Definition:Infimum of Mapping/Real-Valued Function

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

Definition

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

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


Definition 1

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

$\displaystyle \inf_{x \mathop \in S} f \left({x}\right) = \inf f \left[{S}\right]$

where

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


Definition 2

The infimum of $f$ on $S$ is defined as $\displaystyle \inf_{x \mathop \in S} f \left({x}\right) := k \in \R$ such that:

$(1): \quad \forall x \in S: k \le f \left({x}\right)$
$(2): \quad \forall \epsilon \in \R_{>0}: \exists x \in S: f \left({x}\right) < k + \epsilon$


Also known as

Particularly in the field of analysis, the infimum of a set $T$ is often referred to as the greatest lower bound of $T$ and denoted $\map {\operatorname {glb} } T$ or $\map {\operatorname {g.l.b.} } T$.

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


Also defined as

Some sources refer to the infimum as being the lower bound.

Using this convention, any element less than this is not considered to be a lower bound.


Linguistic Note

The plural of infimum is infima, although the (incorrect) form infimums can occasionally be found if you look hard enough.


Also see


Sources