English

Absolutely avoidable order-size pairs in hypergraphs

Combinatorics 2022-08-03 v2

Abstract

For fixed integer r2r\ge 2, we call a pair (m,f)(m,f) of integers, m1m\geq 1, 0f(mr)0\leq f \leq \binom{m}{r}, absolutelyabsolutely avoidableavoidable if there is n0n_0, such that for any pair of integers (n,e)(n,e) with n>n0n>n_0 and 0e(nr)0\leq e\leq \binom{n}{r} there is an rr-uniform hypergraph on nn vertices and ee edges that contains no induced sub-hypergraph on mm vertices and ff edges. Some pairs are clearly not absolutely avoidable, for example (m,0)(m,0) is not absolutely avoidable since any sufficiently sparse hypergraph on at least mm vertices contains independent sets on mm vertices. Here we show that for any r3r\ge 3 and mm0m \ge m_0, either the pair (m,(mr)/2)(m, \lfloor\binom mr/2\rfloor) or the pair (m,(mr)/2m1)(m, \lfloor\binom{m}{r}/2\rfloor-m-1) is absolutely avoidable. Next, following the definition of Erd\H{o}s, F\"uredi, Rothschild and S\'os, we define the densitydensity of a pair (m,f)(m,f) as σr(m,f)=lim supn{e:(n,e)(m,f)}(mr)\sigma_r(m,f) = \limsup_{n \to \infty} \frac{|\{e : (n,e) \to (m,f)\}|}{\binom mr}. We show that for r3 r\ge 3 most pairs (m,f)(m,f) satisfy σr(m,f)=0\sigma_r(m,f)=0, and that for m>rm > r, there exists no pair (m,f)(m,f) of density 1.

Cite

@article{arxiv.2205.15197,
  title  = {Absolutely avoidable order-size pairs in hypergraphs},
  author = {Lea Weber},
  journal= {arXiv preprint arXiv:2205.15197},
  year   = {2022}
}
R2 v1 2026-06-24T11:33:19.300Z