[petsc-users] left and right preconditioning with a constant null space

[Lawrence Mitchell]

On 23/03/17 15:37, Klaij, Christiaan wrote:
> Yes, that's clearer, thanks! I do have is0 and is1 so I can try
> PetscObjectCompose and let you know.
> 
> Note though that the viewer reports that both S and A11 have a
> null space attached... My matrix is a matnest and I've attached a
> null space to A11, so the latter works as expected. But is the viewer
> wrong for S?

No, I think this is a consequence of using a matnest and attaching a
nullspace to A11.  In that case you sort of "can" set a nullspace on
the submatrix returned in MatCreateSubMatrix(Amat, is1, is1), because
you just get a reference.  But if you switched to AIJ then you would
no longer get this.

So it happens that the nullspace you set on A11 /is/ transferred over
to S, but this is luck, rather than design.

So maybe there is something else wrong.  Perhaps you can run with
-fieldsplit_1_ksp_test_null_space to check the nullspace matches
correctly?

Lawrence

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 473 bytes
Desc: OpenPGP digital signature
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20170323/7fa98b79/attachment.pgp>