English

On set systems with strongly restricted intersections

Combinatorics 2024-04-15 v1

Abstract

Set systems with strongly restricted intersections, called α\alpha-intersecting families for a vector α\alpha, were introduced recently as a generalization of several well-studied intersecting families including the classical oddtown and eventown. Given a binary vector α=(a1,,ak)\alpha=(a_1, \ldots, a_k), a collection F\mathcal F of subsets over an nn element set is an α\alpha-intersecting family modulo 22 if for each i=1,2,,ki=1,2,\ldots,k, all ii-wise intersections of distinct members in F\mathcal F have sizes with the same parity as aia_i. Let fα(n)f_\alpha(n) denote the maximum size of such a family. In this paper, we study the asymptotic behavior of fα(n)f_\alpha(n) when nn goes to infinity. We show that if tt is the maximum integer such that at=1a_t=1 and 2tk2t\leq k, then fα(n)(t!n)1tf_{\alpha(n)} \sim {(t! n)}^{\frac 1 t}. More importantly, we show that for any constant cc, as the length kk goes larger, fα(n)f_\alpha(n) is upper bounded by O(nc)O (n^c) for almost all α\alpha. Equivalently, no matter what kk is, there are only finitely many α\alpha satisfying fα(n)=Ω(nc)f_\alpha(n)=\Omega (n^c). This answers an open problem raised by Johnston and O'Neill in 2023. All of our results can be generalized to modulo pp setting for any prime pp smoothly.

Keywords

Cite

@article{arxiv.2404.08280,
  title  = {On set systems with strongly restricted intersections},
  author = {Xin Wei and Xiande Zhang and Gennian Ge},
  journal= {arXiv preprint arXiv:2404.08280},
  year   = {2024}
}

Comments

12 pages, 0 figure

R2 v1 2026-06-28T15:52:13.786Z