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[Anders Logg <question109900@xxxxxxxxxxxxxxxxxxxxx>]

Question #109900 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/109900

Anders Logg proposed the following answer:
On Thu, May 06, 2010 at 02:52:33PM +0200, Marie Rognes wrote:
> Achim Schroll wrote:
> >New question #109900 on DOLFIN:
> >https://answers.launchpad.net/dolfin/+question/109900
> >
> >for a plain Poisson eqn with pure Neumann b.c., how to specify the scaling condition u*dx = 0 ?
>
>
>
> If you have a recent dolfin, you can introduce a constant c acting
> as a Lagrange multiplier corresponding to the constraint. See
> example below.
>
> from dolfin import *
>
> mesh = UnitSquare(32, 32)
> V = FunctionSpace(mesh, "CG", 1)
> Q = FunctionSpace(mesh, "R", 0)
> M = V * Q
>
> (u, c) = TrialFunctions(M)
> (v, d) = TestFunctions(M)
>
> f = Expression("x[0]*x[1]*sin(pi*x[0])")
>
> a = dot(grad(v), grad(u))*dx + d*u*dx + c*v*dx
> L = v*f*dx
>
> pde = VariationalProblem(a, L)
> u_h = pde.solve()
>
> plot(u_h[0])
> interactive()

But make sure to not name that space 'M' if you try this with a UFL
file or you will get a strange error (since 'M' is reserved in form
files).

Anyway, I'm wondering about the dofmap for the "R" element. It sets
_global_dimension to m.num_entities[2] which is the global number of
cells, but then it maps all dofs to 0. Is that right? Shouldn't the
global dimension be 1?

--
Anders

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