Subdofmap of restricted VectorFunctionSpace not restricted

[Chris Richardson]

See https://bitbucket.org/fenics-project/dolfin/issue/444. I'm not sufficiently knowledgeable about ffc to say this is definitively a bug in ffc, but I think this may be the problem.

[Prof Garth Wells]

Removing milestone: 1.7 (automated comment)

[Jakob Maljaars]

I guess you are aware of the 'work-around' by constructing the restricted Vector/TensorFunctionSpace from the ufl_elements? Consider the following minimal example:

from dolfin import *

k  = 4;
nx = 5; ny = 5;
mesh = UnitSquareMesh(nx,ny)

# Building functions space from ufl element, works perfect
Qbard = VectorElement("CG", mesh.ufl_cell(), k)
Qbar   = RestrictedElement(Qbard, 'facet')

# Following approach returns the error (issue #444)
#Qbar2     = VectorFunctionSpace(mesh,'CG', k,restriction = 'facet')

[Jan Blechta]

Does not help:

from dolfin import *
mesh = UnitSquareMesh(3, 3)
e = VectorElement("P", mesh.ufl_cell(), 3)['facet']
FunctionSpace(mesh, e)

suffers from the same problem.

[Jakob Maljaars]

Indeed, it however seems to work for a mixed function space:

from dolfin import *

k  = 3;
mesh = UnitSquareMesh(3,3)

## Building mixed function
V    = VectorElement("DG", mesh.ufl_cell(), k)
Vbar = VectorElement("CG", mesh.ufl_cell(), k)['facet']
mixed = FunctionSpace(mesh, MixedElement([V,Vbar]))

# Just check if we get expected nr of dofs
dofmapVbar     =  mixed.sub(1).dofmap()
cell2dofVbar   =  [ [dof for dof in dofmapVbar.cell_dofs(cell)] \
                    for cell in range(mesh.num_cells())]
print 'Facet dofs cell 0',  cell2dofVbar[0]

Can you confirm that?

[Jan Blechta]

Yes, but how does it workaround the original problem?

[Jakob Maljaars]

Formally, it is not a workaround to the original issue. However, in practice I guess (correct me if I am wrong) facet function spaces generally occur in mixed problems (e.g. HDG type approaches).