[petsc-users] testing for and removing a null space using JFNK

Mark Adams mfadams at lbl.gov
Thu Apr 4 04:26:03 CDT 2019

The Krylov space can not see the null space (by definition) and so getting
a useful near null space from it is not likely.

Getting a null space is a hard problem and bootstrap AMG methods, for
instance, are developed to try to do that. This is an advanced research

You really want to know your null space, what kind of equations do you have?


On Thu, Apr 4, 2019 at 4:28 AM Dave Lee via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Hello PETSc,
> I'm attempting to solve a JFNK problem for a system where I only have a
> function to compute the residual, but no matrix.
> I wanted to know if there exists functionality in PETSc to do the
> following:
> 1) approximate a null space from a set of Krylov vectors
> 2) remove such a null space if it exists
> I'm vaguely familiar with the MatNullSpaceCreate/Remove() functionality,
> however I don't know the precise form of a null space, so I don't have a
> set of vectors I can assemble and pass to this.
> Cheers, Dave.
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