Uniform boundedness

In mathematics, bounded functions are functions for which there exists a lower bound and an upper bound, in other words, a constant which is larger than the absolute value of any value of this function. If we consider a family of bounded functions, this constant can vary between functions. If it is possible to find one constant which bounds all functions, this family of functions is uniformly bounded.

The uniform boundedness principle in functional analysis provides sufficient conditions for uniform boundedness of a family of operators.

Definition

Real line and complex plane

Let :$mathcal\; F=\{f\_i:\; X\; o\; Y,\; iin\; I\}$ be a family of functions with $Y$ being a set of real (or complex) numbers, then we call $mathcal\; F$ uniformly bounded if there exists a real number "c" such that:$|f\_i(x)|leq\; c\; qquad\; i\; in\; I\; quad\; x\; in\; X.$

Metric space

In general let $Y$ be a metric space with metric, then the set:$mathcal\; F=\{f\_i:\; X\; o\; Y,\; iin\; I\}$ is called uniformly bounded if there exists an element $ain\; Y$ and a real number "c" such that:$d(f\_i(x),\; a)\; leq\; c\; qquad\; i\; in\; I\; quad\; x\; in\; X.$

Examples

* The family of functions $f\_n(x)=sin\; nx,$ defined for real $x$ with $n$ traveling through the integers, is uniformly bounded by 1.

* The family of derivatives of the above family, $f\text{'}\_n(x)=n,\; cos\; nx,$ is "not" uniformly bounded. Each $f\text{'}\_n,$ is bounded by $|n|,,$ but there is no real number $M$ such that $|n|le\; M$ for all integers $n.$

References

*cite book
last = Ma
first = Tsoy-Wo
title = Banach-Hilbert spaces, vector measures, group representations
publisher = World Scientific
date = 2002
pages = 620pp
isbn = 9812380388, important to look up the site on its preface