\begin{definition}[Definition:Differential of Mapping/Real-Valued Function/Point]
Let $U \subset \R^n$ be an open set.
Let $f: U \to \R$ be a real-valued function.
Let $f$ be differentiable at a point $x \in U$.
:$\ds \map {\d f} {x; h} := \sum_{i \mathop = 1}^n \frac {\map {\partial f} x} {\partial x_i} h_i = \frac {\map {\partial f} x} {\partial x_1} h_1 + \frac {\map {\partial f} x} {\partial x_2} h_2 + \cdots + \frac {\map {\partial f} x} {\partial x_n} h_n$
where:
:$h = \tuple {h_1, h_2, \ldots, h_n} \in \R^n$
:$\dfrac {\partial f} {\partial x_i}$ is the partial derivative of $f$ {{WRT|Differentiation}} $x_i$.
Category:Definitions/Differentials
\end{definition}