inconsistent singular systems

[Matthew Knepley]

On Tue, Sep 16, 2008 at 11:56 AM, zhifeng sheng <z.sheng at ewi.tudelft.nl> wrote:
> Hi,
>
> how is the nullspace be implemented in the solver? by using Lagrange
> multiplier?

You have to have an explicit basis for the null space in order for
PETSc to use it.

  Matt

> 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
>>>
>>
>
>



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