Complement of Subset with Property (S) is Closed under Directed Suprema
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Theorem
Let $L = \struct {S, \preceq}$ be an up-complete ordered set.
Let $X$ be a subset of $S$ with property (S).
Then $\relcomp S X$ is closed under directed suprema.
Proof
Let $D$ be a directed subset of $S$ such that:
- $D \subseteq \relcomp S X$
Aiming for a contradiction, suppose
- $\sup D \notin \relcomp S X$
By definition of relative complement:
- $\sup D \in X$
By definition of property (S):
- $\exists y \in D: \forall x \in D: y \preceq x \implies x \in X$
By definition of reflexivity:
- $y \in X$
By definitions of intersection and non-empty:
- $D \cap X \ne \O$
Thus this by Empty Intersection iff Subset of Complement contradicts:
- $D \subseteq \relcomp S X$
$\blacksquare$
Sources
- Mizar article WAYBEL11:funcreg 1