# Homework Help: Relation between commutator, unitary matrix, and hermitian exponential operator

1. Jan 26, 2012

### silverdiesel

1. The problem statement, all variables and given/known data
Show that one can write U=exp(iC), where U is a unitary matrix, and C is a hermitian operator. If U=A+iB, show that A and B commute. Express these matrices in terms of C. Assum exp(M) = 1+M+M^2/2!....

2. Relevant equations
U=exp(iC)
C=C*
U*U=I
U=A+iB
exp(M) = sum over n: ((M)^n)/n!

3. The attempt at a solution
I am really stumped. I tried (A+iB)(A*-iB*)=I, and I can get the commutator to come out of that, but I have these A*A and B*B terms which I am unsure how to use. I am also not using the exponential term in any way. I know it has something to do with the taylor expansion, just not sure how to get A+iB into that expansion.

2. Jul 13, 2012

### Latrace

Remember that A and B are real matrices. For the first question, try diagonalizing U first.

Last edited: Jul 13, 2012