UFLACS error for vertex quadrature scheme

[Nico Schlömer]

UFLACS generates for

e = FiniteElement("P", triangle, 1)
u = TrialFunction(e)
v = TestFunction(e)
a = u*v*dx(scheme='vertex')

undefined variable FE3_C0_Q3.

It would be good to add a regression test case with vertex scheme.

[Jan Blechta]

This does not fail in 2016.2.0. The release shall not be blocked due to this regression. The workaround is to use quadrature or tensor representation for this integral.

[Nico Schlömer]

Thanks @blechta for the nice simplification of the MWE.

Using the vertex scheme (with {'quadrature_degree': 1}, actually) is quite important for one of my applications since I need the mass matrix to have no positive entries outside of the diagonal. (This leads to unphysical oscillations in the solution.) The workaround you describe doesn't do the trick.

Is there a workaround that produces a mass matrix with entries only on the diagonal?

[Jan Blechta]

Seems working for me

from dolfin import *

mesh = UnitSquareMesh(10, 10)
V = FunctionSpace(mesh, 'CG', 1)

u = TrialFunction(V)
v = TestFunction(V)

A = assemble(
      u * v * dx,
      form_compiler_parameters={'quadrature_rule': 'vertex',
                                'representation': 'quadrature'}
      )

A = A.array()
for i in range(A.shape[0]):
    A[i, i] = 0.0
assert abs(A).sum() == 0.0

[Jan Blechta]

This issues blocks removing the quadrature representation.