[petsc-dev] fixing inconsistent right hand side

[Barry Smith]

On Dec 4, 2013, at 9:55 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Barry Smith <bsmith at mcs.anl.gov> writes:
> 
>>   Should we add some option to KSP to “fix” the right hand side if it
>>   is inconsistent? 
> 
> Perhaps.
> 
>>   Presumably this can be done if we know the null space of A?
> 
> The right and left null spaces may not match.

  Yeah, we should provide with the MatNullSpace the ability to provide both.

> 
> What is a SNES line search going to do when you come out with a step
> that is the minimum norm solution to a singular system with an
> inconsistent RHS?  (SNES is not going to converge to a small residual or
> the minimum norm solution.)

  I think it could in some circumstance. Anyways I’m not suggesting this option would ALWAYS be used, but it sure would be useful for testing, when you don’t get convergence like this guy’s case.