Ideals form Complete Lattice
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Theorem
Let $L = \struct {S, \vee, \preceq}$ be a bounded below join semilattice.
Let $\II = \struct {\map {\operatorname{Ids} } L, \subseteq}$ be an inclusion ordered set,
where $\map {\operatorname{Ids} } L$ denotes the set of all ideals in $L$.
Then $\II$ is complete lattice.
Proof
Let $X \subseteq \map {\operatorname{Ids} } L$.
By Intersection of Semilattice Ideals is Ideal/Set of Sets:
- $\ds \bigcap X$ is an ideal.
By Intersection is Largest Subset/General Result:
- $\ds \forall A \in \map {\operatorname{Ids} } L: \paren {\forall I \in X: A \subseteq I} \iff A \subseteq \bigcap X$
Thus by definition:
- $X$ admits an infimum.
Thus by dual to Lattice is Complete iff it Admits All Suprema:
- $\II$ is complete lattice.
$\blacksquare$
Sources
- Mizar article YELLOW_2:48