Well some of the zero eigenvectors are rigid body modes, but there are some more which are introduced by lagrange-multiplier based constraint enforcement, which are non trivial.
My final application is for a nonlinear simulation, so I don't mind the extra computational effort initially. Could you have me the suggested solver configurations to get this type of eigenvectors in slepc? Nidish On Aug 16, 2020, 00:17, at 00:17, Jed Brown <j...@jedbrown.org> wrote: >It's possible to use this or a similar algorithm in SLEPc, but keep in >mind that it's more expensive to compute these eigenvectors than to >solve a linear system. Do you have a sequence of systems with the same >null space? > >You referred to the null space as "rigid body modes". Why can't those >be written down? Note that PETSc has convenience routines for >computing rigid body modes from coordinates. > >Nidish <n...@rice.edu> writes: > >> 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 <bsm...@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 <n...@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
