Equivalent Forms
If F is a union-closed family of sets, the family of complement sets to sets in F is closed under intersection, and an element that belongs to at least half of the sets of F 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.
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 lattice, with equality only if the lattice is a Boolean lattice. As Abe (2000) shows, this statement about lattices is equivalent to the Frankl conjecture for union-closed sets: each lattice can be translated into a union-closed set family, and each union-closed set family can be translated into a lattice, such that the truth of the Frankl conjecture for the translated object implies the truth of the conjecture for the original object. This lattice-theoretic version of the conjecture is known to be true for several natural subclasses of lattices (Abe 2000; Poonen 1992; Reinhold 2000) but remains open in the general case.
Read more about this topic: Union-closed Sets Conjecture
Famous quotes containing the words equivalent and/or forms:
“I started off rapping for people just like myself, people who were in awe of wealth and flash. It was a conversation between me and them. But now most of those who buy my records are listening in on others’ conversation. They are the aural equivalent of voyeurs, thrilled at this crazy world that has nothing to do with their experience.”
—Ice-T [Tracy Marrow], U.S. rap musician. Observer (London, Oct. 27, 1991)
“The call of death is a call of love. Death can be sweet if we answer it in the affirmative, if we accept it as one of the great eternal forms of life and transformation.”
—Hermann Hesse (1877–1962)