2018-11-29 10:49:13 +0200 asked a question How to use the divergence of a finite function as the source term of another problem I use RT element to solve problem A and denote the finite element solution by sigma, sigma is a matrix-valued finite element function. I need to use the divergence of sigma to be the source term of another problem, the trial function space is vector-valued functions. But it doesn't work. The code is from __future__ import print_function from fenics import * import matplotlib.pyplot as plt mesh = UnitSquareMesh(8,8) RT = VectorElement("RT",mesh.ufl_cell(), 1,2) Q = FunctionSpace(mesh, RT) V = VectorFunctionSpace(mesh, 'CG',1) f = Expression((('x[0]*x[1]','x[0]'),('x[0]','x[0]*x[1]')),degree=2) sigma = TrialFunction(Q) tau = TestFunction(Q) a = inner(sigma, tau)*dx L = inner(f,tau)*dx sigma = Function(Q) solve(a == L, sigma) def boundary(x, on_boundary): return on_boundary bc = DirichletBC(V, Constant((0.0, 0.0)), boundary) u = TrialFunction(V) v = TestFunction(V) au = inner(grad(u), grad(v))*dx Lu = inner(sigma,grad(v))*dx u = Function(V) solve(au == Lu, u, bc)