English

Counting $r$-graphs without forbidden configurations

Combinatorics 2021-08-02 v1

Abstract

One of the major problems in combinatorics is to determine the number of rr-uniform hypergraphs (rr-graphs) on nn vertices which are free of certain forbidden structures. This problem dates back to the work of Erd\H{o}s, Kleitman and Rothschild, who showed that the number of KrK_r-free graphs on nn vertices is 2ex(n,Kr)+o(n2)2^{\text{ex}(n,K_r)+o(n^2)}. Their work was later extended to forbidding graphs as induced subgraphs by Pr\"omel and Steger. Here, we consider one of the most basic counting problems for 33-graphs. Let E1E_1 be the 33-graph with 44 vertices and 11 edge. What is the number of induced {K43,E1}\{K_4^3,E_1\}-free 33-graphs on nn vertices? We show that the number of such 33-graphs is of order nΘ(n2)n^{\Theta(n^2)}. More generally, we determine asymptotically the number of induced F\mathcal{F}-free 33-graphs on nn vertices for all families F\mathcal{F} of 33-graphs on 44 vertices. We also provide upper bounds on the number of rr-graphs on nn vertices which do not induce iLi \in L edges on any set of kk vertices, where L{0,1,,(kr)}L \subseteq \big \{0,1,\ldots,\binom{k}{r} \big\} is a list which does not contain 33 consecutive integers in its complement. Our bounds are best possible up to a constant multiplicative factor in the exponent when k=r+1k = r+1. The main tool behind our proof is counting the solutions of a constraint satisfaction problem.

Keywords

Cite

@article{arxiv.2107.14798,
  title  = {Counting $r$-graphs without forbidden configurations},
  author = {József Balogh and Felix Christian Clemen and Letícia Mattos},
  journal= {arXiv preprint arXiv:2107.14798},
  year   = {2021}
}

Comments

18 pages

R2 v1 2026-06-24T04:41:58.846Z