Empty Intersection iff Subset of Complement/Corollary
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Corollary to Empty Intersection iff Subset of Complement
Let $A, B, S$ be sets such that $A, B \subseteq S$.
Then:
- $\exists X \in \powerset S: \paren {A \cap X} \cup \paren {B \cap \complement_S \paren X} = \O \iff A \cap B = \O$
where $\overline X$ denotes the relative complement of $X$ in $S$.
Proof 1
Let there exist such a set $X$.
Then:
\(\ds \) | \(\) | \(\ds \paren {A \cap X} \cup \paren {B \cap \complement_S \paren X} = \O\) | ||||||||||||
\(\ds \) | \(\leadstoandfrom\) | \(\ds A \cap X = \O \land B \cap \complement_S \paren X = \O\) | Union is Empty iff Sets are Empty | |||||||||||
\(\ds \) | \(\leadstoandfrom\) | \(\ds A \subseteq \complement_S \paren X \land B \subseteq X\) | Empty Intersection iff Subset of Complement | |||||||||||
\(\ds \) | \(\leadstoandfrom\) | \(\ds A \cap B = \O\) |
$\blacksquare$
Proof 2
We have:
where the universe $\Bbb U$ is posited.
Let $S$ take the position of $\Bbb U$.
Let $C = X$.
Then we have:
- $\paren {A \cap X} \cup \paren {B \cap \relcomp S X} = \O \iff B \subseteq X \subseteq \relcomp S A$
Thus we have shown that:
- $B \subseteq \relcomp S A$
and it follows from Empty Intersection iff Subset of Complement that:
- $A \cap B = \O$
$\blacksquare$
Sources
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): Chapter $1$: Sets and Logic: Exercise $6$