inconsistent singular systems

[zhifeng sheng]

Hi,

how is the nullspace be implemented in the solver? by using Lagrange 
multiplier?

best regards
Zhifeng

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
>>
>