# Definition:Derivative/Real Function

## Contents

## Definition

### At a Point

Let $I$ be an open real interval.

Let $f: I \to \R$ be a real function defined on $I$.

Let $\xi \in I$ be a point in $I$.

Let $f$ be differentiable at the point $\xi$.

#### Definition 1

That is, suppose the limit $\displaystyle \lim_{x \mathop \to \xi} \frac {f \left({x}\right) - f \left({\xi}\right)} {x - \xi}$ exists.

Then this limit is called the **derivative of $f$ at the point $\xi$**.

#### Definition 2

That is, suppose the limit $\displaystyle \lim_{h \mathop \to 0} \frac {f \left({\xi + h}\right) - f \left({\xi}\right)} h$ exists.

Then this limit is called the **derivative of $f$ at the point $\xi$**.

### On an Open Interval

Let $I\subset\R$ be an open interval.

Let $f : I \to \R$ be a real function.

Let $f$ be differentiable on the interval $I$.

Then the **derivative of $f$** is the real function $f': I \to \R$ whose value at each point $x \in I$ is the derivative $f' \left({x}\right)$:

- $\displaystyle \forall x \in I: f' \left({x}\right) := \lim_{h \mathop \to 0} \frac {f \left({x + h}\right) - f \left({x}\right)} h$

### With Respect To

Let $f$ be a real function which is differentiable on an open interval $I$.

Let $f$ be defined as an equation: $y = \map f x$.

Then the **derivative of $y$ with respect to $x$** is defined as:

- $\displaystyle y^\prime = \lim_{h \mathop \to 0} \frac {\map f {x + h} - \map f x} h = D_x \, \map f x$

This is frequently abbreviated as **derivative of $y$** **WRT** or **w.r.t.** **$x$**, and often pronounced something like **wurt**.

We introduce the quantity $\delta y = \map f {x + \delta x} - \map f x$.

This is often referred to as **the small change in $y$ consequent on the small change in $x$**.

Hence the motivation behind the popular and commonly-seen notation:

- $\displaystyle \dfrac {\d y} {\d x} := \lim_{\delta x \mathop \to 0} \dfrac {\map f {x + \delta x} - \map f x} {\delta x} = \lim_{\delta x \mathop \to 0} \dfrac {\delta y} {\delta x}$

Hence the notation $\map {f^\prime} x = \dfrac {\d y} {\d x}$.

This notation is useful and powerful, and emphasizes the concept of a derivative as being the limit of a ratio of very small changes.

However, it has the disadvantage that the variable $x$ is used ambiguously: both as the point at which the derivative is calculated and as the variable with respect to which the derivation is done.

For practical applications, however, this is not usually a problem.

## Notation

There are various notations available to be used for the derivative of a function $f$ with respect to the independent variable $x$:

- $\dfrac {\d f} {\d x}$

- $\dfrac \d {\d x} \left({f}\right)$

- $\dfrac {\d y} {\d x}$ when $y = f \left({x}\right)$

- $f' \left({x}\right)$

- $D f \left({x}\right)$

- $D_x f \left({x}\right)$

When evaluated at the point $\left({x_0, y_0}\right)$, the derivative of $f$ at the point $x_0$ can be variously denoted:

- $f' \left({x_0}\right)$

- $D f \left({x_0}\right)$

- $D_x f \left({x_0}\right)$

- $\dfrac {\d f} {\d x} \left({x_0}\right)$

and so on.

## Sources

- 1968: Murray R. Spiegel:
*Mathematical Handbook of Formulas and Tables*... (previous) ... (next): $\S 13$: Definition of a Derivative: $13.1$ - 2008: Ian Stewart:
*Taming the Infinite*... (previous) ... (next): Chapter $8$: The System of the World: Calculus