Hi again! This is the start of a series where I will post written versions of my presentations. The material below is a presentation I gave at the start of February in 2019 about the abelianness of groups of order using representations. In late 2018 I became involved in my mathematics department’s directed reading program (DRP). In this program I was paired with a graduate student and we chose a topic to study together. We decided on complex representations of where is a finite field. The only requirement for the program was to give a presentation to the rest of the students, and I decided to give a fun little example of using representations to prove group theoretic facts. So without further ado…
Representation Theory and Group Theory
Representation theory of finite groups is a deep theory in its own right, but sometimes we can use representations to give some elegant proofs of statements that belong to group theory. Today I’m going to give an example of such a situation. We’re going to use some representation theory to prove the following proposition:
Proposition: Let be a group of order where is a prime. Then every irreducible representation of is one-dimensional. In particular, is abelian.
Before we can give the proof, we’ll need the definition of a representation and give some simple facts.
Definition: If is a group, a field, and a vector space over , then a representation is a homomorphism . We sometimes write for brevity. A subrepresentation of is a representation with a vector subspace of and for every . The representation is irreducible if it has no proper subrepresentation. The degree of is the dimension of .
Facts: Suppose is a finite group.
- If is an irreducible representation of , then the degree of divides the order of .
- The number of irreducible representations of is equal to the number of conjugacy classes of .
- Let be the number of conjugacy classes of . If is the degree of the -th irreducible representation, then .
These facts are stated without proof. If you take any introductory class in representation theory, then you will very likely prove these facts within the first two weeks of class. They are fairly straightforward, but some other machinery needs to be developed in order to prove them. Now let us prove the proposition.
proof: Let be an irreducible representation of . By fact (1), the degree of is either , , or . We will show it cannot be or . If the degree of is then we contradict fact (3) since . If the degree of is , then fact (3) tells us that is the only irreducible representation of . However the representation , where is a one-dimensional vector space, given by for all (also known as the trivial representation) is an irreducible representation of degree . So, cannot be the only irreducible representation and therefore the degree of cannot be . This forces the degree of to be , and since was an arbitrary irreducible representation we conclude every irreducible representation of is of degree . It follows by facts (3) and (2) that that the number of conjugacy classes of is exactly the order of . This happens if and only if is abelian, so we are done.
There are also some more classical examples of using representation theory to prove facts about group theory. Two of the most common are proofs of Burnside’s theorem and of the Frobenius kernel being a normal subgroup. They mainly use character theory, a subtheory of representations. That concludes this presentation, thanks for reading!