Is true that $\bigcup\Big(\bigcup( A\times B)\Big)=A\cup B$ for any pair of sets $A$ and $B$?

Solution 1:

Aside from the usefulness of the statement, I took it as an intriguing quiz. $\newcommand{\ps}{\mathcal{P}}$ My conclusion is $$\cup (\cup (A \times B)) \subseteq A \cup B$$

Let $x \in \cup (\cup (A \times B))$. Then there is $C \in \cup (A \times B)$ such that $x \in C$. Again, we have $D \in A \times B$ such that $C \in D$. In a single line $$ x \in C \in D \in A \times B $$

From Kuratowski's definition, $$A \times B \subseteq \ps (\ps (A \cup B))$$ Hence, $D \in \ps (\ps (A \cup B))$, $D \subseteq \ps(A \cup B)$, and $C \in \ps(A \cup B)$. Again, $C \subseteq A \cup B$, which leads to $x \in A \cup B$.

(Turned out this is repetition of your proof but in a bit simpler form...)

However, as GEdgar pointed out in the comments, $ A \cup B \subseteq \cup (\cup (A \times B))$ has an obvious counterexample.

Remark: If both $A$ and $B$ are nonempty, for any $x \in A \cup B$ there are two possible cases:

$x \in A \cap B$.
This leads to $(x, x) = \{\{x\}\} \in A \times B$. Hence, $\{x\} \in \cup(A \times B)$, and again, $x \in \cup(\cup(A \times B))$
$x$ is in only one of $A$ and $B$, and the other set has some element $y$ other than $x$.
If $x \in A$ and $ y \in B $, $ (x,y) =\{\{x\},\{x,y\}\} \in A \times B$. Hence, $\{x\} \in \cup(A \times B)$. This give the same conclusion as the case $1$.
On the other hand, if $y \in A$ and $ x \in B $, $ (y,x) =\{\{y\},\{x,y\}\} \in A \times B$. Hence, $\{x,y\} \in \cup (A \times B) $ and $x \in \cup (\cup (A \times B))$.