# Differentiable function

In mathematics, a **differentiable function** of one real variable is a function whose derivative exists at each point in its domain. In other words, the graph of a differentiable function has a non-vertical tangent line at each interior point in its domain. A differentiable function is smooth (the function is locally well approximated as a linear function at each interior point) and does not contain any break, angle, or cusp.

A continuous function is not necessarily differentiable, but a differentiable function is necessarily continuous (at every point where it is differentiable) as being shown below (in the section Differentiability and continuity). A function is said to be *continuously differentiable* if its derivative is also a continuous function; there exists a function that is differentiable but not continuously differentiable as being shown below (in the section Differentiability classes).

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 continuous function need not be differentiable. For example, a function with a bend, cusp, or vertical tangent may be continuous, but fails to be differentiable at the location of the anomaly.

Most functions that occur in practice have derivatives at all points or at almost every point. However, a result of Stefan Banach states that the set of functions that have a derivative at some point is a meagre set in the space of all continuous functions.^{[1]} Informally, this means that differentiable functions are very atypical among continuous functions. The first known example of a function that is continuous everywhere but differentiable nowhere is the Weierstrass function.

A function of several real variables **f**: **R**^{m} → **R**^{n} is said to be differentiable at a point **x**_{0} if there exists a linear map **J**: **R**^{m} → **R**^{n} such that

If a function is differentiable at **x**_{0}, then all of the partial derivatives exist at **x**_{0}, and the linear map **J** is given by the Jacobian matrix. A similar formulation of the higher-dimensional derivative is provided by the fundamental increment lemma found in single-variable calculus.

If all the partial derivatives of a function exist in a neighborhood of a point **x**_{0} and are continuous at the point **x**_{0}, then the function is differentiable at that point **x**_{0}.

However, the existence of the partial derivatives (or even of all the directional derivatives) does not guarantee that a function is differentiable at a point. For example, the function *f*: **R**^{2} → **R** defined by

is not differentiable at (0, 0), but all of the partial derivatives and directional derivatives exist at this point. For a continuous example, the function

is not differentiable at (0, 0), but again all of the partial derivatives and directional derivatives exist.

Any function that is complex-differentiable in a neighborhood of a point is called holomorphic at that point. Such a function is necessarily infinitely differentiable, and in fact analytic.

If *M* is a differentiable manifold, a real or complex-valued function *f* on *M* is said to be differentiable at a point *p* if it is differentiable with respect to some (or any) coordinate chart defined around *p*. If *M* and *N* are differentiable manifolds, a function *f*: *M* → *N* is said to be differentiable at a point *p* if it is differentiable with respect to some (or any) coordinate charts defined around *p* and *f*(*p*).