English

Non-trivial $d$-wise Intersecting families

Combinatorics 2019-11-05 v1

Abstract

For an integer d2d \geq 2, a family F\mathcal{F} of sets is \textit{d-wise intersecting} if for any distinct sets A1,A2,,AdFA_1,A_2,\dots,A_d \in \mathcal{F}, A1A2AdA_1 \cap A_2 \cap \dots \cap A_d \neq \emptyset, and non-trivial\textit{non-trivial} if F=\bigcap \mathcal{F} = \emptyset. Hilton and Milner conjectured that for kd2k \geq d \geq 2 and large enough nn, the extremal non-trivial dd-wise intersecting family of kk-element subsets of [n][n] is one of the following two families: \begin{align*} &\mathcal{H}(k,d) = \{A \in \binom{[n]}{k} : [d-1] \subset A, A \cap [d,k+1] \neq \emptyset\} \cup \{[k+1] \setminus \{i \} : i \in [d - 1]\} \\ &\mathcal{A}(k,d) = \{ A \in \binom{[n]}{k} : |A \cap [d+1]| \geq d \}. \end{align*} The celebrated Hilton-Milner Theorem states that H(k,2)\mathcal{H}(k,2) is the unique extremal non-trivial intersecting family for k>3k>3. We prove the conjecture and prove a stability theorem, stating that any large enough non-trivial dd-wise intersecting family of kk-element subsets of [n][n] is a subfamily of A(k,d)\mathcal{A}(k,d) or H(k,d)\mathcal{H}(k,d).

Keywords

Cite

@article{arxiv.1911.01031,
  title  = {Non-trivial $d$-wise Intersecting families},
  author = {Jason O'Neill and Jacques Verstraete},
  journal= {arXiv preprint arXiv:1911.01031},
  year   = {2019}
}
R2 v1 2026-06-23T12:03:39.856Z