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A Proof of a Frankl–Kupavskii Conjecture on Intersecting Families

  • Agnijo Banerjee

摘要

A family \(\mathcal {F} \subset \mathcal {P}(n)\) F P ( n ) is r-wise k-intersecting if \(|A_1 \cap \dots \cap A_r| \ge k\) | A 1 A r | k for any \(A_1, \dots , A_r \in \mathcal {F}\) A 1 , , A r F . It is easily seen that if \(\mathcal {F}\) F is r-wise k-intersecting for \(r \ge 2\) r 2 , \(k \ge 1\) k 1 then \(|\mathcal {F}| \le 2^{n-1}\) | F | 2 n - 1 . The problem of determining the maximum size of a family \(\mathcal {F}\) F that is both \(r_1\) r 1 -wise \(k_1\) k 1 -intersecting and \(r_2\) r 2 -wise \(k_2\) k 2 -intersecting was raised in 2019 by Frankl and Kupavskii (Combinatorica 39:1255–1266, 2019). They proved the surprising result that, for \((r_1,k_1) = (3,1)\) ( r 1 , k 1 ) = ( 3 , 1 ) and \((r_2,k_2) = (2,32)\) ( r 2 , k 2 ) = ( 2 , 32 ) then this maximum is at most \(2^{n-2}\) 2 n - 2 , and conjectured the same holds if \(k_2\) k 2 is replaced by 3. In this paper we shall not only prove this conjecture but we shall also determine the exact maximum for \((r_1,k_1) = (3,1)\) ( r 1 , k 1 ) = ( 3 , 1 ) and \((r_2,k_2) = (2,3)\) ( r 2 , k 2 ) = ( 2 , 3 ) for all n.