The extremal-shade conjecture for cross-t-intersecting families

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Let C(n,k,l,t)C(n,k,l,t) be the collection of cross-tt-intersecting pairs (A,B)(\mathcal A,\mathcal B), and define

N0(n,mk,ml,k,l,t)=max⁡(A,B)∈C(n,k,l,t)∣∇mk(A)∣ ∣∇ml(B)∣.N_0(n,m_k,m_l,k,l,t)=\max_{(\mathcal A,\mathcal B)\in C(n,k,l,t)}|\nabla_{m_k}(\mathcal A)|\,|\nabla_{m_l}(\mathcal B)|.

For 0≤i+j≤n−t0\le i+j\le n-t, define Gij(n,k,t)={F∈([n]k):∣F∩[t+i+j]∣≥t+i}\mathcal G_{ij}(n,k,t)=\{F\in\binom{[n]}{k}:|F\cap[t+i+j]|\ge t+i\}. The cross-shade extremal-family conjecture. One has

N0(n,mk,ml,k,l,t)=max⁡0≤i≤k−t0≤j≤l−t\i+j≤n−t∣Gij(n,mk,t)∣ ∣Gji(n,ml,t)∣.N_0(n,m_k,m_l,k,l,t)=\max_{\substack{0\le i\le k-t\\0\le j\le l-t\i+j\le n-t}}|\mathcal G_{ij}(n,m_k,t)|\,|\mathcal G_{ji}(n,m_l,t)|.

The conjecture is the shade analogue of the proposed extremal description for cross-tt-intersecting families. The paper gives the definitions and corresponding shade lemma but does not report a resolution.

References

Primary source

James Hirschorn, “Asymptotic upper bounds on the shades of t-intersecting families”, arXiv:0808.1434 (2008).

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