English

Non-uniform skew versions of Bollob\'as' Theorem

Combinatorics 2023-05-24 v1

Abstract

Let A1,,AmA_1, \ldots ,A_m and B1,,BmB_1, \ldots ,B_m be subsets of [n][n] and let tt be a non-negative integer with the following property: AiBit|A_i \cap B_i|\leq t for each ii and AiBj>t|A_i\cap B_j|>t whenever i<ji< j. Then m2ntm\leq 2^{n-t}. Our proof uses Lov\'asz' tensor product method. We prove the following skew version of Bollob\'as' Theorem. Let A1,,AmA_1, \ldots ,A_m and B1,,BmB_1, \ldots ,B_m be finite sets of [n][n] satisfying the conditions AiBi=A_i \cap B_i =\emptyset for each ii and AiBjA_i\cap B_j\ne \emptyset for each i<ji< j. Then i=1m1(Ai+BiAi)n+1. \sum_{i=1}^m \frac{1}{{|A_i|+|B_i| \choose |A_i|}}\leq n+1. Both upper bounds are sharp.

Keywords

Cite

@article{arxiv.2305.14191,
  title  = {Non-uniform skew versions of Bollob\'as' Theorem},
  author = {Gábor Hegedüs},
  journal= {arXiv preprint arXiv:2305.14191},
  year   = {2023}
}