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

[Mark F. Adams]

On Sep 19, 2013, at 7:05 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

> 
> On Sep 19, 2013, at 4:28 PM, "Mark F. Adams" <mfadams at lbl.gov> wrote:
> 
>> 
>> 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.  
> 
>  Really, but (for periodic or N. bc) the null space has three vectors, you don't need all of them in your near null space?
> 

we are probably not understanding each other.  If you are just stacking 3 Laplacians in a matrix, uncoupled, then you could do three independent solves with one (near) null space vector for each solve.  The constant function.  When stacked you are just doing all three solves simultaneously.  Norms might be a little different and the three matrices might have different largest eigenvalues so that will make the SA solver a little different.

> 
>> 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?
>>> 
>> 
>