Borg's strong form for extremal cross-intersecting levels

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Let H\mathcal{H} be a hereditary family, with H(r)\mathcal{H}^{(r)} and H(s)\mathcal{H}^{(s)} its rrth and ssth levels. For cross-intersecting subfamilies, let M(H(r),H(s),1)M(\mathcal{H}^{(r)},\mathcal{H}^{(s)},1) denote the pairs maximizing the sum of their sizes. A family is trivial 11-intersecting if all its members contain a common element. Borg's weak form. If 1≤r≤s1 \leq r \leq s and

μ(H)≥r+s,\mu(\mathcal{H}) \geq r+s,

then for some (A,B)∈M(H(r),H(s),1)(\mathcal{A},\mathcal{B}) \in M(\mathcal{H}^{(r)},\mathcal{H}^{(s)},1), A\mathcal{A} is a trivial 11-intersecting family. The source reports that this weak form is disproved by Borg's result, which gives counterexamples for 2≤r≤s2 \leq r \leq s and suitable hereditary families.

References

Primary source

Peter Borg, “Cross-intersecting non-empty uniform subfamilies of hereditary families”, arXiv:1806.01093 (2018).

Additional references

2 papers in this index state this conjecture (2011–2018). The statement above is taken from the most recent of them; the others are arXiv:1106.6144.

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