# Local boundedness

In mathematics, a function is locally bounded if it is bounded around every point. A family of functions is locally bounded if for any point in their domain all the functions are bounded around that point and by the same number.

## Locally bounded function

A real-valued or complex-valued function f defined on some topological space X is called locally bounded if for any x0 in X there exists a neighborhood A of x0 such that f (A) is a bounded set, that is, for some number M>0 one has

for all x in A.

That is to say, for each x one can find a constant, depending on x, which is larger than all the values of the function in the neighborhood of x. Compare this with a bounded function, for which the constant does not depend on x. Obviously, if a function is bounded then it is locally bounded. The converse is not true in general.

This definition can be extended to the case when f takes values in some metric space. Then the inequality above needs to be replaced with

for all x in A, where d is the distance function in the metric space, and a is some point in the metric space. The choice of a does not affect the definition. Choosing a different a will at most increase the constant M for which this inequality is true.

## Examples

• The function f: RR defined by

is bounded, because 0≤ f (x) ≤ 1 for all x. Therefore, it is also locally bounded.

• The function f: RR defined by

is not bounded, as it becomes arbitrarily large. However, it is locally bounded because for each a, |f(x)| ≤ M in the neighborhood (a - 1,a + 1), where M = 2|a|+3.

• The function f:RR defined by

for x ≠ 0 and taking the value 0 for x=0 is not locally bounded. In any neighborhood of 0 this function takes values of arbitrarily large magnitude.

## Locally bounded family

A set (also called a family) U of real-valued or complex-valued functions defined on some topological space X is called locally bounded if for any x0 in X there exists a neighborhood A of x0 and a positive number M such that

for all x in A and f in U. In other words, all the functions in the family must be locally bounded, and around each point they need to be bounded by the same constant.

This definition can also be extended to the case when the functions in the family U take values in some metric space, by again replacing the absolute value with the distance function.

## Examples

• The family of functions fn:RR

where n = 1, 2, ... is locally bounded. Indeed, if x0 is a real number, one can choose the neighborhood A to be the interval (x0-1, x0+1). Then for all x in this interval and for all n≥1 one has

with M=|x0|+1. Moreover, the family is not uniformly bounded, because there is no way that both the neighborhood A and the constant M can be chosen so that they are independent of the point x0.

• The family of functions fn:RR

is locally bounded, if n is greater than zero. For any x0 one can choose the neighborhood A to be R itself. Then we have

with M=1. Note that the value of M does not depend on the choice of x0 or its neighborhood A. This family is then not only locally bounded, it is also uniformly bounded.

• The family of functions fn:RR

is not locally bounded. Indeed, for any x0 the values fn(x0) cannot be bounded as n tends toward infinity.

## Topological vector spaces

Local boundedness may also refer to a property of topological vector spaces, or of functions from a topological space into a topological vector space.

### Locally bounded topological vector spaces

Let X be a topological vector space. Then a subset BX is bounded if for each neighborhood U of 0 in X there exists a number s > 0 such that

B tU for all t > s.

A topological vector space is said to be locally bounded if X admits a bounded neighborhood of 0.

### Locally bounded functions

Let X be a topological space, Y a topological vector space, and f : XY a function. Then f is locally bounded if each point of X has a neighborhood whose image under f is bounded.

The following theorem relates local boundedness of functions with the local boundedness of topological vector spaces:

Theorem. A topological vector space X is locally bounded if and only if the identity mapping 1 : X X is locally bounded.