[petsc-dev] Attaching a near null space to an IS

[Barry Smith]

  I think it belongs in the DM (or the matrix if it was put in the matrix). When you select a subpart of the DM that new DM should contain the near null space for the subpart.  The DM associated with the field defining IS is where all properties should go, not the IS itself. Of course, we really have not yet solidified the getting of the new DM for the field yet, maybe after the release.

   Barry

On May 6, 2012, at 6:30 PM, Matthew Knepley wrote:

> On Sun, May 6, 2012 at 7:15 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> Matt introduced this concept, he says the IS is a better place to attach things.
> 
> http://petsc.cs.iit.edu/petsc/petsc-dev/rev/2ad289ac99e0
> 
> I don't understand why the IS is better (because it's mostly immutable?). I'm worried that putting it there is going to be fragile because the near null space is not a property of an IS at all.
> 
> Its not a property of your matrix either, or you would not need me to tell you. Its a property of the operator. The operator
> is defined by the DM. The near null space is actually a property of a suboperator, defined by the DM using a field (we are
> not allowing arbitrary divisions). The representation of a field in PETSc is an IS (http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/DM/DMCreateFieldIS.html) so it makes sense to attach field information to the IS. Moreover, it
> makes a hell of a lat more sense to attach an auxiliary operator (like L_p) to this IS than to a matrix.
> 
> Furthermore, this scheme is completely workable in a nested context. The user can specify the IS, or pull out the DM IS
> and play with it, without a bunch of cumbersome copies hanging around that we do not want and can't destroy. That is what
> would happen with persistent submatrices. Lastly, I am running this for PyLith and it works great.
> 
>    Matt
> 
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener