[SciPy-User] Help required regding Eigenvectors

[Charles R Harris]

On Tue, Sep 21, 2010 at 11:28 AM, Andrew Dessler <adessler at tamu.edu> wrote:

> Sudheer Phani <bpsu <at> james.hut.fi> writes:
>
> >
> > Hello All,
> >
> >    I have been encountering some strange problems with the calculation of
> > eigen vectors using python. The problem is with the sign of the eigen
> > vectors calculated. On Very few occasions the eigen vectors calculated
> > using python have an opposite sign than the actual eigenvectors, though
> > the magnitude is correct.
> >
> > Can any one of you plz help me in this regard and suggest me some
> solution
> > if you have encoutered similar problems .
> >
> > Any help in this regd is appreciated
> >
> > Thanks in Advace
> >
> > regds
> > Sudheer
> >
>
>
> I had a similar experience.  However, I solved it when I realized that the
> eig
> routine in scipy.linalg does not necessarily return the biggest
> eigenvalue/eigenvector first.  It often does, but not always.  Once I
> realized I
> had to find the biggest one on my own, the problem was solved.
>
>
Note that eigen vectors don't *have* a sign and this is the case even if
they are normalized. It is always possible to multiply them by +/- 1, or
even by a complex number of magnitude 1 if you are working with complex
arrays.

Chuck
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