Derivative

Definition at a point

Algebraic definition

Suppose $f$ is a function defined on a subset of the reals and $x_{0}$ is a point in the interior of the domain of $f$, i.e., the domain of $f$ contains an open interval surrounding $x_{0}$. The derivative (also called first derivative) of $f$ at $x_{0}$, denoted $f'(x_{0})$ is defined as the limit of the difference quotient of $f$ between $x_{0}$ and $x$, as $x\to x_{0}$. Explicitly:

$\!f'(x_{0}):=\lim _{x\to x_{0}}\Delta f(x,x_{0})=\lim _{x\to x_{0}}{\frac {f(x)-f(x_{0})}{x-x_{0}}}$

If this limit exists, then we say that the derivative exists and has this value, and we say that the function is differentiable at the point. If the limit does not exist, then we say that the function is not differentiable at the point and the derivative does not exist.

Computationally useful version of algebraic definition

This is obtained from the previous definition by the variable substitution $h:=x-x_{0}$ so $x=x_{0}+h$. Explicitly:

$\!f'(x_{0}):=\lim _{h\to 0}\Delta f(x_{0}+h,x_{0})=\lim _{h\to 0}{\frac {f(x_{0}+h)-f(x_{0})}{h}}$

Geometric definition

Suppose $f$ is a function and $x_{0}$ is a point in the interior of the domain of $f$, i.e., the domain of $f$ contains an open interval surrounding $x_{0}$. The derivative of $f$ at $x_{0}$ is the slope of the tangent line to the graph of $f$ through the point $(x_{0},f(x_{0}))$.

Definition as a function

Suppose $f$ is a function defined on a subset of the reals. Its derivative or first derivative, denoted $\!f'$, is a function defined as follows:

• The domain is the following subset of the domain of $\!f$: An element in the domain of $\!f$ is in the domain of $f'$ if and only if it is in the interior of the domain of $\!f$ and the derivative of $\!f$ exists at the point.
• The function value at any point in the domain is simply the value of the derivative of $\!f$ at that point.

MORE ON THE WAY THIS DEFINITION OR FACT IS PRESENTED: We first present the version that deals with a specific point (typically with a $\{\}_{0}$ subscript) in the domain of the relevant functions, and then discuss the version that deals with a point that is free to move in the domain, by dropping the subscript. Why do we do this?
The purpose of the specific point version is to emphasize that the point is fixed for the duration of the definition, i.e., it does not move around while we are defining the construct or applying the fact. However, the definition or fact applies not just for a single point but for all points satisfying certain criteria, and thus we can get further interesting perspectives on it by varying the point we are considering. This is the purpose of the second, generic point version.

One-sided notions

Left hand derivative

Suppose $f$ is a function defined at a point $x_{0}\in \mathbb {R}$ and also to the immediate left of $x_{0}$. The left hand derivative of $f$ at $x_{0}$ is defined as the left hand limit for the difference quotient between $x$ and $x_{0}$. In other words, it is:

$\operatorname {LHD} (f)(x_{0})=f'_{-}(x_{0}):=\lim _{x\to x_{0}^{-}}{\frac {f(x)-f(x_{0})}{x-x_{0}}}=\lim _{h\to 0^{-}}{\frac {f(x+h)-f(x)}{h}}$

Right hand derivative

Suppose $f$ is a function defined at a point $x_{0}\in \mathbb {R}$ and also to the immediate right of $x_{0}$. The right hand derivative of $f$ at $x_{0}$ is defined as the right hand limit for the difference quotient between $x$ and $x_{0}$. In other words, it is:

$\operatorname {RHD} (f)(x_{0})=f'_{+}(x_{0}):=\lim _{x\to x_{0}^{+}}{\frac {f(x)-f(x_{0})}{x-x_{0}}}=\lim _{h\to 0^{+}}{\frac {f(x+h)-f(x)}{h}}$

Relation between one-sided derivatives and the usual (two-sided) derivative

The derivative $f'(x_{0})$ exists if and only if (both the left hand derivative and the right hand derivative exist at $x_{0}$ and their values are equal). Further, the value of the derivative equals both these equals values.

