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

Let n,s,kn,s,k be positive integers with k2k\geq 2 and nks+k1n\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 2ik12\leq i\leq k-1, define

Aik(n,s):=e([n]k):e[(s+1)i1]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[s1]Se([n]k):se,eS.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):eUD^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)maxA2k(n,s),,Ak1k(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.

Sources & referencesView supporting material

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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