That's interesting, because in class when we did similar problems but D=/=I we had to use the transitions matrices (S and S^-1) in order to get the correct answer, but its possible that I am misunderstanding.
For example, when we have [; A = \left[ \begin{array}{ccc}3 & 1 & 0 \\
0 & 3 & 0 \\
0...