# Complex-valued function

In mathematics, a **complex-valued function** (not to be confused with **complex variable function**) is a function whose values are complex numbers. Its domain does not necessarily have any structure related to complex numbers. Most important uses of such functions in complex analysis and in functional analysis are explained below.

A vector space and a commutative algebra of functions over complex numbers can be defined in the same way as for real-valued functions. Also, any complex-valued function f on an arbitrary set X can be considered as an ordered pair of two real-valued functions: (Re*f*, Im*f*) or, alternatively, as a real-valued function φ on *X* × {0, 1} (the disjoint union of two copies of X) such that for any x:

- Re
*f*(*x*) =*F*(*x*, 0) - Im
*f*(*x*) =*F*(*x*, 1)

Some properties of complex-valued functions (such as measurability and continuity) are nothing more than corresponding properties of real-valued functions.

## Complex analysis

Complex analysis considers holomorphic functions on complex manifolds, such as Riemann surfaces. The property of analytic continuation makes them very dissimilar from smooth functions, for example. Namely, if a function defined in a neighborhood can be continued to a wider domain, then this continuation is unique.

As real functions, any holomorphic function is infinitely smooth and analytic. But there is much less freedom in construction of a holomorphic function than in one of a smooth function.

## Functional analysis

Complex-valued L^{2} spaces on sets with a measure have a particular importance because they are Hilbert spaces. They often appear in functional analysis (for example, in relation with Fourier transform) and operator theory. A major user of such spaces is quantum mechanics, as wave functions.

The sets on which the complex-valued L^{2} is constructed have the potential to be more exotic than their real-valued analog. For example, complex-valued function spaces are used in some branches of p-adic analysis for algebraic reasons: complex numbers form an algebraically closed field (which facilitates operator theory), whereas neither real numbers nor p-adic numbers are not.

Also, complex-valued continuous functions are an important example in the theory of C*-algebras: see Gelfand representation.

## See also

- Function of a complex variable, the dual concept

## External links

Weisstein, Eric W.. "Complex Function". http://mathworld.wolfram.com/ComplexFunction.html.