Kleitman's conjecture about families of given size minimizing the number of $k$-chains

TL;DR

This paper proves Kleitman's conjecture asymptotically for large $n$, showing that families of size up to nearly half of the power set minimize the number of $k$-chains, extending previous results for 2-chains.

Contribution

The paper establishes an asymptotic proof of Kleitman's conjecture for all fixed $k$, for sufficiently large $n$, for families of size up to nearly half of the power set.

Findings

Kleitman's conjecture holds asymptotically for large $n$

Families of size up to $(1- ext{epsilon})2^n$ minimize $k$-chains

The proof extends previous results for 2-chains to general $k$

Abstract

A central theorem in combinatorics is Sperner's Theorem, which determines the maximum size of a family $\mathcal{F}\subseteq \mathcal{P}(n)$ that does not contain a $2$-chain $F_1\subsetneq F_2$. Erd\H{o}s later extended this result and determined the largest family not containing a $k$-chain $F_1\subsetneq \ldots \subsetneq F_k$. Erd\H{o}s and Katona and later Kleitman asked how many such chains must appear in families whose size is larger than the corresponding extremal result. This question was resolved for $2$-chains by Kleitman in $1966$, who showed that amongst families of size $M$ in $\mathcal{P}(n)$, the number of $2$-chains is minimized by a family whose sets are taken as close to the middle layer as possible. He also conjectured that the same conclusion should hold for all $k$, not just $2$. The best result on this question is due to Das, Gan and Sudakov who showed that Kleitman's conjecture holds for families whose size is at most the size of the $k+1$ middle layers of $\mathcal{P}(n)$, provided $k\leq n-6$. Our main result is that for every fixed $k$ and $\epsilon>0$, if $n$ is sufficiently large then Kleitman's conjecture holds for families of size at most $(1-\epsilon)2^n$, thereby establishing Kleitman's conjecture asymptotically. Our proof is based on ideas of Kleitman and Das, Gan and Sudakov. Several open problems are also given.