Definition:Pointwise Supremum
Definition
Let $S$ be a set.
Let $\struct {T, \preceq}$ be an ordered set.
Let $\family {f_i}_{i \mathop \in I}, f_i: S \to T$ be an $I$-indexed collection of mappings.
Suppose that for all $s \in S$, it holds that:
- $\ds \sup_{i \mathop \in I} \map {f_i} s \in T$
where the supremum is taken in $T$.
Then the pointwise supremum of $\family {f_i}_{i \mathop \in I}$, denoted $\ds \sup_{i \mathop \in I} f_i: S \to T$, is defined by:
- $\ds \map {\paren {\sup_{i \mathop \in I} f_i} } s := \sup_{i \mathop \in I} \map {f_i} s$
where the latter supremum is again taken in $T$.
By assumption, this supremum is guaranteed to exist.
Thence it can be seen that pointwise supremum is an instance of a pointwise operation.
Also known as
Because of the way $\ds \sup_{i \mathop \in I} f_i$ is defined, there is usually no need to distinguish between the left hand side and right hand side of the definition.
Thus $\ds \map {\sup_{i \mathop \in I} f_i} s$ is commonly used instead of $\ds \map {\paren {\sup_{i \mathop \in I} f_i} } s$.
Also defined as
Sometimes the imposition that all suprema exist in $T$ is considered too strong.
In these cases, some suitable extension of $\preceq$ to a suitable ordered set $\overline T$ may be created, in which the suprema do exist.
This $\overline T$ is then set to be the codomain of the pointwise supremum.
For example, this is done on Pointwise Supremum of Real-Valued Functions.
Examples
- Definition:Pointwise Supremum of Real-Valued Functions, taking $T$ to be $\R$
- Definition:Pointwise Supremum of Extended Real-Valued Functions, taking $T$ to be the extended real numbers $\overline \R$