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

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Let $U$ be an open subset of $\R^n$.

Let $\norm \cdot $ denote the Euclidean norm on $\R^n$.

Let $f: U \to \R$ be a real-valued function.

Let $x \in U$.

$f$ is differentiable at $x$ if and only if there exist $\alpha_1, \ldots, \alpha_n \in \R$ and a real-valued function $r: U \setminus \set x \to \R$ such that:

$(1):\quad \map f {x + h} = \map f x + \alpha_1 h_1 + \cdots + \alpha_n h_n + \map r h \norm h$
$(2):\quad \ds \lim_{h \mathop \to 0} \map r h = 0$