[petsc-users] question about CISS

[Povolotskyi, Mykhailo]

Hello,

I have been experimenting with CISS by computing part of the spectrum of 
a complex matrix of rather small size (774).

I have compared two spectral regions: the circle and the ring.

The results were compared against LAPACK.

The attached figure shows that the accuracy with the ring is low, 
comparatively to a circle.

It looks like if the ring is used, the eigenvalues are found, but not 
accurately.

The circle area works quite okay.

This is what I'm doing:

       EPS            eps;
       EPSType        type;
       RG             rg;

       EPSCreate(MPI_COMM_SELF,&eps);
       EPSSetOperators( eps,matrix_petsc,NULL);
       EPSSetType(eps,EPSCISS);
       EPSSetProblemType(eps, EPS_NHEP);

       EPSSetFromOptions(eps);


       EPSGetRG(eps,&rg);
       RGSetType(rg,RGRING);

       double vscale(1.0);

       double start_ang(0);
       double end_ang(1.0);
RGRingSetParameters(rg,center,radius,vscale,start_ang,end_ang,width);

       EPSSolve(eps);


Could you, please, advise me on this problem?

Thank you,

Michael.

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