# Power Set is Complete Lattice

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## Theorem

Let $S$ be a set.

Let $\struct {\powerset S, \subseteq}$ be the relational structure defined on $\powerset S$ by the relation $\subseteq$.

Then $\struct {\powerset S, \subseteq}$ is a complete lattice.

## Proof

From Subset Relation on Power Set is Partial Ordering, we have that $\subseteq$ is a partial ordering.

We note in passing that for any set $S$:

- From Supremum of Power Set, $\powerset S$ has a supremum, that is, $S$ itself
- From Infimum of Power Set, $\powerset S$ has an infimum, that is, $\O$.

These are also the maximal and minimal elements of $\powerset S$.

Let $\mathbb S$ be a subset of $\powerset S$.

Then from Union is Smallest Superset:

- $\paren {\forall X \in \mathbb S: X \subseteq T} \iff \bigcup \mathbb S \subseteq T$

and from Intersection is Largest Subset:

- $\paren {\forall X \in \mathbb S: T \subseteq X} \iff T \subseteq \bigcap \mathbb S$

So $\bigcap \mathbb S$ is the infimum and $\bigcup \mathbb S$ is the supremum of $\struct {\mathbb S, \subseteq}$.

Hence by definition $\powerset S$ is a complete lattice.

$\blacksquare$

## Also see

## Sources

- 1965: Seth Warner:
*Modern Algebra*... (previous) ... (next): Chapter $\text {III}$: The Natural Numbers: $\S 14$: Orderings: Example $14.3$