Largest Families Without an r-Fork

Let [n] = { 1,2,...,n} be a finite set, a family of its subsets, 2 ≤ r a fixed integer. Suppose that contains no r + 1 distinct members F, G ₁,..., G _r such that F ⊂ G ₁,...,F ⊂ G _r all hold. The maximum size is asymptotically determined up to the second term, improving the result of Tran. The work of the second author was supported by the Hungarian National Foundation for Scientific Research grant numbers NK0621321, AT048826, the Bulgarian National Science Fund under Grant IO-03/2005 and the projects of the European Community: INTAS 04-77-7171, COMBSTRU–HPRN-CT-2002-000278, FIST–MTKD-CT-2004-003006.