Intersection is Decreasing
Jump to navigation
Jump to search
Theorem
Let $U$ be a set.
Let $\mathcal F$ and $\mathcal G$ be sets of subsets of $U$.
Then $\mathcal F \subseteq \mathcal G \implies \bigcap \mathcal G \subseteq \bigcap \mathcal F$, where by convention $\bigcap \varnothing = U$.
That is, $\bigcap$ is a decreasing mapping from $(\mathcal P(\mathcal P(U)), \subseteq)$ to $(\mathcal P(U), \subseteq)$, where $\mathcal P(U)$ is the power set of $U$.
Proof
Let $\mathcal F \subseteq \mathcal G$.
Let $x \in \bigcap \mathcal G$.
Then for each $S \in \mathcal F$, $S \in \mathcal G$.
By the definition of intersection, $x \in S$.
Since this holds for all $S \in \mathcal F$, $x \in \bigcap \mathcal F$.
Since this holds for all $ x \in \bigcap \mathcal G$:
- $\bigcap \mathcal G \subseteq \bigcap \mathcal F$
$\blacksquare$