English

Minimum blocking sets for families of partitions

Combinatorics 2025-08-20 v2 Discrete Mathematics

Abstract

A 33-partition of an nn-element set VV is a triple of pairwise disjoint nonempty subsets X,Y,ZX,Y,Z such that V=XYZV=X\cup Y\cup Z. We determine the minimum size φ3(n)\varphi_3(n) of a set E\mathcal{E} of triples such that for every 3-partition X,Y,ZX,Y,Z of the set {1,,n}\{1,\dots,n\}, there is some {x,y,z}E\{x,y,z\}\in \mathcal{E} with xXx\in X, yYy\in Y, and zZz\in Z. In particular, φ3(n)=n(n2)3.\varphi_3(n)=\left\lceil{\frac{n(n-2)}{3}}\right\rceil. For d>3d>3, one may define an analogous number φd(n)\varphi_d(n). We determine the order of magnitude of φd(n)\varphi_d(n), and prove the following upper and lower bounds, for d>3d>3: 2nd1d!o(nd1)φd(n)0.86(d1)!nd1+o(nd1).\frac{2 n^{d-1}}{d!} -o(n^{d-1}) \leq \varphi_d(n) \leq \frac{0.86}{(d-1)!}n^{d-1}+o(n^{d-1}).

Keywords

Cite

@article{arxiv.2505.15362,
  title  = {Minimum blocking sets for families of partitions},
  author = {Guillermo Gamboa Quintero and Ida Kantor},
  journal= {arXiv preprint arXiv:2505.15362},
  year   = {2025}
}