# Definition:Derivative/Function With Values in Normed Space

## Definition

Let $U \subset \R$ be an open set.

Let $\struct {X, \norm {\, \cdot \,}_X}$ be a normed vector space.

Let $f : U \to X$ be differentiable at $x \in U$.

The derivative of $f$ at $x$ is defined as the element $\map {f'} x \in X$ which satisfies:

$\ds \lim_{h \mathop \to 0} \norm {\frac {\map f {x + h} - \map f x} h - \map {f'} x}_X = 0$