The extremal-family conjecture for shades of t-intersecting families

For 1tkmn1\le t\le k\le m\le n, let I(n,k,t)I(n,k,t) be the family of all tt-intersecting subfamilies of ([n]k)\binom{[n]}{k}, and let

M0(n,m,k,t)=maxAI(n,k,t)m(A),M_0(n,m,k,t)=\max_{\mathcal A\in I(n,k,t)}|\nabla_m(\mathcal A)|,

where m(A)\nabla_m(\mathcal A) is the mm-shade of A\mathcal A. Define

Fi(n,m,t)={F([n]m):F[t+2i]t+i}.\mathcal F_i(n,m,t)=\left\{F\in\binom{[n]}{m}:|F\cap[t+2i]|\ge t+i\right\}.

The shade extremal-family conjecture. One has

M0(n,m,k,t)=max0imin(kt,nt2)Fi(n,m,t).M_0(n,m,k,t)=\max_{0\le i\le\min\left(k-t,\frac{n-t}{2}\right)}|\mathcal F_i(n,m,t)|.

The conjecture proposes that the largest shade is attained by one of the standard extremal families. The paper notes that the analogous assertion is false, since the optimal family for the 4m4m-conjecture yields an asymptotic ratio of 1/21/2 in a case where the proposed bound would not allow it.

Sources & referencesView supporting material

Primary source

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

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