# Definition:Supremum of Mapping/Real-Valued Function

This page is about suprema of real-valued functions. For other uses, see Definition:Supremum.

## Definition

Let $f: S \to \R$ be a real-valued function.

Let $f$ be bounded above on $S$.

### Definition 1

The supremum of $f$ on $S$ is defined by:

$\displaystyle \sup_{x \mathop \in S} f \left({x}\right) := \sup f \left[{S}\right]$

where

$\sup f \left[{S}\right]$ is the supremum in $\R$ of the image of $S$ under $f$.

### Definition 2

The supremum of $f$ on $S$ is defined as $\displaystyle \sup_{x \mathop \in S} \map f x := K \in \R$ such that:

$(1): \quad \forall x \in S: \map f x \le K$
$(2): \quad \exists x \in S: \forall \epsilon \in \R_{>0}: \map f x > K - \epsilon$

## Also known as

Particularly in the field of analysis, the supremum of a set $T$ is often referred to as the least upper bound of $T$ and denoted $\map {\operatorname {lub} } T$ or $\map {\operatorname {l.u.b.} } T$.

Some sources refer to the supremum of a set as the supremum on a set.

## Also defined as

Some sources refer to the supremum as being the upper bound.

Using this convention, any element greater than this is not considered to be an upper bound.

## Linguistic Note

The plural of supremum is suprema, although the (incorrect) form supremums can occasionally be found if you look hard enough.