The union-closed sets conjecture states that every finite nontrivial union-closed family has an element contained in at least half of its members.
Every finite union-closed family other than has an element belonging to at leastof its members. The proof applies the binary entropy product inequality coordinate by coordinate to the union of two independent uniform members of the family.
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The Union-Closed Sets Conjecture is a problem in combinatorial set theory that deals with the properties of families of sets.