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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:

  1. $A_k \not= \mathcal{A}(\mathcal{F})$, $k=1,\ldots,n$
  2. $A_k \not\subseteq \mathcal{A}(\mathcal{F})$, $k=1,\ldots,n$

Both of them do exist.

With this question I am asking for examples of families such that:

  1. $A_j \cap A_k \not= \mathcal{A}(\mathcal{F})$, $j,k=1,\ldots,n$
  2. $A_j \cap A_k \not\subseteq \mathcal{A}(\mathcal{F})$, $j,k=1,\ldots,n$
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1 Answer 1

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The family in this answer is an example for both cases 1. and 2. in the question.

It has $135$ sets and $105$ elements.

This is its Hasse diagram:

enter image description here

For more details see the linked answer.

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