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    $\begingroup$ Actually, I get the same answer regardless of whether or not I assume the elements are real. I wonder if you're running into memory problems or if you're using an older version of Mathematica? I'm using 8.0.4. $\endgroup$ Commented Feb 8, 2012 at 1:19
  • $\begingroup$ Hi guys, this was my post originally but I lost my account name form yesterday. The matrix comes from the solution to a algebraic ricatti equation. Now, I know F is positive semi definite since it comes from some transition and diffusion matrices in a statespace representation. Obviously this is going to be an ugly output, but thinking about it , I only need to know which elements of the solution are different from zero. $\endgroup$ Commented Feb 8, 2012 at 21:04
  • $\begingroup$ Is there a mapping rule I can use to test this? $\endgroup$ Commented Feb 8, 2012 at 21:15
  • $\begingroup$ I'm afraid I don't understand your question. Test what? $\endgroup$ Commented Feb 9, 2012 at 1:49
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    $\begingroup$ If the entries of the original matrix are all exact numbers, I'd suggest processing the output of MatrixPower[] with RootReduce[] before submitting to Position[]. $\endgroup$ Commented Feb 9, 2012 at 12:30