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[Marie Rognes <question109900@xxxxxxxxxxxxxxxxxxxxx>]

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

Marie Rognes proposed the following answer:
Anders Logg wrote:
> 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?
>
>   


Yes, it is a bug for non-mixed elements(!). I noticed but didn't really 
want to start fixing before
the weekend. It's on the todo-list though.

--
Marie

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