Michal,
Have you tried to use ARPACK to get the smallest eigenvalues? That's what
we are doing in one of our code and it works pretty well. We have a patch
here
<https://github.com/ORNL-CEES/mfmg/blob/master/scripts/0001-Enable-ARPACK-in-regular-mode.patch>
so that you don't need to provide the inverse of the operator to use
ARPACK. You still need to provide a mass matrix but it's no used.
Best,
Bruno
On Thursday, October 18, 2018 at 8:55:23 AM UTC-4, Michał Wichrowski wrote:
>
> Dear all,
> I need to compute the minimum and maximum eigenvalues of the matrix-free
> operator (vmult is only available ). For the largest eigenvalue, I'm using
> the power method, but I have some problems with minimal eigenvalue. The
> condition number of the operator is close to 1, so I tried using the power
> method on inverse obtained by CG. This solution is time-consuming and for
> some cases I got strange results.
> I have seen that CG can output eigenvalues:
> solver_outer.connect_eigenvalues_slot( [](const std::vector< double > &
> vec)->void { std::cout<< " "<<vec.front()<< " , " <<vec.back()<< " size:
> "<< vec.size()<<std::endl; } , false);
> solver_outer.solve (shat, sol, rhs,
> PreconditionIdentity()
> );
>
> but the estimate of largest eigenvalue obtained this way does not match
> one from power method:
> Maximum eigenvalue from power method: 1.001
> Maximum eigenvalues from CG: 0.0071333 , 1793.58 size: 249
>
> Michał
>
>
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