On Wednesday October 20 2010 12:54:07 Marie Rognes wrote: > On 20. okt. 2010 21:20, Johan Hake wrote: > > On Wednesday October 20 2010 10:39:11 Marie Rognes wrote: > >> A while back (in connection with the blueprint > >> https://blueprints.launchpad.net/dolfin/+spec/solver-interfaces) there > >> was a discussion regarding the interface to VariationalProblem. Anders > >> and Marie have discussed this a bit further, in particular with regard > >> to the adaptive solution of variational problems, and suggest the > >> interface outlined below. (This suggestion involves a change in the > >> interface, and hence the double post to both the dolfin and > >> fenics-mailinglists) > > > > I guess this discussion comes on top of the previous one? Because that > > blueprint mentioned a lot more than what is mentioned here. > > Yes. > > > I also assume this is limited to the Python interface as doing stuff like > > derivative behind the scene is limited to PyDOLFIN? > > Yes and no. > > Using derivative is limited to Python (for now...). However, the > suggested change allows a clean common interface for c++ and Python, > incorporating the desired implicit derivative for Python users.
Ok. > >> >From Marie's perspective, the main reasons for changing the interface > >> >are > >> > > >> (a) The current "nonlinear=true" variable seems superfluous and > >> > >> suboptimal > >> > >> (b) We should allow for automated computation of the Jacobian (when > >> > >> needed). > >> > >> For (nonlinear) variational problems, the interface should read > >> > >> pde = VariationalProblem(Form F, Form jacobian=None, ...) > >> pde.solve(u) > >> > >> where "F" is a Form of rank 1, "jacobian" is a Form of rank 2, and "u" > >> is a Function. Such a pde will be treated as a nonlinear variational > >> problem. The "jacobian" would be an optional argument. If not given, > >> > >> jacobian = derivative(F, u) > >> > >> will be used for the nonlinear solve if needed (for instance as the > >> left-hand side of the Newton iteration). > >> > >> Additionally, we have the interface for linear problems (as before) > >> > >> pde = VariationalProblem(Form a, Form L, ...) > >> pde.solve(u) / u = pde.solve() > >> > >> where "a" is a Form of rank 2 and "L" is a form of rank 1. Such as pde > >> will be treated as a linear variational problem. > > > > Some wild thoughts... > > > > Couldn't we just lump a and L into one form F, and let VariationalProblem > > then figure out what kindoff problem the user would like to solve? > > Basically VariationalProblem then solve F=0. > > > > Differentiate F if the form is of rank 1, (or take an optional Jacobian), > > or split it into a linear problem using lhs and rhs? > > This is possible in the Python interface with the model suggested above. > (Just implies adding a bit more intelligence than indicated) However, > for the sake of the c++ interface (and many application codes), not > removing the (a, L) option seems like a good idea to me. Agree. However, I am not using VariationalProblem to anything :) > >> Philosophical question: Should u be given as an argument to the > >> > >> VariationalProblem instead of to the call to solve? > > > > It is more natural to give u to solve, as that is what you solve for. > > Then you can differentiate wrt to u in the solve function. However, it > > makes it more difficult to make u an optional argument to solve. > > I'm rather flexible at this point. Me too. > Note that u is only an optional argument if the problem is linear, > right. For my purposes, no differentiation is needed in such cases (and > hence u can be created and not given). Sure, but how does a VariationalProblem know it is nonlinear so it can tell a user to provide u in solve? Johan > > -- > Marie > > _______________________________________________ > Mailing list: https://launchpad.net/~dolfin > Post to : [email protected] > Unsubscribe : https://launchpad.net/~dolfin > More help : https://help.launchpad.net/ListHelp _______________________________________________ Mailing list: https://launchpad.net/~dolfin Post to : [email protected] Unsubscribe : https://launchpad.net/~dolfin More help : https://help.launchpad.net/ListHelp
