A sharp threshold for a random version of Sperner's Theorem

Abstract

The Boolean lattice $\mathcal{P}(n)$ consists of all subsets of $[n]=\{1,\dots ,n\}$ partially ordered under the containment relation. Sperner's Theorem states that the largest antichain of the Boolean lattice is given by a middle layer: the collection of all sets of size $\lfloor n/2\rfloor$, or also, if $n$ is odd, the collection of all sets of size $\lceil n/2\rceil$. Given $p$, choose each subset of $[n]$ with probability $p$ independently. We show that for every constant $p>3/4$, the largest antichain among these subsets is also given by a middle layer, with probability tending to $1$ as $n$ tends to infinity. This $3/4$ is best possible, and we also characterize the largest antichains for every constant $p>1/2$. Our proof is based on some new variations of Sapozhenko's graph container method.

24 pages, 3 figures; added more details in new version