[petsc-users] Convergence Criteria?

[Matthew Knepley]

On Wed, Sep 21, 2011 at 4:16 PM, Alan Wei <zhenglun.wei at gmail.com> wrote:

> However, why signs for v[] in ComputeMatrix, which contains the values of
> a row of the matrix. They all have a negative signs. Therefore, I got
> confused which equation does this program solve for:
> 1)  u[j][i] = (u[j+1][i] + u[j-1][i] + u[j][i+1] + u[j][i-1] - rhs *
> dx*dy)/4
> or
> 2)  4u[j][i] - u[j+1][i] - u[j-1][i] - u[j][i+1] - u[j][i-1] + rhs * dx*dy
> = 0
>

The Laplacian is a negative definite operator, so we usually solver -\Delta
u = f since that
is a positive definite problem.

  Thanks,

     Matt


> thanks,
> Alan
>
>
> On Wed, Sep 21, 2011 at 8:22 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>> On Sep 20, 2011, at 10:47 PM, Alan Wei wrote:
>>
>> > Dear Dr. Smith,
>> >     I figure out this problem. Actually, I made my own RHS, but I did
>> not multiply them by the volume (dx*dy).
>> >     However, I met another problem. All values calculated from this
>> example are exactly opposite to values from my own code. I wonder if the RHS
>> I input by ComputeRHS are the really RHS or -1.*RHS?
>>
>>    We do not change the sign of the right hand side.
>>
>>   Barry
>>
>> >
>> > thanks in advance,
>> > Alan
>> >
>> > On Mon, Sep 19, 2011 at 8:43 PM, Barry Smith <bsmith at mcs.anl.gov>
>> wrote:
>> >
>> > On Sep 19, 2011, at 6:25 PM, Alan Wei wrote:
>> >
>> > > Dear folks,
>> > >     I hope you guys are having a nice day.
>> > >     I'm reading the /src/ksp/ksp/examples/tutorials/ex29.c.html and
>> wonder how to set up the convergence criteria?
>> >
>> >   -ksp_rtol 1.e-10 for example
>> >
>> >   Run with -ksp_monitor_true_residual -ksp_converged_reason
>> >
>> >
>> > > Currently I use it as a poisson solver to solve a Poisson Equation
>> with three direction Neumann BC's and one direction Diriechlet BC's. It
>> seems very bad on it.
>> >
>> >    Hmm, multigrid should likely converge well. Are you sure you've set
>> the BC's correctly?
>> >
>> >   Barry
>> >
>> > >
>> > > thanks in advance,
>> > > Alan
>> >
>> >
>>
>>
>


-- 
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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