Definition:Differentiable Mapping

At a Point
Let $f$ be a real function defined on an open interval $I$.

Let $\xi \in I$ be a point in $I$.

Then $f$ is differentiable at the point $\xi$ iff the limit:
 * $\displaystyle \lim_{x \to \xi} \frac {f \left({x}\right) - f \left({\xi}\right)} {x - \xi}$

exists.

This limit, if it exists, is called the derivative of $f$ at $\xi$.

On an Interval
Let $f$ be a real function defined on an open interval $I$.

Let $f$ be differentiable at each point of $I$.

Then $f$ is differentiable on $I$.

On the Real Number Line
In the definition of differentiable on an interval let that interval be the real number line $\R$.

Let $f$ be differentiable at each point of $\R$.

Then $f$ is differentiable everywhere (on $\R$).

In the Complex Plane
Let $f \left({z}\right): \C \to \C$ be a single-valued continuous complex function in a domain $D \subseteq \C$.

Let $z_0 \in D$ be a point in $D$.

Then $f \left({z}\right)$ is complex-differentiable at $z_0$ iff the limit:
 * $\displaystyle \lim_{h \to 0} \frac {f \left({z_0+h}\right) - f \left({z_0}\right)} h$

exists as a finite number and is independent of how the complex increment $h$ tends to $0$.

If such a limit exists, it is called the derivative of $f$ at $z_0$.

If $f \left({z}\right)$ is complex-differentiable at every point in $D$, it is differentiable in $D$. Such a function is called analytic.