inconsistent singular systems

Barry Smith bsmith at
Tue Sep 16 11:35:39 CDT 2008

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.


> Thanks
> Håkan Jakobsson

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