For a function f, a local maximum (resp. minimum) is a point c that has a neighbourhood at every point of which f(x) ≤ f(c) (resp. f(x) ≥ f(c)). If f is differentiable at an interior local maximum or minimum c, then f′(c) = 0; that is, an interior local maximum or minimum is a stationary point. This is Fermat’s Theorem.