[Getfem-users] mesh_fem_sum question

[Roman Putanowicz]

Dear All,

I have the following problem:

I would like to build an element defined over 1D segment, with two nodes.
At each node I would like to have 3 DOFs -- 1 Lagrange DOF and 2 Hermite
DOFs, so my element will be composed from linear Lagrange element and 
cubic Hermite element (or simply it will be so called plane frame element).

I thought that I could use the class mesh_fem_sum to build such element
without introducing new element kind. It works, but partially --
inside the assembly routine the tensor vGrad is correctly calculated
as [0..6][0..1][0..2] tensor but  the method  nb_dof returns 4 (four)
as the number of degrees of freedom and the resulting stiffness matrix
is assembled as 4x4.

The documentation of mesh_fem_sum says : "Implement a special mesh_fem with 
merges the FEMs of two (or more) mesh_fems."

Could you please explain me shortly, what exactly is the nature of the 
merging two mesh_fems, and why I got wrong (in my view) number of degrees
of freedom in the code attached below?

Thank you in advance for any hint, 

Regards

Roman
--------------------8<------------
int main() {
 getfem::mesh mesh;

 std::vector<bgeot::size_type> ind(2);
 ind[0] = mesh.add_point(bgeot::base_node(0.0, 0.0));
 ind[1] = mesh.add_point(bgeot::base_node(2.0, 2.0));

 mesh.add_segment(ind[0], ind[1]);

 getfem::mesh_fem femBeam(mesh);
 getfem::mesh_fem femBar(mesh);

 femBar.set_finite_element(getfem::fem_descriptor("FEM_PK(1,1)"));
 femBeam.set_finite_element(getfem::fem_descriptor("FEM_HERMITE(1)"));

 femFrame.set_mesh_fems(femBeam, femBar);
 getfem::size_type Ndof =  femFrame.nb_dof();
 std::cout << "NUMBER OF DEGREES OF FREEDOM " << Ndof << "\n";

 return 0;
}