[petsc-users] Null space correction depends on solver?

[Jed Brown]

On Tue, Aug 28, 2012 at 12:25 PM, Thomas Witkowski <
thomas.witkowski at tu-dresden.de> wrote:

>
>        GMRES uses left preconditioning by default which forces the null
>> space into the Krylov space at each iteration. FGMRES uses right
>> preconditioning and doesn't  do anything to the null space because it is
>> naturally in the Krylov space (since the result with fgmres contains the
>> constants this is why I suspect the matrix actually doesn't have the null
>> space you think it has). You can try running -ksp_type gmres -ksp_pc_side
>> right      my guess is that you will not get near zero for the dot.
>>
> Any idea how to debug this problem? Lets concentrate on the gmres/no
> precon case first.


Left preconditioning and anything using -pc_type none works correctly. It
is only the case of right preconditioning that is incorrect (and needs to
be fixed in PETSc).
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