On Tue, Nov 15, 2011 at 5:43 PM, Jed Brown <jedbrown@mcs.anl.gov> wrote:
On Tue, Nov 15, 2011 at 17:37, Matthew Knepley <knepley@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?

What are you on? It is F(x) = 0. This embedding will not work for all problems, but it will work for many, and be easy. If you
have a more complicated system, by all means us TS, however I am pointing out that there are useful regimes where this
kind of embedding would work.

   Matt
 
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.



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