[petsc-dev] snes/examples/tutorials/ex27.c

[Jed Brown]

On Tue, Nov 15, 2011 at 17:37, Matthew Knepley <knepley at gmail.com> wrote:

>   b) FormFunction and FormJacobian in a SNES could be used by an enclosing
> TS to do pseudo-timestepping


This can't work. What equation would the FormFunction and FormJacobian
correspond to?

The TS *defines* the equation that needs to be solved. The user provides
information according to an interface. The TS interface is more general,
and really is required for transient things. The SNESFormFunction isn't
enough because you haven't identified positivity and constraints. Add that
transient term to all variables equally and you are going to see some
confusing stuff like huge conservation error and incorrect boundary
conditions, such that you could easily end up on the wrong branch or
mis-diagnose a limit cycle versus steady state.
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