inconsistent singular systems

[Håkan Jakobsson]

Thanks for the quick response. I've tried both orthogonalizing and  
not, having convergence in both cases and very similar solutions.  
Likely since b has been almost orthogonal to Null(A) already.
/Håkan
On Sep 16, 2008, at 6:35 PM, Barry Smith wrote:

>
> On Sep 16, 2008, at 10:10 AM, Håkan Jakobsson wrote:
>
>> Hi,
>>
>> How does PETSc handle the solution of a singular system Ax=b which  
>> also happens to be inconsistent? More specifically: is b  
>> orthogonalized against Null(A) internally by PETSc?
>
>   No, you are expected to do this before calling the solver on the  
> right hand side. Note: if A is not symmetric then I think you want  
> to apply the Null(A')
>
>   MatNullSpaceCreate() and KSPSetNullSpace() are a way you can tell  
> the iterative solver the null space, otherwise most iterative  
> solvers will create a giant
> solution in the null space direction and not converge.
>
>    Barry
>
>>
>>
>> Thanks
>>
>> Håkan Jakobsson
>>
>