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Arpack usage


Hello,

I am now involved in using arpack to solve a large complex matrix's
eigenvalues and corresponding eigenvectors. The solver I used is
arpack, it's really great, fast and convenient.

I compare the eigenvalues and eigenvectors from arpack solver and
matlab 'eigs' function (though I know it also uses arpack), however, I
found a big mismatch on eigenvectors (eigenvalue is OK, they match!)
Can anyone who have used arpack tell me why such big mismatch exist?

Thanks a lot,

lorin

On May 14, 9:29 am, lorin <wwang@gmail.com> wrote:

> Hello,

> I am now involved in using arpack to solve a large complex matrix's
> eigenvalues and corresponding eigenvectors. The solver I used is
> arpack, it's really great, fast and convenient.

> I compare the eigenvalues and eigenvectors from arpack solver and
> matlab 'eigs' function (though I know it also uses arpack), however, I
> found a big mismatch on eigenvectors (eigenvalue is OK, they match!)
> Can anyone who have used arpack tell me why such big mismatch exist?

If X is an eigenvector, so c*X (a multiple of X), and arpack and
Matlab may use different conventions in scaling the eigenvectors. Are
the arpack eigenvectors proportional to those of Matlab?
On 14 mei, 15:29, lorin <wwang@gmail.com> wrote:

> Hello,

> I am now involved in using arpack to solve a large complex matrix's
> eigenvalues and corresponding eigenvectors. The solver I used is
> arpack, it's really great, fast and convenient.

> I compare the eigenvalues and eigenvectors from arpack solver and
> matlab 'eigs' function (though I know it also uses arpack), however, I
> found a big mismatch on eigenvectors (eigenvalue is OK, they match!)
> Can anyone who have used arpack tell me why such big mismatch exist?

> Thanks a lot,

> lorin

Perhaps you have eigenvalues that (nearly) coincide? Then you could
get any
linear combination of the eigenvectors. That is one possible
explanation.
Another is: have you normalised the vectors before comparing them?
There
could be a (complex) factor between them.

Regards,

Arjen

On May 14, 9:42 pm, Arjen Markus <arjen.mar@wldelft.nl> wrote:

Thanks, Arjen and Beliavsky... I have recompare the eigenvectors from
Matlab and ARPACK. There is a factor between them, for different
matrix, the factor is not the same, but all factors' absolute
magnitude is unit length. I try to find out why such factor exists,
but it seems difficult...
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