*-Sylow subgroups are cyclic is a semidirect product of two cyclic groups, in particular solvable. Such groups are called Z-groups.

For any positive integer *N*, the solvable groups of derived length at most *N* form a subvariety of the variety of groups, as they are closed under the taking of homomorphic images, subalgebras, and (direct) products. The direct product of a sequence of solvable groups with unbounded derived length is not solvable, so the class of all solvable groups is not a variety.

Burnside's theorem states that if *G* is a finite group of order *p ^{a}q^{b}* where

*p*and

*q*are prime numbers, and

*a*and

*b*are non-negative integers, then

*G*is solvable.

As a strengthening of solvability, a group *G* is called **supersolvable** (or **supersoluble**) if it has an *invariant* normal series whose factors are all cyclic. Since a normal series has finite length by definition, uncountable groups are not supersolvable. In fact, all supersolvable groups are finitely generated, and an abelian group is supersolvable if and only if it is finitely generated. The alternating group *A*_{4} is an example of a finite solvable group that is not supersolvable.

If we restrict ourselves to finitely generated groups, we can consider the following arrangement of classes of groups:

A group *G* is called **virtually solvable** if it has a solvable subgroup of finite index. This is similar to virtually abelian. Clearly all solvable groups are virtually solvable, since one can just choose the group itself, which has index 1.

A solvable group is one whose derived series reaches the trivial subgroup at a *finite* stage. For an infinite group, the finite derived series may not stabilize, but the transfinite derived series always stabilizes. A group whose transfinite derived series reaches the trivial group is called a **hypoabelian group**, and every solvable group is a hypoabelian group. The first ordinal *α* such that *G*^{(α)} = *G*^{(α+1)} is called the (transfinite) derived length of the group *G*, and it has been shown that every ordinal is the derived length of some group (Malcev 1949).