\( \bigcap_{i \in I} A_{i} \subset A_{j} \subset \bigcup_{i \in I} A_{i} . \)
Bew.: Sei \( x \in \bigcap_{i \in I} A_{i} \) und \( j \in I \).
==> \( \forall i \in I x \in A_{i} \).
wegen \( j \in I \) also \( x \in A_{j} \).
==> \( \exists i \in I x \in A_{i} \)
==> \( x \in \bigcup_{i \in I} A_{i} \). q.e.d.