I'm attempting a homework problem and have an idea for proving the result that relies on whether or not matrices with positive entries have only real eigenvalues. Is this true?

For $2\times 2$ matrices this is easy to show. However I'm not sure how to decide if it is true for general $n \times n$ matrices.

I know that the Perron-Frobenius Theorem says the spectral radius of a positive matrix is itself an eigenvalue. If the result I'm asking about is true, can I use this to prove it?


1 Answer 1


As pointed out in this answer https://mathoverflow.net/questions/99845/properties-of-eigenvalues-of-general-nonnegative-matrices

the matrix $$ \begin{pmatrix} 0&0&0&1 \\ 1&0&0&1 \\ 0&1&0&1 \\ 0&0&1&1 \end{pmatrix} $$ has some complex eigenvalues. This remains true, if we add $0.01$ to every entry, making the matrix positive with some complex eigenvalues. (Of course, calculations have to be done by some software...)

  • $\begingroup$ +1 for genuinely add that link! $\endgroup$ Sep 26, 2018 at 6:28
  • $\begingroup$ Awesome. Thank you. $\endgroup$ Sep 26, 2018 at 6:30

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