The question is that D1 and D2 are any two diagonal 3x3 matrices and

P = AD1Ainv

Q = AD2Ainv

Show that PQ = QP.

Now, I know that D1 and D2 commute, but im not sure how I can show this algebraically. Any ideas?

BTW: A is a 3 x 3 matrix and is the inverse of itself. So, really, P = AD1A and Q = AD2A.

Many thanks

MB