On a restricted cross-intersection problem

  • Authors:
  • Peter Keevash;Benny Sudakov

  • Affiliations:
  • Department of Mathematics, Caltech, Pasadena, CA;Department of Mathematics, Princeton University, Princeton, NJ

  • Venue:
  • Journal of Combinatorial Theory Series A
  • Year:
  • 2006

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Abstract

Suppose A and B are families of subsets of an n-element set and L is a set of s numbers. We say that the pair (A, B) is L-cross-intersecting if |A ∩ B| ∈ L for every A ∈ A and B ∈ B. Among such pairs (A,B) we write PL(n) for the maximum possible value of |A||B|. In this paper we find an exact bound for PL(n) when n is sufficiently large, improving earlier work of Sgall. We also determine P{2}(n) and P{1, 2}(n) exactly, which respectively confirm special cases of a conjecture of Ahlswede, Cai and Zhang and a conjecture of Sgall.