hi all I have a simple question relating to permutation matrices. We have an a matrix, X. We have a permutation matrix, P. We can get the permuted version of X by doing permutedX = P*X*P'. Now, I want to represent the matrices in vector form. The way the books mention it as follows. They define vectors p = vec(P) by putting each row of P one after the other. Then constructing Y = p*transpose(p). So, P is of size n by n and Y is of size n^2 by n^2. they then construct vec(X) and define a matrix M of size n^2 by n^2 by putting elements of vec(X) on its main diagonal. The following statement confuses me. M*Y is equivalent to the first equation, I tried doing this in matlab considering P to be a simple identity matrix of size 3 by 3. does not work. i am possibly missing something really simple. can somebody tell me how this reasoning works. thanks.