Each term must be a valid term in a weak equation, i.e., it must contains exactly one test function.
For example, the weak form of Laplace's equation
on the interior of a domain with Robin boundary condition
on an inflow surface, is written as
// create first and second order Gaussian quadrature objects QuadratureFamily quad1 = new GaussianQuadrature(1); QuadratureFamily quad2 = new GaussianQuadrature(2); // write the weak form. The interior term can use order 1 quadrature, // but the surface term involves quadratic functions and must use // order 2 quadrature Expr eqn = Integral(interior, (grad*phiHat)*(grad*phi), quad1) + Integral(in, phiHat*(x-phi)/L, quad2) ;
Dirichlet boundary conditions are not usually written in terms of Integral operators; see the page on boundary conditions for more information on that subject.