# Example: Deduction of the Weak Form

While partial differential equations are typically posed in the strong form, e.g.

the finite element method is based on the weak form, which is under the assumption of homogeneous Neumann boundary conditions in the above case given by

where is a suitable test space. By an extension of the functionality in ViennaMath, ViennaFEM can also deduce the weak form automatically:

Deducing the weak form out of the strong form

using namespace viennamath; function_symbol u; // instantiate the unknown // instantiate Poisson equation with f=1: equation poisson_eq = make_equation(laplace(u), -1.0); std::cout << "Strong form: " << poisson_eq << std::endl; std::cout << "Weak form: " << viennafem::make_weak_form(poisson_eq) << std::endl; // Inhomogeneous permittivity: div( eps * grad(u)) = -1.0: viennafem::cell_quan<CellType> eps; eps.wrap_constant( my_key() ); equation poisson_eq2 = make_equation( div( eps * grad(u)), -1); std::cout << "Strong form: " << poisson_eq2 << std::endl; std::cout << "Weak form: " << viennafem::make_weak_form(poisson_eq2) << std::endl;

The second form of the Poisson equation includes a diffusion coefficient 'eps', which is constant within each ViennaGrid cell (of type 'CellType'), but may vary among the cells inside the mesh.