A function is a relation that uniquely associates members of one set with members of another set. More formally, a function from A to B is an object f such that every a in A is uniquely associated with an object f(a) in B. A function is therefore a many-to-one (or sometimes one-to-one) relation. The set A of values at which a function is defined is called its domain, while the set f(A) subset B of values that the function can produce is called its range. Here, the set B is called the codomain of f.

In the context of univariate, real-valued functions f:A subset R->R, the fact that domain elements are mapped to unique range elements can be expressed graphically by way of the vertical line test.

In some literature, the term "map" is synonymous with function. Some caution must be exhibited, however, as it is not uncommon for the term map to denote a function with some sort of unspoken regularity assumption, e.g., in point-set topology, where "map" sometimes refers to a function which is continuous with respect to some topology.


Examples of functions over the reals R include sinx (many-to-one), x (one-to-one), x^2 (two-to-one except for the single point x=0), etc.

Unfortunately, the term "function" is also used to refer to relations that map single points in the domain to possibly multiple points in the range. These "functions" are called multivalued functions (or multiple-valued functions), and arise prominently in the theory of complex functions, where the presence of multiple values engenders the use of so-called branch cuts.

Several notations are commonly used to represent (non-multivalued) functions. The most rigorous notation is f:x->f(x), which specifies that f is function acting upon a single number x (i.e., f is a univariate, or one-variable, function) and returning a value f(x). To be even more precise, a notation like "f:R->R, where f(x)=x^2" is sometimes used to explicitly specify the domain and codomain of the function. The slightly different "maps to" notation f:x|->f(x) is sometimes also used when the function is explicitly considered as a "map."

Generally speaking, the symbol f refers to the function itself, while f(x) refers to the value taken by the function when evaluated at a point x. However, especially in more introductory texts, the notation f(x) is commonly used to refer to the function f itself (as opposed to the value of the function evaluated at x). In this context, the argument x is considered to be a dummy variable whose presence indicates that the function f takes a single argument (as opposed to f(x,y), etc.). While this notation is deprecated by professional mathematicians, it is the more familiar one for most nonprofessionals. Therefore, unless indicated otherwise by context, the notation f(x) is taken in this work to be a shorthand for the more rigorous f:x->f(x).

See also

Complex Function, Map, Multivalued Function, Pathological, Real Function, Single-Valued Function, Special Function Explore this topic in the MathWorld classroom

Portions of this entry contributed by Christopher Stover

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Abramowitz, M. and Stegun, I. A. (Eds.). "Miscellaneous Functions." Ch. 27 in Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, 9th printing. New York: Dover, pp. 997-1010, 1972.Arfken, G. "Special Functions." Ch. 13 in Mathematical Methods for Physicists, 3rd ed. Orlando, FL: Academic Press, pp. 712-759, 1985.Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; and Vetterling, W. T. "Special Functions." Ch. 6 in Numerical Recipes in FORTRAN: The Art of Scientific Computing, 2nd ed. Cambridge, England: Cambridge University Press, pp. 205-265, 1992.Weisstein, E. W. "Books about Special Functions."

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Cite this as:

Stover, Christopher and Weisstein, Eric W. "Function." From MathWorld--A Wolfram Web Resource.

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