Re: eigs compatibility

[Daniel J Sebald]

On 07/04/2013 05:52 PM, Ed Meyer wrote:
> On Thu, Jul 4, 2013 at 12:54 PM, Daniel J Sebald <address@hidden
> <mailto:address@hidden>> wrote:
>
>     On 07/04/2013 02:21 PM, John W. Eaton wrote:
>
>         On 07/04/2013 03:03 PM, Daniel J Sebald wrote:
>
>         I have no clue why ARPACK has the limitation of providing at
>         most N-2
>         eigenvalues, but that's the way it works. I'm not going to try
>         to fix
>         that problem.
>
>
> they use a shifting strategy to get eigenvalues clustered nearest a
> given shift, so
> if you want all the eigenvalues for a large sparse matrix you can call
> arpack twice,
> once for k/2 eigenvalues at the low end of the spectrum, and again for
> the k/2 at
> the high end.

Creative idea.  I might try a time comparison.


>  "Large" is greater than 300th order even on my klunky
> laptop; otherwise
> it makes more sense to just call the lapack routines if the user wants a
> large percentage
> of the eigenvalues.

Thanks Ed.

Dan