Definitions

# Composition series

In abstract algebra, a composition series provides a way to break up an algebraic structure, such as a group or a module, into simple pieces. The need for considering composition series in the context of modules arises from the fact that many naturally occurring modules are not semisimple, hence cannot be decomposed into a direct sum of simple modules. A composition series of a module M is a finite increasing filtration of M by submodules such that the successive quotients are simple and serves as a replacement of the direct sum decomposition of M into its simple constituents.

A composition series may not even exist, and when it does, it need not be unique. Nevertheless, a group of results known under the general name Jordan-Hölder theorem asserts that whenever composition series exist, the isomorphism classes of simple pieces (although, perhaps, not their location in the group or the module in question) and their multiplicities are uniquely determined. Composition series may thus be used to define invariants of finite groups and Artinian modules.

A related but distinct concept is a chief series: a composition series is a maximal subnormal series, while a chief series is a maximal normal series.

## For groups

If a group G has a normal subgroup N which is neither the identity subgroup nor G itself, then the factor group G/N may be formed, and some aspects of the study of the structure of G may be broken down by studying the "smaller" groups G/N and N. If G has no such normal subgroup, then G is a simple group. Otherwise, the question naturally arises as to whether G can be reduced to simple "pieces", and if so, are there any unique features of the way this can be done?

More formally, a composition series of a group G is a subnormal series

$1 = H_0triangleleft H_1triangleleft cdots triangleleft H_n = G,$
with strict inclusions, such that each Hi is a maximal normal subgroup of Hi+1. Equivalently, a composition series is a subnormal series such that each factor group Hi+1 / Hi is simple. The factor groups are called composition factors.

A subnormal series is a composition series if and only if it is of maximal length. That is, there are no additional subgroups which can be "inserted" into a composition series. The length n of the series is called the composition length.

If a composition series exists for a group G, then any subnormal series of G can be refined to a composition series, informally, by inserting subgroups into the series up to maximality. Every finite group has a composition series, but not every infinite group has one. For example, the infinite cyclic group has no composition series.

### Uniqueness: Jordan-Hölder theorem

A group may have more than one composition series. However, the Jordan-Hölder theorem (named after Camille Jordan and Otto Hölder) states that any two composition series of a given group are equivalent. That is, they have the same composition length and the same composition factors, up to permutation and isomorphism. This theorem can be proved using the Schreier refinement theorem. The Jordan-Hölder is also true for transfinite ascending composition series, but not transfinite descending composition series .

#### Example

For a cyclic group of order n, composition series correspond to ordered prime factorizations of n. For example, the cyclic group C12 has
$C_1triangleleft C_2triangleleft C_6 triangleleft C_\left\{12\right\}$,
$C_1triangleleft C_2triangleleft C_4triangleleft C_\left\{12\right\}$,
$C_1triangleleft C_3triangleleft C_6 triangleleft C_\left\{12\right\}$
as different composition series. The sequences of composition factors obtained in the respective cases are
$C_2,C_3,C_2$
$C_2,C_2,C_3$ and
$C_3,C_2,C_2$.

## For modules

Given a ring R and an R-module M, a composition series for M is a series of submodules

$\left\{0\right\} = J_0 subset cdots subset J_n = M$

where all inclusions are strict and Jk is a maximal submodule of Jk+1 for each k. As for groups, if M has a composition series at all, then any finite strictly increasing series of submodules of M may be refined to a composition series, and any two composition series for M are equivalent. In that case, the (simple) quotient modules Jk+1/Jk are known as the composition factors of M, and the number of occurrences of each isomorphism type of simple R-module as a composition factor does not depend on the choice of composition series. If R is an Artinian ring, then every finitely generated R-module has a composition series by the Hopkins-Levitzki theorem. In particular, for any field K, any finite-dimensional module for a finite-dimensional algebra over K has a composition series, unique up to equivalence.

A middle ground between modules and groups is the concept of groups with a set of operators. Restricting attention to subgroups invariant under the action of the operators, nearly identical proofs establish the corresponding results, such as Jordan-Hölder. An important case occurs where the set of operators is the set of inner automorphisms, then the corresponding idea of a composition series is instead a chief series. A unified approach to both groups and modules can be followed, simplifying some of the exposition .

## References

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