Domain of a function

Summary

In mathematics, the domain of a function is the set of inputs accepted by the function. It is sometimes denoted by or , where f is the function. In layman's terms, the domain of a function can generally be thought of as "what x can be".[1]

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 , the domain of f is X. 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 sets of real numbers, 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 , 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 to , where , is written as .

Natural domain edit

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 edit

  • The function   defined by   cannot be evaluated at 0. Therefore, the natural domain of   is the set of real numbers excluding 0, which can be denoted by   or  .
  • The piecewise function   defined by   has as its natural domain the set   of real numbers.
  • The square root function   has as its natural domain the set of non-negative real numbers, which can be denoted by  , the interval  , or  .
  • The tangent function, denoted  , has as its natural domain the set of all real numbers which are not of the form   for some integer  , which can be written as  .

Other uses edit

The term domain is also commonly used in a different sense in mathematical analysis: a domain is a non-empty connected open set in a topological space. In particular, in real and complex analysis, a domain is a non-empty connected open subset of the real coordinate space   or the complex coordinate space  

Sometimes such a domain is used as the domain of a function, although functions may be defined on more general sets. The two concepts are sometimes conflated as in, for example, the study of partial differential equations: in that case, a domain is the open connected subset of   where a problem is posed, making it both an analysis-style domain and also the domain of the unknown function(s) sought.

Set theoretical notions edit

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: XY.[2]

See also edit

Notes edit

  1. ^ "Domain, Range, Inverse of Functions". Easy Sevens Education. Retrieved 2023-04-13.
  2. ^ Eccles 1997, p. 91 (quote 1, quote 2); Mac Lane 1998, p. 8; Mac Lane, in Scott & Jech 1971, p. 232; Sharma 2010, p. 91; Stewart & Tall 1977, p. 89

References edit

  • Bourbaki, Nicolas (1970). Théorie des ensembles. Éléments de mathématique. Springer. ISBN 9783540340348.
  • Eccles, Peter J. (11 December 1997). An Introduction to Mathematical Reasoning: Numbers, Sets and Functions. Cambridge University Press. ISBN 978-0-521-59718-0.
  • Mac Lane, Saunders (25 September 1998). Categories for the Working Mathematician. Springer Science & Business Media. ISBN 978-0-387-98403-2.
  • Scott, Dana S.; Jech, Thomas J. (31 December 1971). Axiomatic Set Theory, Part 1. American Mathematical Soc. ISBN 978-0-8218-0245-8.
  • Sharma, A. K. (2010). Introduction To Set Theory. Discovery Publishing House. ISBN 978-81-7141-877-0.
  • Stewart, Ian; Tall, David (1977). The Foundations of Mathematics. Oxford University Press. ISBN 978-0-19-853165-4.