English

Two-part set systems

Combinatorics 2016-08-14 v1

Abstract

The two part Sperner theorem of Katona and Kleitman states that if XX is an nn-element set with partition X1X2X_1 \cup X_2, and \cF\cF is a family of subsets of XX such that no two sets A,B\cFA, B \in \cF satisfy ABA \subset B (or BAB \subset A) and AXi=BXiA \cap X_i=B \cap X_i for some ii, then \cF(nn/2)|\cF| \le {n \choose \lfloor n/2 \rfloor}. We consider variations of this problem by replacing the Sperner property with the intersection property and considering families that satisfiy various combinations of these properties on one or both parts X1X_1, X2X_2. Along the way, we prove the following new result which may be of independent interest: let \cF,\cG\cF, \cG be families of subsets of an nn-element set such that \cF\cF and \cG\cG are both intersecting and cross-Sperner, meaning that if A\cFA \in \cF and B\cGB \in \cG, then A⊄BA \not\subset B and B⊄AB \not\subset A. Then \cF+\cG<2n1|\cF| +|\cG| < 2^{n-1} and there are exponentially many examples showing that this bound is tight.

Keywords

Cite

@article{arxiv.1110.0099,
  title  = {Two-part set systems},
  author = {Dániel Gerbner and Péter L. Erdős and Nathan Lemons and Dhruv Mubayi and Cory Palmer and Balázs Patkós},
  journal= {arXiv preprint arXiv:1110.0099},
  year   = {2016}
}
R2 v1 2026-06-21T19:13:40.037Z