# Upper and lower bounds

2010 Mathematics Subject Classification: Primary: 26A03 Secondary: 06A [MSN][ZBL]

Characteristics of sets on the real line. The least upper bound of a given set of real numbers is the smallest number bounding this set from above; its greatest lower bound is the largest number bounding it from below. This will now be restated in more detail. Let there be given a subset $X$ of the real numbers. A number $\beta$ is said to be its least upper bound, denoted by $\sup X$ (from the Latin "supremum" — largest), if every number $x\in X$ satisfies the inequality $x\leq\beta$, and if for any $\beta'<\beta$ there exists an $x'\in X$ such that $x'>\beta'$. A number $\alpha$ is said to be the greatest lower bound of $X$, denoted by $\inf X$ (from the Latin "infimum" — smallest), if every $x\in X$ satisfies the inequality $x\geq\alpha$, and if for any $\alpha'>\alpha$ there exists an $x'\in X$ such that $x'<\alpha'$.

Examples.

$$\inf[a,b]=a,\quad\sup[a,b]=b;$$

$$\inf(a,b)=a,\quad\sup(a,b)=b;$$

if the set $X$ consists of two points $a$ and $b$, $a<b$, then

$$\inf X=a,\quad\sup X=b.$$

These examples show, in particular, that the least upper bound (greatest lower bound) may either belong to the set (e.g. in the case of the interval $[a,b]$) or not belong to it (e.g. in the case of the interval $(a,b)$). If a set has a largest (smallest) member, this number will clearly be the least upper bound (greatest lower bound) of the set.

The least upper bound (greatest lower bound) of a set not bounded from above (from below) is denoted by the symbol $+\infty$ (respectively, by the symbol $-\infty$). If $\mathbf N=\{1,2,\dots\}$ is the set of natural numbers, then

$$\inf\mathbf N=1,\quad\sup\mathbf N=+\infty.$$

If $\mathbf Z$ is the set of all integers, both positive and negative, then

$$\inf\mathbf Z=-\infty,\quad\sup\mathbf Z=+\infty.$$

Each non-empty set of real numbers has a unique least upper bound (greatest lower bound), finite or infinite. All non-empty sets bounded from above have finite least upper bounds, while all those bounded from below have finite greatest lower bounds.

The terms least upper (greatest lower) limit of a set are also sometimes used instead of the least upper bound (greatest lower bound) of a set, in one of the senses defined above. By the least upper bound (greatest lower bound) of a real-valued function, in particular of a sequence of real numbers, one means the least upper bound (greatest lower bound) of the set of its values (cf. also Upper and lower limits).

How to Cite This Entry:
Upper and lower bounds. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Upper_and_lower_bounds&oldid=36143
This article was adapted from an original article by L.D. Kudryavtsev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article