Definition:Differential of Mapping/Real-Valued Function/Point

Definition
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 $x \in U$.


 * $\displaystyle \rd f \left({x; h}\right) := \sum_{i \mathop = 1}^n \frac {\partial f \left({x}\right)} {\partial x_i} h_i = \frac {\partial f \left({x}\right)} {\partial x_1} h_1 + \frac {\partial f \left({x}\right)} {\partial x_2} h_2 + \cdots + \frac {\partial f \left({x}\right)} {\partial x_n} h_n$

where:
 * $h = \left({h_1, h_2, \ldots, h_n}\right) \in \R^n$
 * $\dfrac {\partial f} {\partial x_i}$ is the partial derivative of $f$ $x_i$.