# Domain of a function

__: Mathematical concept__

**Short description**In mathematics, the **domain** of a function is the set of inputs accepted by the function. It is sometimes denoted by [math]\displaystyle{ \operatorname{dom}(f) }[/math], where *f* is the function.

More precisely, given a function [math]\displaystyle{ f\colon X\to Y }[/math], 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 [math]\displaystyle{ \R }[/math], 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 [math]\displaystyle{ f\colon X\to Y }[/math], 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 [math]\displaystyle{ f \colon X \to Y }[/math] to [math]\displaystyle{ A }[/math], where [math]\displaystyle{ A\subseteq X }[/math], is written as [math]\displaystyle{ \left. f \right|_A \colon A \to Y }[/math].

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

## 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 [math]\displaystyle{ \mathbb{R}^{n} }[/math] 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* → *Y*.^{[2]}

## See also

- Attribute domain
- Bijection, injection and surjection
- Codomain
- Domain decomposition
- Effective domain
- Image (mathematics)
- Lipschitz domain
- Naive set theory
- Support (mathematics)

## Notes

- ↑ Weisstein, Eric W.. "Domain" (in en). https://mathworld.wolfram.com/Domain.html.
- ↑ 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.

Original source: https://en.wikipedia.org/wiki/Domain of a function.
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