[petsc-dev] coarse grid in gamg (for the OPTE paper)

[Mark F. Adams]

On Sep 19, 2013, at 3:53 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

> 
>   It is essentially 3 Laplacians

If you mean literally 3 scalar laplacians packed in one matrix for some reason then the one constant vector is fine.  Is this a block diagonal matrix with 3 big blocks, if you order it correctly of course?

> so I think the default null space of 3 constant vectors is fine.

You get this if you set the block size to 3.  Constant vectors in each of the three components.

> The problem is without the block information presumably GAMG is using only a single constant vector over all variables?  So maybe we need to construct a 3 vector null space which just marks in the reduced vector from which of the 3 components each entry came from.
> 
>   Barry
> 
> On Sep 19, 2013, at 3:48 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> 
>> Barry Smith <bsmith at mcs.anl.gov> writes:
>>>  That is what it is doing and apparently it doesn't result in a good
>>>  preconditioner; I don't know why off hand. One thing is it no
>>>  longer knows about the block structure.
>> 
>> How is the near null space being specified?
>