 The maximum theorem of Claude Berge (1963) describes the continuity of an optimal solution as a function of underlying parameters.For unconstrained problems with twice-differentiable functions, some critical points can be found by finding the points where the gradient of the objective function is zero (that is, the stationary points).More generally, a zero subgradient certifies that a local minimum has been found for minimization problems with convex functions and other locally Lipschitz functions.