English

On a problem of Henning and Yeo about the transversal number of uniform linear systems whose 2-packing number is fixed

Combinatorics 2020-02-28 v3

Abstract

A linear system is a pair (P,L)(P,\mathcal{L}) where L\mathcal{L} is a family of subsets on a ground finite set PP such that ll1|l\cap l^\prime|\leq 1, for every l,lLl,l^\prime \in \mathcal{L}. If all elements of L\mathcal{L} of a linear system (P,L)(P,\mathcal{L}), then the linear system is called rr-uniform linear system. The transversal number of a linear system (P,L)(P,\mathcal{L}), τ(P,L)\tau(P,\mathcal{L}), is the minimum cardinality of a subset P^P\hat{P}\subseteq P satisfying lP^l\cap\hat{P}\neq\emptyset, for every lLl\in\mathcal{L}. The 2-packing number of a linear system (P,L)(P,\mathcal{L}), ν2(P,L)\nu_2(P,\mathcal{L}), is the maximum cardinality of a subset RLR\subseteq\mathcal{L} such that, any three elements of RR don't have a common point (are triplewise disjoint), that is, if three elements are chosen in RR, then they are not incidents in a common point. For r2r\geq2, let (P,L)(P,\mathcal{L}) be an rr-uniform linear system. In "{\sc M. A. Henning and A. Yeo:} {\it Hypergraphs with large transversal number,} Discrete Math. {\bf 313} (2013), no. 8, 959--966." Henning and Yeo state the following question: Is it true that if (P,L)(P,\mathcal{L}) is an rr-uniform linear system then τ(P,L)P+Lr+1\tau(P,\mathcal{L})\leq\displaystyle\frac{|P|+|\mathcal{L}|}{r+1} holds for all r2r\geq2?. In this note, we give some results of rr-uniform linear systems, whose 2-packing number is fixed, satisfying the inequality.

Keywords

Cite

@article{arxiv.1710.02501,
  title  = {On a problem of Henning and Yeo about the transversal number of uniform linear systems whose 2-packing number is fixed},
  author = {Carlos A. Alfaro and Adrián Vázquez-Ávila},
  journal= {arXiv preprint arXiv:1710.02501},
  year   = {2020}
}