[petsc-users] Solve for all repeated eigenvalues

[Vinh Pham-Xuan]

I do not know about -st_type but the other options are ok (I guess).
With these options, you are looking for eigenvalues on the real axis
between [-13, -7]. Each linear system involved in the contour integral
method is solved by using cholesky factorization.
CISS in SLEPC can also find complex-valued eigenvalues which are inside a
rectangle or ellipse. Therefore, the matrix is not restricted to real and
symmetric.


On Fri, 29 Jun 2018 at 00:15, murat keçeli <keceli at gmail.com> wrote:

> How about spectrum-slicing (i.e. -eps_interval -13,-7 -st_type sinvert
> -st_ksp_type preonly -st_pc_type cholesky)? I guess you'd need real
> symmetric matrices though.
>
> On Thu, Jun 28, 2018 at 4:51 PM, Vinh Pham-Xuan
> <vinh.phamxuan2 at mail.dcu.ie> wrote:
> > You may try the contour integral method (CISS). Obviously, it is
> > computationally more expensive. I used the contour integral method on
> MATLAB
> > and it worked quite well. I expected the same result with SLEPc.
> >
> > On Thu, 28 Jun 2018 at 23:22, Jose E. Roman <jroman at dsic.upv.es> wrote:
> >>
> >> This is a known issue of Krylov solvers. Sometimes you can workaround it
> >> by increasing the number of eigenvalues to compute (nev, for instance
> >> request twice as much as you need) and maybe reduce the mpd parameter to
> >> force more restarts. This approach is quite wasteful and still there is
> no
> >> guarantee that you get the full multiplicities right.
> >>
> >> To avoid the issue completely, use the lobpcg solver. However, SLEPc's
> >> implementation of lobpcg is not very well tuned, I have to take some
> time to
> >> improve it. You can also try the BLOPEX interface (configure
> >> --download-blopex in SLEPc).
> >>
> >> Jose
> >>
> >>
> >> > El 28 jun 2018, a las 22:53, Greg Meyer <gregory.meyer at gmail.com>
> >> > escribió:
> >> >
> >> > Hi all,
> >> >
> >> > I'm using SLEPc to solve for some eigenvalues. If I ask for the
> >> > eigenvalues with the leftmost real part (using SMALLEST_REAL), and
> there are
> >> > repeated eigenvalues, it appears that the solver does not always find
> all of
> >> > the repeated values before jumping up to the next unique eigenvalue.
> For
> >> > example if the lowest eigenvalues of my matrix are [-12, -10, -10,
> -10, -8],
> >> > and I request 4 eigenvalues, I may get [-12, -10, -10, -8]. I'm using
> the
> >> > default solver. Is there any way, or any solver, that ensures that no
> >> > eigenvalues are skipped?
> >> >
> >> > Thanks in advance,
> >> > Greg
> >>
> >
> >
> > --
> > Vinh Pham-Xuan, Ph.D
> > School of Electronic Engineering
> > Dublin City University
> > Ireland
> >
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-- 
Vinh Pham-Xuan, Ph.D
School of Electronic Engineering
Dublin City University
Ireland

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