Definition:Infimum
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Definition
Ordered Set
Let $\left({S, \preceq}\right)$ be a poset.
Let $T \subseteq S$.
An element $c \in S$ is the infimum of $T$ in $S$ if:
- $(1): \quad c$ is a lower bound of $T$ in $S$
- $(2): \quad d \preceq c$ for all lower bounds $d$ of $T$ in $S$.
The infimum of $T$ is denoted $\inf \left({T}\right)$.
The infimum of $x_1, x_2, \ldots, x_n$ is denoted $\inf \left\{{x_1, x_2, \ldots, x_n}\right\}$.
If there exists an infimum of $T$ (in $S$), we say that $T$ admits an infimum (in $S$).
The infimum of $T$ is often called the greatest lower bound of $T$ and denoted $\operatorname{glb} \left({T}\right)$.
Mapping
Let $f$ be a mapping defined on a subset of the real numbers $S \subseteq \R$.
Let $f$ be bounded below on $S$.
It follows from the Continuum Property that the codomain of $f$ has an infimum on $S$.
Thus:
- $\displaystyle \inf_{x \in S} f \left({x}\right) = \inf f \left({S}\right)$
Also defined as
Some sources refer to the infimum as being the lower bound. Using this convention, any element greater 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.
