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:

If R:U(1)GL(n,𝐂) is a smooth representation then there exists a basis of 𝐂n with respect to which R(eiθ)=(eim1θ00eimnθ) 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=R1Rn with Ri=R|Vi .

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 v1V1 and v2V2 . 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 .