A combination of the function and the flux at the boundary. For example, a function φ‎ might satisfy Laplace’s equation inside a region and the Robin condition cφ‎ + ∂φ‎/∂n = f on the boundary where c is a constant and f a specified function; here ∂φ‎/∂n denotes the directional derivative of φ‎ in the direction of the outward pointing normal. The existence and uniqueness of any solution depends on the sign of c. Compare Dirichlet problem, Neumann condition.