English

Intersection patterns of planar sets

Combinatorics 2019-12-17 v2

Abstract

Let A={A1,,An}\mathcal A=\{A_1,\ldots,A_n\} be a family of sets in the plane. For 0i<n0 \leq i < n, denote by fif_i the number of subsets σ\sigma of {1,,n}\{1,\ldots,n\} of cardinality i+1i+1 that satisfy iσAi\bigcap_{i \in \sigma} A_i \neq \emptyset. Let k2k \geq 2 be an integer. We prove that if each kk-wise and (k+1)(k+1)-wise intersection of sets from A\mathcal A is empty, or a single point, or both open and path-connected, then fk+1=0f_{k+1}=0 implies fkcfk1f_k \leq cf_{k-1} for some positive constant cc depending only on kk. Similarly, let b2,k>2bb \geq 2, k > 2b be integers. We prove that if each kk-wise or (k+1)(k+1)-wise intersection of sets from A\mathcal A has at most bb path-connected components, which all are open, then fk+1=0f_{k+1}=0 implies fkcfk1f_k \leq cf_{k-1} for some positive constant cc depending only on bb and kk. These results also extend to two-dimensional compact surfaces.

Keywords

Cite

@article{arxiv.1907.00885,
  title  = {Intersection patterns of planar sets},
  author = {Gil Kalai and Zuzana Patáková},
  journal= {arXiv preprint arXiv:1907.00885},
  year   = {2019}
}
R2 v1 2026-06-23T10:08:57.333Z