A note on large H-intersecting families

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A family F of graphs on a fixed set of n vertices is called triangle-intersecting if for any G1,G2 ∈ F , the intersection G1 ∩ G2 contains a triangle. More generally, for a fixed graph H, a family F is H-intersecting if the intersection of any two graphs in F contains a subgraph isomorphic to H. In [D. Ellis, Y. Filmus, and E. Friedgut, J. Eur. Math. Soc., 14 (2012), pp. 841-885], a 36-year old conjecture of Simonovits and Sós was proved stating that the maximal size of a triangleintersecting family is (1/8)2n (n-1)/2. Furthermore, they proved a p-biased generalization, stating that for any p ≤ 1/2, we have μp (F ) ≤ p3, where μ p (F ) is the probability that the random graph G(n, p) belongs to \scrF . In the same paper, the authors conjectured that the assertion of their biased theorem holds also for 1/2 < p ≤ 3/4, and more generally, that for any non-t-colorable graph H and any H-intersecting family F , we have μ p (F ) ≤ pt (t+1)/2 for all p ≤ (2t 1)/(2t). In this note we construct, for any fixed H and any p > 1/2, an H-intersecting family F of graphs such that μ p (F ) ≥ 1 e-n2/C, where C depends only on H and p, thus disproving both conjectures.

Original languageEnglish
Pages (from-to)398-401
Number of pages4
JournalSIAM Journal on Discrete Mathematics
Issue number1
StatePublished - 2019


  • Extremal set theory
  • Intersecting families
  • Probabilistic methods
  • Triangle intersecting

All Science Journal Classification (ASJC) codes

  • General Mathematics


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