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| Title of object: |
function |
| Canonical Name: |
Function |
| Type: |
Definition |
| Created on: |
2001-10-19 00:47:07 |
| Modified on: |
2005-05-18 06:04:38 |
| Classification: |
msc:03E20 |
| Defines: |
domain, codomain, composition, image, range |
| Synonyms: |
function=map |
Revision comment (for changes between this and next version):
| Changes for correction #6571 ('image'). |
Preamble:
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{graphicx}
\usepackage{xypic} |
Content:
A \emph{function} is a triple $(f,A,B)$ where:
\begin{enumerate}
\item $A$ is a set (called the \emph{domain} of the function).
\item $B$ is a set (called the \emph{codomain} of the function).
\item $f$ is a relation from $A$ to $B$.
\item For every $a \in A$, there exists $b \in B$ such that $(a,b) \in f$.
\item If $a \in A$, $b_1,b_2 \in B$, and $(a,b_1) \in f$ and $(a,b_2) \in f$, then $b_1 = b_2$.
\end{enumerate}
The triple $(f,A,B)$ is usually written with the specialized notation $f\colon A \to B$. This notation visually conveys the fact that $f$ maps elements of $A$ into elements of $B$.
Other standard notations for functions are as follows:
\begin{itemize}
\item For $a \in A$, one denotes by $f(a)$ the unique element $b \in B$ such that $(a,b) \in f$.
\item The \emph{image} of $f$, denoted $f(A)$, is the set
$$
\{b \in B \mid f(a) = b \text{ for some } a \in A\}
$$
consisting of all elements of $B$ which equal $f(a)$ for some element $a \in A$.
\item In cases where the function $f$ is clear from context, the notation $a \mapsto b$ is equivalent to the statement $f(a) = b$.
\item Given two functions $f\colon A \to B$ and $g\colon B \to C$, there exists a unique function $g \circ f\colon A \to C$ satisfying the equation $g \circ f(a) = g(f(a))$. The function $g \circ f$ is called the \emph{composition} of $f$ and $g$.
\item When a function $f\colon A \to A$ has its domain equal to its codomain, one often writes $f^n$ for the $n$-fold composition
$$
\underbrace{f \circ f \circ \cdots \circ f}_{n\text{ times}}
$$
where $n$ is any natural number. Occasionally this can be confused with ordinary exponentiation (for example the function $x\mapsto (\sin x)(\sin x)$ is conventionally written as $\sin^2$); in such cases one usually writes $f^{[n]}$ to mean the $n$-fold composition.
\end{itemize}
There is no universal agreement as to the definition of the \emph{range} of a function. Some authors define the range of a function to be equal to the codomain, and others define the range of a function to be equal to the image. |
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