[petsc-users] Null space/Near null space

[Barry Smith]

> On Jul 23, 2015, at 3:42 AM, Lawrence Mitchell <lawrence.mitchell at imperial.ac.uk> wrote:
> 
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> On 22/07/15 23:57, Barry Smith wrote:
>> 
>> If your matrix has a null space you should always be setting 
>> MatSetNullSpace.
>> 
>> If you know the near null space then you should also always set it 
>> (it cannot do any harm) because some preconditioners can take 
>> advantage of it.
>> 
>> I do not think PETSc currently has code that transfers near null 
>> space information to hypre; I do not know if hypre has any
>> algorithms that can take advantage of it. Only some AMG multigrid
>> methods can utilize near null space information.
> 
> FWIW, the HYPRE manual suggests yes:
> 
> If the block size is set (PETSc already does this) and nodal systems
> coarsening is requested (PETSc has an option for this) then one can
> provide the near null modes with HYPRE_BoomerAMGSetInterpVectors,
> passing an array of Vecs (PETSc does not do this last step).

   Thanks. So someone should make this small code addition plus a test case that utilizes it to demonstrate that it functions and wrap that up in a pull request.

  Barry

> 
> Lawrence
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