Frankl and Kupavskii's matching–cover conjecture for uniform hypergraphs

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Let n,s,kn,s,k be positive integers with k≥2k\geq 2 and n≥ks+k−1n\geq ks+k-1. Let HH be a kk-graph on vertex set [n][n], and let ν(H)\nu(H) and τ(H)\tau(H) denote its matching number and vertex-cover number. For 2≤i≤k−12\leq i\leq k-1, define

Aik(n,s):=e∈([n]k):∣e∩[(s+1)i−1]∣≥i.A^k_i(n,s):=\\{e\in\binom{[n]}{k}:|e\cap[(s+1)i-1]|\geq i\\}.

Let S=s+1,…,s+kS=\\{s+1,\ldots,s+k\\} and define

HMn,sk:=e∈([n]k):e∩[s−1]≠∅∪S∪e∈([n]k):s∈e,e∩S≠∅.HM^k_{n,s}:=\\{e\in\binom{[n]}{k}:e\cap[s-1]\neq\emptyset\\}\cup\\{S\\}\cup\\{e\in\binom{[n]}{k}:s\in e,\\ e\cap S\neq\emptyset\\}.

For a set U⊆[n]U\subseteq[n] of size k(s+1)−1k(s+1)-1, let Dn,sk(U):=e∈([n]k):e⊆UD^k_{n,s}(U):=\\{e\in\binom{[n]}{k}:e\subseteq U\\}, and write Dn,skD^k_{n,s} for this construction. Frankl and Kupavskii's conjecture. If ν(H)=s\nu(H)=s and τ(H)>s\tau(H)>s, then

e(H)≤max⁡∣A2k(n,s)∣,…,∣Ak−1k(n,s)∣,∣HMn,sk∣,∣Dn,sk∣.e(H)\leq\max\\{|A^k_2(n,s)|,\ldots,|A^k_{k-1}(n,s)|,|HM^k_{n,s}|,|D^k_{n,s}|\\}.

The listed constructions have matching number ss and vertex-cover number greater than ss, so the conjecture proposes that they determine the maximum edge count among such hypergraphs. Its status is not resolved by the supplied text.

References

Primary source

Mingyang Guo, Hongliang Lu and Dingjia Mao, “A stability result on matchings in 3-uniform hypergraphs”, arXiv:2103.15127 (2021).

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