Leibniz notation for derivative

The Leibniz notation for derivative views the derivative as the relative rate of change of two variables and is thus a somewhat different perspective on the derivative.

Suppose $f$ is a function, and $x,y$ are variables related by $y:=f(x)$. Here, $x$ is an independent variable and $y$ is the dependent variable (with the dependency being described by the function $f$). We then define:

${\frac {dy}{dx}}:=f'(x)$

In particular, $dy/dx$ is a function of $x$. Its value at $x=x_{0}$ is defined as $f'(x_{0})$ and is denoted as follows:

$\!{\frac {dy}{dx}}|_{x=x_{0}}:=f'(x_{0})$

Note that the $dy/dx$ notation does not mean that a number $dy$ is being divided by a number $dx$. One way of justifying this notation is by expressing it as a limit of a difference quotient:

$\!{\frac {dy}{dx}}|_{x=x_{0}}:=\lim _{x\to x_{0}}{\frac {y-y_{0}}{x-x_{0}}}=\lim _{x\to x_{0}}{\frac {\Delta y}{\Delta x}}$

where $\Delta y=y-y_{0}$ denotes the difference in $y$-values and $\Delta x=x-x_{0}$ denotes the difference in $x$-values. The difference quotient is actually a quotient of numbers, and the derivative is a limit of this. Hence, many of the formal manipulations involving fractions of numbers work with this notation, even though $dy/dx$ itself is not a quotient of numbers (see chain rule for differentiation and inverse function theorem).

Computation of derivative

For a full list, see Category:Differentiation rules.

Method for constructing new functions from old	In symbols	Derivative in terms of derivative of the old functions	Proof
pointwise sum	$f+g$ is the function $x\mapsto f(x)+g(x)$ $f_{1}+f_{2}+\dots +f_{n}$ is the function $x\mapsto f_{1}(x)+f_{2}(x)+\dots +f_{n}(x)$	Sum of the derivatives of the functions being added (the derivative of the sum is the sum of the derivatives) $\!f'+g'$ $\!f_{1}'+f_{2}'+\dots +f_{n}'$	differentiation is linear
pointwise difference	$f-g$ is the function $x\mapsto f(x)-g(x)$	Difference of the derivatives, i.e., $f'-g'$	differentiation is linear
scalar multiple by a constant	$af$ is the function $x\mapsto af(x)$ where $a$ is a real number	$x\mapsto af'(x)$	differentiation is linear
pointwise product	$f\cdot g$ (sometimes denoted $fg$) is the function $x\mapsto f(x)g(x)$ $f_{1}\cdot f_{2}\cdot \dots f_{n}$ (sometimes denoted $f_{1}f_{2}\dots f_{n}$ is the function $x\mapsto f_{1}(x)f_{2}(x)\dots f_{n}(x)$	For two functions, $x\mapsto f(x)g'(x)+f'(x)g(x)$ For multiple functions, $x\mapsto f_{1}'(x)f_{2}(x)\dots f_{n}(x)+f_{1}(x)f_{2}'(x)\dots f_{n}(x)+\dots +f_{1}(x)f_{2}(x)\dots f_{n}'(x)$	product rule for differentiation
pointwise quotient	$f/g$ is the function $x\mapsto f(x)/g(x)$	$x\mapsto {\frac {g(x)f'(x)-f(x)g'(x)}{(g(x))^{2}}}$	quotient rule for differentiation
composite of two functions	$f\circ g$ is the function $x\mapsto f(g(x))$	$x\mapsto f'(g(x))g'(x)$	chain rule for differentiation
inverse function of a one-one function	$f^{-1}$ sends $x$ to the unique $y$ such that $f(y)=x$	$\!{\frac {1}{f'(f^{-1}(x))}}$	inverse function theorem
piecewise definition	Fill this in later	Fill this in later	differentiation rule for piecewise definition by interval