# Bounded function

In mathematics, a function *f* defined on some set *X* with real or complex values is called **bounded** if the set of its values is bounded. In other words, there exists a real number *M* such that

for all *x* in *X*. A function that is *not* bounded is said to be **unbounded**.

If *f* is real-valued and *f*(*x*) ≤ *A* for all *x* in *X*, then the function is said to be **bounded (from) above** by *A*. If *f*(*x*) ≥ *B* for all *x* in *X*, then the function is said to be **bounded (from) below** by *B*. A real-valued function is bounded if and only if it is bounded from above and below.

An important special case is a **bounded sequence**, where *X* is taken to be the set **N** of natural numbers. Thus a sequence *f* = (*a*_{0}, *a*_{1}, *a*_{2}, ...) is bounded if there exists a real number *M* such that

for every natural number *n*. The set of all bounded sequences forms the sequence space .

The definition of boundedness can be **generalized** to functions *f : X → Y* taking values in a more general space *Y* by requiring that the image *f(X)* is a bounded set in *Y*.

## Related Notions

Weaker than boundedness is local boundedness. A family of bounded functions may be uniformly bounded.

A bounded operator *T : X → Y* is not a bounded function in the sense of this page's definition (unless *T = 0*), but has the weaker property of **preserving boundedness**: Bounded sets *M ⊆ X* are mapped to bounded sets *T(M) ⊆ Y.* This definition can be extended to any function *f* : *X* → *Y* if *X* and *Y* allow for the concept of a bounded set.

## Examples

- The function sin :
**R**→**R**is bounded. - The function defined for all real
*x*except for −1 and 1 is unbounded. As*x*approaches −1 or 1, the values of this function get larger and larger in magnitude. This function can be made bounded if one considers its domain to be, for example, [2, ∞) or (−∞, −2].

- The function defined for all real
*x**is*bounded. - The inverse trigonometric function arctangent defined as:
*y*= arctan(*x*) or*x*= tan(*y*) is increasing for all real numbers*x*and bounded with −π/2 <*y*< π/2 radians - Every continuous function
*f*: [0, 1] →**R**is bounded. More generally, any continuous function from a compact space into a metric space is bounded. - All complex-valued functions
*f*:**C**→**C**which are entire are either unbounded or constant as a consequence of Liouville's theorem. In particular, the complex sin :**C**→**C**must be unbounded since it's entire. - The function
*f*which takes the value 0 for*x*rational number and 1 for*x*irrational number (cf. Dirichlet function)*is*bounded. Thus, a function does not need to be "nice" in order to be bounded. The set of all bounded functions defined on [0, 1] is much bigger than the set of continuous functions on that interval.