The colorful fractional Helly conjecture for d-Leray complexes

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A simplicial complex K\mathsf{K} is dd-Leray if, for every induced subcomplex L≤K\mathsf{L}\leq\mathsf{K} and every i≥di\ge d, its reduced homology group H~i(L)\widetilde H_i(\mathsf{L}) over Q\mathbb{Q} vanishes. Let the vertex set be partitioned as N=N1⊔⋯⊔Nd+1N=N_1\sqcup\cdots\sqcup N_{d+1}, and write ni:=∣Ni∣n_i:=|N_i| for i∈[d+1]i\in[d+1]. A colorful dd-face is a dd-dimensional face containing one vertex from each NiN_i. The colorful fractional Helly conjecture for dd-Leray complexes. If K\mathsf{K} contains at least αn1⋯nd+1\alpha n_1\cdots n_{d+1} colorful dd-faces for some α∈(0,1]\alpha\in(0,1], then there is an i∈[d+1]i\in[d+1] such that

dim⁡K[Ni]≥(1−(1−α)1/(d+1))ni−1.\dim\mathsf{K}[N_i]\geq\bigl(1-(1-\alpha)^{1/(d+1)}\bigr)n_i-1.

This would extend the proved optimal theorem for dd-collapsible complexes to the broader class of dd-Leray complexes, providing a topological analogue of the convex-set result; the source does not report a resolution.

References

Primary source

Denys Bulavka, Afshin Goodarzi and Martin Tancer, “Optimal bounds for the colorful fractional Helly theorem”, arXiv:2010.15765 (2020).

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