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I would like to know if exists an algorithm to find the solutions (if any) of the following matrix equation: $$A\otimes B=B\otimes A$$ in which the $N \times N$ matrix $B$ is given and the $A$ ($A\neq0$) is the unknown $N\times N$ matrix. The conditions on B are: $$B\neq 1, B\neq 0$$... Thanks.

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$A=B$ is a solution. –  Henning Makholm Jul 4 '12 at 10:47
    
@Henning Makholm: thanks. This is a trivial solution. Is it the only possible solution? –  Riccardo.Alestra Jul 4 '12 at 10:59
    
Constant multiples of $A$ as well. –  anon Jul 4 '12 at 11:02

1 Answer 1

up vote 2 down vote accepted

$A=cB$ is a solution for any scalar $c$, but these are all the solutions as long as $B\ne0$.

As relevant here, the Kronecker product is just an outer product (since matrix multiplication is not relevant), so you can model the equation as $A (B^T) = B (A^T)$ which again boils down to a number of equations $a_ib_j=a_jb_i$ for all $i$ and $j$. From there it is easy to see that if there are nonzero entries anywhere, the two inputs must be related by a common proportionality constant.

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