is closed under intersection. Proof. We define the complement of the set system as . Let , be arbitrary sets in and so and are both in . Since is union-closed, is in , and therefore the complement of , is in , the elements in neither , nor . And this is exactly the intersection of the complements of and , . Therefore, is union-closed if and only if the complement of , is intersection closed. Secondly, we show that if a set system contains an element in at least half the sets, then its complement has an element in at most half. Lemma 2. A set system contains an element in half of its sets if and only if the complement set system , contains an element in at most half of its sets. Proof. Trivial. Therefore, if is a union-closed family of sets, the family of complement sets to sets in relative to the universe , is closed under intersection, and an element that belongs to at least half of the sets of belongs to at most half of the complement sets. Thus, an equivalent form of the conjecture (the form in which it was originally stated) is that, for any intersection-closed family of sets that contains more than one set, there exists an element that belongs to at most half of the sets in the family. Lattice formulation Although stated above in terms of families of sets, Frankl's conjecture has also been formulated and studied as a question in lattice theory. A lattice is a partially ordered set in which for two elements x and y there is a unique greatest element less than or equal to both of them (the meet of x and y) and a unique least element greater than or equal to both of them (the join of x and y). The family of all subsets of a set S, ordered by set inclusion, forms a lattice in which the meet is represented by the set-theoretic intersection and the join is represented by the set-theoretic union; a lattice formed in this way is called a Boolean lattice. The lattice-theoretic version of Frankl's conjecture is that in any finite lattice there exists an element x that is not the join of any two smaller elements, and such that the number of elements greater than or equal to x totals at most half the