Algebra III/Introduction to Algebra III: Representation Theory Due: Please upload solutions to NUCT by Tuesday, June 16, 2020.
Problem 1. Let G be a finite group, and let G b be the set of isomorphism classes of irreducible finite dimensional complex representations of G. For every σ ∈ G, we b choose a representative (V
σ, π
σ) of the class σ. We define the Fourier transform of f ∈ C [G] to be the “function”
1f b that to σ ∈ G b assigns the endomorphism
f b (σ) = X
g∈G
f (g)π
σ(g) ∈ End
C(V
σ).
Prove the following statements:
(a) For all f
1, f
2∈ C [G] and σ ∈ G, b
f \
1∗ f
2(σ) = f b
1(σ) ◦ f b
2(σ),
where f
1∗ f
2∈ C [G] is the convolution product of f
1and f
2defined by (f
1∗ f
2)(g) = X
hk=g
f
1(h)f
2(k).
Here the sum ranges over pairs (h, k) ∈ G × G such that hk = g.
(b) For all f ∈ C [G], the Frobenius inversion formula f (g) = 1
|G|
X
σ∈Gb
n
σtr(π
σ(g)
−1◦ f b (σ))
holds. Here n
σ= dim
C(V
σ) and we recall that |G| = P
σ∈Gb
n
2σ.
1The Fourier transformfbis really not a function, but rather a section of a bundle, where the fiber ofσ is EndC(Vσ). Also, to avoid making the choice of (Vσ, πσ), one should work with the
∞-category of representations instead.
1