Note: On averaging Frankl's conjecture for large union-closed-sets

• Authors: Gábor Czédli
• Venue: Journal of Combinatorial Theory Series A
• Year: 2009

Let F be a union-closed family of subsets of an m-element set A. Let n=|F|=2 and for a@?A let s(a) denote the number of sets in F that contain a. Frankl's conjecture from 1979, also known as the union-closed sets conjecture, states that there exists an element a@?A with n-2s(a)==3 and n=2^m-2^m^/^2. Moreover, for these ''large'' families F we prove an even stronger version via averaging. Namely, the sum of the n-2s(a), for all a@?A, is shown to be non-positive. Notice that this stronger version does not hold for all union-closed families; however we conjecture that it holds for a much wider class of families than considered here. Although the proof of the result is based on elementary lattice theory, the paper is self-contained and the reader is not assumed to be familiar with lattices.