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A cusp on the graph of a continuous function. At zero, the function is continuous but not differentiable. If f is differentiable at a point x 0, then f must also be continuous at x 0. In particular, any differentiable function must be continuous at every point in its domain. The converse does not hold: a
It states that if f is continuously differentiable, then around most points, the zero set of f looks like graphs of functions pasted together. The points where this is not true are determined by a condition on the derivative of f. The circle, for instance, can be pasted together from the graphs of the two functions ± √ 1 - x 2.
This function is continuous on the closed interval [−r, r] and differentiable in the open interval (−r, r), but not differentiable at the endpoints −r and r. Since f (− r ) = f ( r ) , Rolle's theorem applies, and indeed, there is a point where the derivative of f is zero.
In a similar vein, Milgrom and Segal's (2002) Theorem 3 implies that the value function must be differentiable at = and hence satisfy the envelope formula when the family {(,)} is equi-differentiable at (,) and ((),) is single-valued and continuous at =, even if the maximizer is not differentiable at (e.g., if is described by a set of ...
For functions of a single variable, the theorem states that if is a continuously differentiable function with nonzero derivative at the point ; then is injective (or bijective onto the image) in a neighborhood of , the inverse is continuously differentiable near = (), and the derivative of the inverse function at is the reciprocal of the derivative of at : ′ = ′ = ′ (()).
The definitions are applied to graphs as follows. If a function (a -cochain) is defined at the nodes of a graph: ,,, … then its exterior derivative (or the differential) is the difference, i.e., the following function defined on the edges of the graph (-cochain):
It allows the translation of various partial differential equations, e.g., the heat equation, to the graph setting. Based on the first-order partial difference operators on graphs one can formally derive a family of weighted graph -Laplace operators,: () for < by minimization of the discrete -Dirichlet energy functional
To be a C r-loop, the function γ must be r-times continuously differentiable and satisfy γ (k) (a) = γ (k) (b) for 0 ≤ k ≤ r. The parametric curve is simple if | (,): (,) is injective. It is analytic if each component function of γ is an analytic function, that is, it is of class C ω.