Domain of a function

In mathematics, the domain of a function is the set of inputs accepted by the function. It is sometimes denoted by $\operatorname {dom} (f)$ or $\operatorname {dom} f$ , where $f$ is the function.

A function

f

from

X

to

Y

. The set of points in the red oval

X

is the domain of

f

.

Graph of the real-valued square root function, f(x) = √x, whose domain consists of all nonnegative real numbers

More precisely, given a function $f\colon X\to Y$ , the domain of $f$ is $X$ . Note that in modern mathematical language, the domain is part of the definition of a function rather than a property of it.

In the special case that $X$ and $Y$ are both subsets of $\mathbb {R}$ , the function $f$ can be graphed in the Cartesian coordinate system. In this case, the domain is represented on the $x$ -axis of the graph, as the projection of the graph of the function onto the $x$ -axis.

For a function $f\colon X\to Y$ , the set $Y$ is called the codomain, and the set of values attained by the function (which is a subset of $Y$ ) is called its range or image.

Any function can be restricted to a subset of its domain. The restriction of $f\colon X\to Y$ to $A$ , where $A\subseteq X$ , is written as $\left.f\right|_{A}\colon A\to Y$ .

Natural domain

If a real function $f$ is given by a formula, it may be not defined for some values of the variable. In this case, it is a partial function, and the set of real numbers on which the formula can be evaluated to a real number is called the natural domain or domain of definition of $f$ . In many contexts, a partial function is called simply a function, and its natural domain is called simply its domain.

Examples

The function $f$ defined by $f(x)={\frac {1}{x}}$ cannot be evaluated at 0. Therefore the natural domain of $f$ is the set of real numbers excluding 0, which can be denoted by $\mathbb {R} \setminus \{0\}$ or $\{x\in \mathbb {R} :x\neq 0\}$ .
The piecewise function $f$ defined by $f(x)={\begin{cases}1/x&x\not =0\\0&x=0\end{cases}},$ has as its natural domain the set $\mathbb {R}$ of real numbers.
The square root function $f(x)={\sqrt {x}}$ has as its natural domain the set of non-negative real numbers, which can be denoted by $\mathbb {R} _{\geq 0}$ , the interval $[0,\infty )$ , or $\{x\in \mathbb {R} :x\geq 0\}$ .
The tangent function, denoted $\tan$ , has as its natural domain the set of all real numbers which are not of the form ${\tfrac {\pi }{2}}+k\pi$ for some integer $k$ , which can be written as $\mathbb {R} \setminus \{{\tfrac {\pi }{2}}+k\pi :k\in \mathbb {Z} \}$ .

Other uses

The word "domain" is used with other related meanings in some areas of mathematics. In topology, a domain is a connected open set.[1] In real and complex analysis, a domain is an open connected subset of a real or complex vector space. In the study of partial differential equations, a domain is the open connected subset of the Euclidean space $\mathbb {R} ^{n}$ where a problem is posed (i.e., where the unknown function(s) are defined).

Set theoretical notions

For example, it is sometimes convenient in set theory to permit the domain of a function to be a proper class $X$ , in which case there is formally no such thing as a triple $(X, Y, G)$ . With such a definition, functions do not have a domain, although some authors still use it informally after introducing a function in the form $f : X \to Y$ .[2]

Notes

Weisstein, Eric W. "Domain". mathworld.wolfram.com. Retrieved 2020-08-28.
Eccles 1997, p. 91 (quote 1, quote 2); Mac Lane 1998, p. 8; Mac Lane, in Scott & Jech 1967, p. 232; Sharma 2004, p. 91; Stewart & Tall 1977, p. 89

References

Bourbaki, Nicolas (1970). Théorie des ensembles. Éléments de mathématique. Springer. ISBN 9783540340348.

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[1] Weisstein, Eric W. "Domain". mathworld.wolfram.com. Retrieved 2020-08-28.

[2] Eccles 1997, p. 91 (quote 1, quote 2); Mac Lane 1998, p. 8; Mac Lane, in Scott & Jech 1967, p. 232; Sharma 2004, p. 91; Stewart & Tall 1977, p. 89