[petsc-users] Nullspace issue

[Jed Brown]

SVD works. Black box solvers aren't really.

You can try sparse approximate inverse, but good luck. ;-D

If you specify a "near null space", you can try smoothed aggregation.

Or just run domain decomposition, the decomposition generally breaks to
null space so that factorization can be used on subdomains.
On Jun 12, 2012 8:19 AM, "Thomas Witkowski" <thomas.witkowski at tu-dresden.de>
wrote:

>  Am 12.06.2012 15:04, schrieb Jed Brown:
>
> On Tue, Jun 12, 2012 at 7:56 AM, Thomas Witkowski <
> thomas.witkowski at tu-dresden.de> wrote:
>
>> There should be no null space from the Cahn-Hilliard equation.
>
>
>  You said all those boundary conditions are either Neumann or periodic. I
> guess it couples to the fluid variables without any null space?
>
> yes.
>
>
>
>> Is there some black-box preconditioner that does not relay on LU
>> factorization at some point? I know that black-box approaches are mostly
>> not efficient, but I would have something I can work with.
>
>
> The SVD always works and will tell you about a null space, but of course
> it's very expensive.
>
> So assume I have a basis for the null space of the system that should be
> solved. Is there any block-box solver/preconditioner approach that does not
> make use of (I)LU factorization at any point?
>
> Thomas
>
>
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