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Assume $A$ is $m \times n$ and $B$ is $m \times n$.

Is there a connection between the eigenvalues of AB' and the eigenvalues of B'A?

One is an $m \times m$ and the other is $n \times n$.

($B'$ stands for the transpose of $B$)

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    I missed a $X^m$ in the last comment.2012-01-27

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It seems easier for me to assume that $B$ is an $n \times m$ matrix. In that case, a classical argument shows that $AB$ and $BA$ have the same nonzero eigenvalues, not counting multiplicity. The case that these eigenvalues are distinct is dense in the general case, so $AB$ and $BA$ have the same nonzero eigenvalues counting multiplicity. Of course one of them has $|n-m|$ more zero eigenvalues than the other.