English

Refinements of Alon-Babai-Suzuki-type intersection theorems via non-shadows and binomial support

Combinatorics 2026-03-16 v2

Abstract

We prove a multilevel non-shadow refinement of the Alon--Babai--Suzuki (ABS) nonuniform restricted-intersection theorem. Let K={k1,,kr}K=\{k_1,\dots,k_r\} and let LL be a set with L=s|L|=s. If FkK([n]k)\mathcal{F}\subseteq \bigcup_{k\in K}\binom{[n]}{k} is LL-intersecting and ki>srk_i>s-r for every ii, then F+j=sr+1sNj(F)N(n,s,r),|\mathcal{F}| + \sum_{j=s-r+1}^{s} |\mathcal{N}_j(\mathcal{F})| \le N(n,s,r), equivalently Fj=sr+1sjF.|\mathcal{F}| \le \sum_{j=s-r+1}^{s} |\partial_j\mathcal{F}|. Thus the ABS bound is sharpened by the total non-shadow deficit on the top rr levels. In the modular setting, we take a coefficient-sensitive viewpoint: the polynomial method depends not just on the degree of the annihilator polynomial PL(t)=L(t)Fp[t]P_L(t)=\prod_{\ell\in L}(t-\ell)\in\mathbb{F}_p[t], but on which binomial terms actually appear in it. This yields a gap-free modular bound depending only on the active support levels of PLP_L. For almost-initial residue patterns L={0,1,,sm1}R(modp)L=\{0,1,\dots,s-m-1\}\cup R \pmod p we obtain the collapse Fi=0m(nsi).|\mathcal{F}|\le \sum_{i=0}^{m}\binom{n}{s-i}. In particular, for consecutive residues L={0,1,,s1}(modp)L=\{0,1,\dots,s-1\}\pmod p we get the sharp bound F(ns)|\mathcal{F}|\le \binom{n}{s}, giving a partial negative answer to a question of Alon--Babai--Suzuki: the modular ABS bound N(n,s,r)N(n,s,r) is not attainable in the consecutive-residue regime whenever r2r\ge 2.

Keywords

Cite

@article{arxiv.2603.10309,
  title  = {Refinements of Alon-Babai-Suzuki-type intersection theorems via non-shadows and binomial support},
  author = {Jiangdong Ai and Mingyu Liu},
  journal= {arXiv preprint arXiv:2603.10309},
  year   = {2026}
}

Comments

version 2, 12 pages, fixed some typos and applied the binomial-support viewpoint to a non-modular setting