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Re: Forcing real eigenvalues with 'eigs'?


From: Bob Odom
Subject: Re: Forcing real eigenvalues with 'eigs'?
Date: Thu, 22 Jan 2009 10:38:01 -0800 (PST)

My guess is that with finite precision arithmetic, you may be stuck with
very small imaginary parts. If there are real stability problems they
could get larger. If you think the computation is stable, I would also
suggest just using real().

Bob


On Thu, 22 Jan 2009, S?ren Hauberg wrote:

tor, 22 01 2009 kl. 13:35 +0100, skrev Francesco Potort??:
>Is it possible to force 'eigs' to produce real eigenvalues? I'm
>computing eigenvalues of a real symmetric matrix, but I'm still getting
>complex eigenvalues (numerical issues I guess), so it would be nice if I
>could force them into being real.

Have you looked at the numbers?  If the imaginary part is very small
compared to the real one, you can just discard it using real().  If not,
you may have stability problems.

Yeah, I can just discard them using real (they are indeed quite small).
It just seems to me that it would be better if I could constraint 'eigs'
to working only with real values.

S??ren

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