[petsc-users] KSPBuildSolution

Matthew Knepley knepley at gmail.com
Wed Feb 16 18:30:45 CST 2011

On Wed, Feb 16, 2011 at 6:11 PM, Juha Jäykkä <juhaj at iki.fi> wrote:

> Hi again.
> I do not know which one to answer to, but thanks to all. I got one step
> further with your help:
> [0]PETSC ERROR: Zero pivot row 1 value 1.90611e-13 tolerance 1e-12!

You have a singular Jacobian, which leads me to believe that your boundary
conditions are incorrect.


> Adding -pc_factor_shift_type POSITIVE_DEFINITE helps and now I get at least
> some progress. It does not matter if I use -snes_fd, coloring or my hand-
> written Jacobian. If I start from a non-solution initial guess, they all
> progress somewhat towards what I believe is more or less a real solution.
> This
> is all ran with -pc_type lu -ksp_type preonly, btw.
> However, they do not seem to change the second to the last (counting from
> left
> to right) value on the lattice (this is 1D). It does change, but only by
> 0.02%
> while values elsewhere in the lattice change much more significantly.
> Again, I
> am running with the parameters where I know the exact solution and my
> initial
> guess is, on purpose, 0.01 off the correct one (except at the endpoints).
> What
> could be causing this?
> One thing strikes me as odd. I tried checking the value of the function at
> the
> first iteration and at that particular point it is of the order of -1.e-9,
> whereas in the next point to the left, it is +1.e-9 and then starts
> increasing
> towards the left before it starts decreasing again around half-way through
> the
> lattice.
> Cheers,
> Juha
> --
>                 -----------------------------------------------
>                | Juha Jäykkä, juhaj at iki.fi                     |
>                | http://www.maths.leeds.ac.uk/~juhaj           |
>                 -----------------------------------------------

What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
-- Norbert Wiener
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