[petsc-users] Solving singular systems with petsc

[Nidish]

I just use the standard eigs function (https://www.mathworks.com/help/matlab/ref/eigs.html) as a black box. I think it uses a lanczos type method under the hood.

Nidish

On Aug 15, 2020, 21:42, at 21:42, Barry Smith <bsmith at petsc.dev> wrote:
>
>Exactly what algorithm are you using in Matlab to get the 10 smallest
>eigenvalues and their corresponding eigenvectors? 
>
>  Barry
>
>
>> On Aug 15, 2020, at 8:53 PM, Nidish <nb25 at rice.edu> wrote:
>> 
>> The section on solving singular systems in the manual starts with
>assuming that the singular eigenvectors are already known.
>> 
>> I have a large system where finding the singular eigenvectors is not
>trivially written down. How would you recommend I proceed with making
>initial estimates? In MATLAB (with MUCH smaller matrices), I conduct an
>eigensolve for the first 10 smallest eigenvalues and take the
>eigenvectors corresponding to the zero eigenvalues from this. This
>approach doesn't work here since I'm unable to use SLEPc for solving
>> 
>> K.v = lam*M.v
>> 
>> for cases where K is positive semi-definite (contains a few "rigid
>body modes") and M is strictly positive definite.
>> 
>> I'd appreciate any assistance you may provide with this.
>> 
>> Thank you,
>> Nidish
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