[petsc-users] Dealing with 2nd Derivatives on Boundaries

[Jed Brown]

On Thu, Jan 10, 2013 at 2:35 PM, David Scott <d.scott at ed.ac.uk> wrote:

> Thanks for the replies. I obviously need to think about the problem more.
> I was aware that the solution was not fully determined and I had created a
> MatNullSpace to deal with the arbitrary constant.
>
> I have contacted the original author of the code (I am just modifying it
> to use PETSc) to find out more about the formulation of the problem. I
> should point out that the the actual problem I want to deal with does not
> have a zero RHS.
>
> This systems of equations is only part of the complete system and only the
> gradient of the solution is required elsewhere so the arbitrary constant
> has no physical significance.
>

You are in 2D and the dimension of your null space is the number of grid
points on that "non-boundary". It can change your solution in a myriad of
ways.
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