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Let M = P*D, where P is a permutation matrix and D diagonal. If P is also symmetric, then does M have all real eigenvalues?

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2 Answers 2

up vote 5 down vote accepted

How about $M = \begin{pmatrix}0&1\\\1&0\end{pmatrix}\begin{pmatrix}-1&0\\\0&1\end{pmatrix}$?

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No. If $P$ is the matrix of a transposition (2 by 2) and $D$ is $diag(1, -1)$ the eigenvlues are imaginary.

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