English

On the Hilton-Spencer intersection theorems for unions of cycles

Combinatorics 2019-08-26 v1

Abstract

A family A\mathcal{A} of sets is said to be intersecting if every two sets in A\mathcal{A} intersect. An intersecting family is said to be \emph{trivial} it its sets have a common element. A graph GG is said to be rr-EKR if at least one of the largest intersecting families of independent rr-element sets of GG is trivial. Let α(G)\alpha(G) and ω(G)\omega(G) denote the independence number and the clique number of GG, respectively. Hilton and Spencer recently showed that if GG is the vertex-disjoint union of a cycle C{_*C} raised to the power kk^* and ss cycles 1C,,sC{_1C}, \dots, {_sC} raised to the powers k1,,ksk_1, \dots, k_s, respectively, 1rα(G)1 \leq r \leq \alpha(G), and min(ω(1Ck1),,ω(sCks))2k+1,\min\big(\omega(_1C^{k_1}), \dots, \omega(_sC^{k_s})\big) \geq 2k^* + 1, then GG is rr-EKR. They had shown that the same holds if C{_*C} is replaced by a path and the condition on the clique numbers is relaxed to min(ω(1Ck1),,ω(sCks))k+1.\min\big(\omega(_1C^{k_1}), \dots, \omega(_sC^{k_s})\big) \geq k^* + 1. We use the classical Shadow Intersection Theorem of Katona to obtain a short proof of each result for the case where the inequality for the minimum clique number is strict.

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Cite

@article{arxiv.1908.08825,
  title  = {On the Hilton-Spencer intersection theorems for unions of cycles},
  author = {Peter Borg and Carl Feghali},
  journal= {arXiv preprint arXiv:1908.08825},
  year   = {2019}
}

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9 pages