Let’s take the left regular representation of a finite group on its group algebra and decompose it into irreducible representations.
Our first step is to compute the character of as a left -module. The nice thing here is that it’s a permutation representation, and that means we have a shortcut to calculating its character: is the number of fixed point of the action of on the standard basis of . That is, it counts the number of with . But this can only happen if is the group identity, and in that case every element is a fixed point. Thus we conclude
where in the last line we use the fact that evaluating the character of any representation at the identity element gives the degree of that representation.
So, what does this tell us? Every irreducible representation shows up in with a multiplicity equal to its degree. In particular, it must show up at least once. That is, the left regular representation contains all the irreducible representations.
Thus if are the irreducible representations of , we have a decomposition.
Taking dimensions on either side, we find