If R:U(1)→GL(n,𝐂) is a smooth representation then there exists a basis of 𝐂n with respect to which R(eiθ)=(eim1θ0⋱0eimnθ) where m1,…,mn are integers called the weights of the representation. A fancier way of saying this is that 𝐂n=⊕ni=1Vi where each Vi is a 1-dimensional subrepresentation and R=R1⊕⋯⊕Rn with Ri=R|Vi .
Representations of U(1), part 1
Representations of U(1), part 1
We now state the classification theorem for representations of U(1) and illustrate it with an example. We will prove the theorem next time.
Theorem:
This means that the basis with respect to which R has this form is a basis of eigenvectors v1,…,vn . Moreover, vk is simultaneously an eigenvector of all the matrices R(eiθ) with eigenvalue eimkθ .
Example:
Take R(eiθ)=(cosθ-sinθsinθcosθ)∈GL(2,𝐂) . The characteristic polynomial of this matrix is det(cosθ-λ-sinθsinθcosθ-λ)=λ2-2λcosθ+1,
so the eigenvalues are λ=2cosθ±√4cos2θ-42=cosθ±isinθ=e±iθ
. Therefore the weights of this representation are ±1
.
The eigenvectors are (i,1) and (-i,1) . These are therefore our vectors v1∈V1 and v2∈V2 . With respect to this basis of eigenvectors, R(eiθ)=(eiθ00e-iθ) .
Pre-class exercise
Exercise:
Check that (±i,1) is an eigenvector of (cosθ-sinθsinθcosθ) with eigenvalue e±iθ .