Representations

Representations

In the next part of the course, we're going to focus on representations of Lie groups. Remember that a (complex) representation of a group is a homomorphism R:GGL(n,𝐂) , that is an assignment of a matrix R(g) to each group element such that R(g1g2)=R(g1)R(g2) and R(1)=I . The image of this representation is a group of matrices which is a quotient of G . In this course, we'll focus on smooth representations of matrix groups.

Why are we focusing on representations? There are many fantastic applications:

Recap

I just want to recap the punchline of the first half of the course, as this will be the basis for everything that comes after.

Given a smooth representation R:GGL(n,𝐂) we get a Lie algebra representation R*:𝔤𝔤𝔩(n,𝐂) , that is a linear map such that R*[X,Y]=[R*X,R*Y] .

Remark:

Here, 𝔤𝔩(n,𝐂) is a complex vector space, but 𝔤 is a vector space over the real numbers. When I say that R*:𝔤𝔤𝔩(n,𝐂) is linear, the only thing that makes sense is for it to be real linear, i.e. R*(λX)=λR*(X)λ𝐑.

We will later complexify 𝔤 to obtain a complex vector space 𝔤𝐂 and get an associated complex linear map R𝐂*:𝔤𝐂𝔤𝔩(n,𝐂) .

The key property of R* was the equation R(expX)=exp(R*X).

This tells us that R determines R* by differentiation: R*(X)=ddt|t=0R(exp(tX)).
Using the formula R(expX)=exp(R*X) , we see that R* determines R(g) for all gexp(𝔤) . Does that mean R* determines R(g) for all gG ?

Lemma:

If G is a path-connected group (i.e. any two matrices in G are connected by a smooth path of matrices in G ) then R is determined by R* .

This is because, in this case, G is generated as a group by exp(𝔤)G . The proof of this lemma is an exercise.

Given R*:𝔤𝔤𝔩(n,𝐂) , does R(expX)=exp(R*X) give a well-defined representation R:GGL(n,𝐂) ? Lie's theorem told us that this is true if G is simply-connected. If G is not simply-connected, we need to think. The first example we'll consider is U(1) , which is not simply-connected, but all the other examples we will consider are simply-connected.

Plan

Our plan for the rest of the course is:

Pre-class exercise

Exercise:

In an earlier video, we constructed a map R:SU(2)SO(3) . Check that R(M1)=R(M2) if and only if M1=±M2 . Given a representation S:SO(3)GL(n,𝐂) , we get a representation RS:SU(2)SO(3) . Show that a representation T:SU(2)SO(3) has this form if and only if T(-M)=T(M) for all MSU(2) .

Exercise:

Suppose we have a Lie algebra 𝔤𝔤𝔩(n,𝐑) consisting of real matrices. Consider the subspace 𝔤𝐂𝔤𝔩(n,𝐂) consisting of matrices of the form M+iN with M and N in 𝔤 . Show that:

  • this is a Lie subalgebra, i.e. that it is preserved by Lie bracket

  • if f:𝔤𝔤𝔩(m,𝐂) is a real-linear Lie algebra homomorphism then f𝐂:𝔤𝐂𝔤𝔩(m,𝐂) defined by f𝐂(M+iN)=f(M)+if(N) for M and N in 𝔤 is also a Lie algebra homomorphism.