[petsc-users] newbie questions on preconditioner LU

[Jed Brown]

On Mon, Jan 28, 2013 at 1:46 PM, Ling Zou <lingzou80 at gmail.com> wrote:

> As you explained, M is not explicitly computed. However, when setup the ksp
>
> KSPSetOperators(KSP ksp,Mat Amat,Mat Pmat,MatStructure flag);
>
> We need a Pmat (the M we are talking about here). In the solver, we
> actually need M^{-1}
>

The two statements above contradict. You can *either* have M=Pmat or you
can have "M^{-1}" is the preconditioning operation used in the Krylov
method.


> as the preconditioning matrix, so we don't need to compute the M matrix
> but need PC to get the approximated M^{-1}, right? Even we don't explicitly
> compute M, but we still need provide non-zero entries for this M, is it
> correct?
>

You need to provide a matrix that will be used to compute a preconditioning
operation. The preconditioning operation will *not* be the inverse of the
matrix you provide, but it may be "close".
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