Domain (mathematics)

In mathematics, the domain of a given function is the set of "input" values for which the function is defined. In a representation of a function in a xy Cartesian coordinate system, the domain is represented on the abscissa (the x axis).

Domain of a function
Given a function f:X→Y, the set X of input values is the domain of f; the set Y is the codomain of f.

The range of f is the set of all output values of f; this is the set $$\{ f(x) : x \in X \}$$. The range of f can be the same set as the codomain or it can be a proper subset of it. It is in general smaller than the codomain unless f is a surjective function.

A well defined function must map every element of its domain to an element of its codomain. For example, the function f defined by
 * f(x) = 1/x

has no value for f(0). Thus, the set of real numbers, $$\mathbb{R}$$, cannot be its domain. In cases like this, the function is either defined on $$\mathbb{R} \backslash \{0 \}$$ or the "gap is plugged" by explicitly defining f(0). If we extend the definition of f to
 * f(x) = 1/x, for x ≠ 0
 * f(0) = 0,

then f is defined for all real numbers, and its domain is $$\mathbb{R}$$.

Any function can be restricted to a subset of its domain. The restriction of g : A → B to S, where S ⊆ A, is written g |S : S → B.

Domain of a partial function
There are two distinct meanings in current mathematical usage for the notion of the domain of a partial function. Most mathematicians, including recursion theorists, use the term "domain of f" for the set of all values x such that f(x) is defined. But some, particularly category theorists, consider the domain of a partial function f:X→Y to be X, irrespective of whether f(x) exists for every x in X.

Category theory
In category theory one deals with morphisms instead of functions. Morphisms are arrows from one object to another. The domain of any morphism is the object from which an arrow starts. In this context, many set theoretic ideas about domains must be abandoned or at least formulated more abstractly. For example, the notion of restricting a morphism to a subset of its domain must be modified. See subobject for more.

Real and complex analysis
In real and complex analysis, a domain is an open connected subset of a real or complex vector space.