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In mathematics, a **unary function** is a function that takes one argument. A unary operator belongs to a subset of unary functions, in that its codomain coincides with its domain. In contrast, a unary function's domain need not coincide with its range.

The successor function, denoted , is a unary operator. Its domain and codomain are the natural numbers; its definition is as follows:

In some programming languages such as C, executing this operation is denoted by postfixing `++`

to the operand, i.e. the use of `n++`

is equivalent to executing the assignment .

Many of the elementary functions are unary functions, including the trigonometric functions, logarithm with a specified base, exponentiation to a particular power or base, and hyperbolic functions.

- Foundations of Genetic Programming