Kruskal-Katona's function and a variation of cross-intersecting antichains

TL;DR

This paper explores properties of the Kruskal-Katona function and establishes optimal bounds for cross-intersecting antichains with limited disjoint pairs, revealing extremal structures involving sets of size n/2 and n/2+1.

Contribution

It provides new bounds on the size of cross-intersecting antichains with restricted disjoint pairs and characterizes the extremal families.

Findings

Established a best possible upper bound on |A| + |B|.

Identified extremal families containing only n/2 and (n/2+1)-sets.

Extended understanding of cross-intersecting antichains with limited disjoint pairs.

Abstract

We prove some properties of the Kruskal-Katona function, and apply to the following variation of cross-intersecting antichains. Let $n\ge 4$ be an even integer and $\mathscr{A}$ and $\mathscr{B}$ be two cross-intersecting antichains of $\mathbb{N}_n$ with at most $k$ disjoint pairs, i.e. for all $A_i\in \mathscr{A}$, $B_j\in\mathscr{B}$, $A_i\cap B_j=\emptyset$ only if $i=j\le k$. We prove a best possible upper bound on $|\mathscr{A}|+|\mathscr{B}|$. Furthermore, we show that the extremal families contain only $\frac{n}{2}$ and $(\frac{n}{2}+1)$-sets.