a question about tolerance

[Ryan Yan]

On Tue, Sep 15, 2009 at 8:29 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Tue, Sep 15, 2009 at 6:04 PM, Ryan Yan <vyan2000 at gmail.com> wrote:
>
>> Thanks Jed.
>>
>> And in the term ||Ae||/||Ax||,  PETSc impilictly assume that we use zero
>> as initial guess and here the x really means exact solution, right?
>>
>
> This has nothing to do with the initial guess. Ae is the residual, and x is
> the current guess. We do not have the exact solution,
> but I guess A x^* = b, so that could have been used.
>
>

Thanks Matt.

I agree that in the print out ||Ae||/||Ax||,  x here just represents the x*.

Yan




>    Matt
>
>
>>
>> Yan
>>
>>
>>
>>
>> On Tue, Sep 15, 2009 at 6:41 PM, Jed Brown <jed at 59a2.org> wrote:
>>
>>> Ryan Yan wrote:
>>> > But can you say a little more about what happened in the first
>>> scenario, I
>>> > did not quite fellow you. Let's say, If I set -ksp_rtol 1e-2, what does
>>> this
>>> > mean in the left PC case. More specifically, which term will be
>>> considered
>>> > as a stop creteria for the KSP solve.
>>>
>>> Look at the first column (preconditioned residual norm).  Notice that
>>> this is decreasing by 1e-2 and 1e-3 respectively in your examples.
>>>
>>> Jed
>>>
>>>
>>
>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
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