inconsistent singular systems

[Barry Smith]

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
>