# The Jordan-Hölder Theorem

In order to state the main theorem of this section, we first need two definitions.

Definition 11.3.1   Let be a group with . Then is called maximal in if properly (i.e., ) and there does not exist any normal subgroup where the inclusions are all meant to be proper.

On the basis of Corollary 8.3.3 another way of characterizing a maximal normal subgroup is as follows: is a maximal normal subgroup of if and only if is simple. (See the exercises below.)

We now state our last definition.

Definition 11.3.2   A composition series for a group is a normal series as in (11.7), where all the inclusions are proper and such that is maximal in (in other words, each factor is simple).

For example, in the case of the previously given normal series for in (11.8), only the first is a composition series for . A composition series for would be: . Note that would not be a composition series for (Why?). Unlike the case of normal series, it is possible that an arbitrary group does not have a composition series (see exercise 1 for this section) or even if it does have one a subgroup of it may not have one. Of course, a finite group does have a composition series.

We now consider the case in which a group, , does have a composition series, and we prove the following important theorem.

Theorem 11.3.3 (Jordan-Hölder)   : If a group has a composition series, then any two composition series are equivalent (i.e., the composition factors are unique).

Proof: Suppose we are given two composition series. Applying Schreier's refinement theorem (Theorem 11.2.2), we get that the two composition series have equivalent refinements. But the only refinement of a composition series is one obtained by introducing repetitions. If in the 1-1 correspondnece between the factors of these refinements, the paired factors equal to are disregarded (i.e., if we drop the repetitions), we get clearly that the original composition series are equivalent.

It was mentioned in the introduction to Chapter 6 that the simple groups are important because they play a role in finite group theory somewhat analogous to that of the primes in number theory.'' In particular, an arbitrary finite group, , can be broken down into simple components. These uniquely determined simple components are, according to the Jordan-Hölder, the factors of a composition series for .

We close by giving an application of this theorem. In particular, we use the Jordan-Hölder Theorem to prove the uniqueness part of the Fundamental Theorem of Arithmetic. The Fundamental Theorem of Arithmetic states that every positive integer not equal to a prime can be factored uniquely (up to order) into a product of primes.

First, we claim that such a factorization exists. Indeed, suppose is composite (i.e., and is not a prime). Then an easy induction shows that has a prime divisor and we can write , where is an integer satisfying . If is prime, the claim holds. Otherwise, has a prime factor , and where is an integer. Continuing in this fashion, we must come to an equation , where is a prime , since the sequence of decreasing positive integers

cannot continue indefinitely. We now have that is a product of primes. This proves the existence claim.

On the basis of the Jordan-Hölder Theorem, we can easily show the other part of the Fundamental Theorem of Arithmetic, i.e., apart from order of the factors, the representation of as product of primes is unique. To do this suppose that

and

where the and are primes. Then denoting, as usual, by the cyclic group of order , we have

and

as two composition series for . But the Jordan-Hölder Theorem implies these must be equivalent; hence we must have and by suitably arranging , . Thus we have established the unique factorization theorem for positive integers as an application of the Jordan-Hölder Theroem.

Subsections

David Joyner 2007-08-06