Definition:Differentiable Mapping/Real Function/Point/Definition 2
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Definition
Let $f$ be a real function defined on an open interval $\openint a b$.
Let $\xi$ be a point in $\openint a b$.
$f$ is differentiable at the point $\xi$ if and only if the limit:
- $\ds \lim_{h \mathop \to 0} \frac {\map f {\xi + h} - \map f \xi} h$
exists.