# Maschke's theorem

In mathematics, Maschke's theorem, named after Heinrich Maschke, is a theorem in group representation theory that concerns the decomposition of representations of a finite group into irreducible pieces. Maschke's theorem allow one to make general conclusions about representations of a finite group G without actually computing them. It reduces the task of classifying all representations to a more manageable task of classifying irreducible representations, since when the theorem applies, any representation is a direct sum of irreducible pieces (constituents). Moreover, it follows from the Jordan–Hölder theorem that, while the decomposition into a direct sum of irreducible subrepresentations may not be unique, the irreducible pieces have well-defined multiplicities. In particular, a representation of a finite group over a field of characteristic zero is determined up to isomorphism by its character.

## Formulations

Maschke's theorem addresses the question: when is a general (finite-dimensional) representation built from irreducible subrepresentations using the direct sum operation? This question (and its answer) are formulated differently for different perspectives on group representation theory.

### Group-theoretic

Maschke's theorem is commonly formulated as a corollary to the following result:

Theorem. If V is a complex representation of a finite group G with a subrepresentation W, then there is another subrepresentation U of V such that V=WU.

Then the corollary is

Corollary (Maschke's theorem). Every representation of a finite group G over a field F with characteristic not dividing the order of G is a direct sum of irreducible representations.

The vector space of complex-valued class functions of a group G has a natural G-invariant inner product structure, described in the article Schur orthogonality relations. Maschke's theorem was originally proved for the case of representations over $\mathbb {C}$ by constructing U as the orthogonal complement of W under this inner product.

### Module-theoretic

One of the approaches to representations of finite groups is through module theory. Representations of a group G are replaced by modules over its group algebra K[G] (to be precise, there is an isomorphism of categories between K[G]-Mod and RepG, the category of representations of G). Irreducible representations correspond to simple modules. In the module-theoretic language, Maschke's theorem asks: is an arbitrary module semisimple? In this context, the theorem can be reformulated as follows:

Maschke's Theorem. Let G be a finite group and K a field whose characteristic does not divide the order of G. Then K[G], the group algebra of G, is semisimple.

The importance of this result stems from the well developed theory of semisimple rings, in particular, the Artin–Wedderburn theorem (sometimes referred to as Wedderburn's Structure Theorem). When K is the field of complex numbers, this shows that the algebra K[G] is a product of several copies of complex matrix algebras, one for each irreducible representation. If the field K has characteristic zero, but is not algebraically closed, for example, K is a field of real or rational numbers, then a somewhat more complicated statement holds: the group algebra K[G] is a product of matrix algebras over division rings over K. The summands correspond to irreducible representations of G over K.

### Category-theoretic

Reformulated in the language of semi-simple categories, Maschke's theorem states

Maschke's theorem. If G is a group and F is a field with characteristic not dividing the order of G, then the category of representations of G over F is semi-simple.

## Proofs

### Module-theoretic

Let V be a K[G]-submodule. We will prove that V is a direct summand. Let π be any K-linear projection of K[G] onto V. Consider the map $\varphi :K[G]\to V$ given by $\varphi (x)={\frac {1}{\#G}}\sum _{s\in G}s\cdot \pi (s^{-1}\cdot x).$ Then φ is again a projection: it is clearly K-linear, maps K[G] onto V, and induces the identity on V. Moreover we have

{\begin{aligned}\varphi (t\cdot x)&={\frac {1}{\#G}}\sum _{s\in G}s\cdot \pi (s^{-1}\cdot t\cdot x)\\{}&={\frac {1}{\#G}}\sum _{u\in G}t\cdot u\cdot \pi (u^{-1}\cdot x)\\{}&=t\cdot \varphi (x),\end{aligned}} so φ is in fact K[G]-linear. By the splitting lemma, $K[G]=V\oplus \ker \varphi$ . This proves that every submodule is a direct summand, that is, K[G] is semisimple.

## Converse statement

The above proof depends on the fact that #G is invertible in K. This might lead one to ask if the converse of Maschke's theorem also holds: if the characteristic of K divides the order of G, does it follow that K[G] is not semisimple? The answer is yes.

Proof. For $x=\sum \lambda _{g}g\in K[G]$ define $\epsilon (x)=\sum \lambda _{g}$ . Let $I=\ker \epsilon$ . Then I is a K[G]-submodule. We will prove that for every nontrivial submodule V of K[G], $I\cap V\neq 0$ . Let V be given, and let $v=\sum \mu _{g}g$ be any nonzero element of V. If $\epsilon (v)=0$ , the claim is immediate. Otherwise, let $s=\sum 1g$ . Then $\epsilon (s)=\#G\cdot 1=0$ so $s\in I$ and $sv=\left(\sum 1g\right)\left(\sum \mu _{g}g\right)=\sum \epsilon (v)g=\epsilon (v)s$ so that $sv$ is an element of both I and V. This proves that V is not a direct complement of I for all V, so K[G] is not semisimple.