Definition:Differentiable Mapping/Real Function/Point/Definition 1

Definition
Let $f$ be a real function defined on an open interval $\left({a \,.\,.\, b}\right)$.

Let $\xi$ be a point in $\left({a \,.\,.\, b}\right)$.

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

exists.