native eigs versus arpack eigs

[David Bateman]

brendan detracey wrote:
> Hello,
> Finally got Octave compiled on CentOs 5.2 x86_64, thanks to help from this mailing list. Did not bother with "experimental" 64-bit compile, yet.
>
> 1) Now I wonder, what is the difference between the eigs command in the ARPACK interface vs. the native eigs command? Is it worth the bother?
>   
As Soren said, eigs is only available through the ARPACK package. So I 
presume you meant eigs versus eig. The eigs command uses an iterative 
Arnoldi approach to calculate a small subset of the 
eigenvalue/eigenvectors. This subset can either be the larger/smaller 
eigenvalues, or the eigenvalues that are closest to a particular value. 
However the eig command calculates all of the eigenvalues/eigenvectors 
of a matrix directly with a Hessenberg decomposition. The advantage of 
the eigs command is that it is that it isw ell suited for use with large 
sparse matrices, and is more rapid than eig when a small subset of 
eigenvectors/eigenvalues are needed. The disadvantage is that eigs, 
being an iterative approach, can have some stability issues, 
particularly if you specify a value of lambda that is exactly an eigenvalue.

The advantage of eig is that it gives all eigenvalues at the same time 
has better stability properties than eigs. Whereas its disadvantage is 
that there is no sparse version.

> 2) I can't for the life of me find the Octave equivalent of the Matlab command "display"
>   
As Soren says "display" is a function that is used of OOP and so is only 
available in recent developmental versions of Octave. Do you mean you 
need OOP?

D.

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David Bateman                                David.Bateman at motorola.com
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