Consider a union-closed family $\mathcal{F}=\{A_1,…,A_n\}$ of $n$ finite sets. Let $\mathcal{A}(\mathcal{F})$ be the set of elements that belong to at least half of the sets of $\mathcal{F}$.
Another question asked for examples of families $\mathcal{F}$ such that:
- $A_k \not= \mathcal{A}(\mathcal{F})$, $k=1,\ldots,n$
- $A_k \not\subseteq \mathcal{A}(\mathcal{F})$, $k=1,\ldots,n$
Both of them do exist.
With this question I am asking for examples of families such that:
- $A_j \cap A_k \not= \mathcal{A}(\mathcal{F})$, $j,k=1,\ldots,n$
- $A_j \cap A_k \not\subseteq \mathcal{A}(\mathcal{F})$, $j,k=1,\ldots,n$