On some extremal and probabilistic questions for tree posets

TL;DR

This paper investigates extremal and probabilistic properties of posets with tree-shaped Hasse diagrams, confirming conjectures for certain cases and extending classical theorems like Sperner's to these structures.

Contribution

It proves the exponential growth rate of P-free families for posets with tree-shaped Hasse diagrams of radius at most 2 and resolves the random P-free problem for these cases.

Findings

Confirmed conjecture for posets with tree-shaped Hasse diagrams of radius ≤ 2.

Established the exponential size of P-free families in these cases.

Extended the random Sperner theorem to posets with tree-shaped Hasse diagrams.

Abstract

Given two posets $P,Q$ we say that $Q$ is $P$-free if $Q$ does not contain a copy of $P$. The size of the largest $P$-free family in $2^{[n]}$, denoted by $La(n,P)$, has been extensively studied since the 1980s. We consider several related problems. Indeed, for posets $P$ whose Hasse diagrams are trees and have radius at most $2$, we prove that there are $2^{(1+o(1))La(n,P)}$ $P$-free families in $2^{[n]}$, thereby confirming a conjecture of Gerbner, Nagy, Patk\'os and Vizer [Electronic Journal of Combinatorics, 2021] in these cases. For such $P$ we also resolve the random version of the $P$-free problem, thus generalising the random version of Sperner's theorem due to Balogh, Mycroft and Treglown [Journal of Combinatorial Theory Series A, 2014], and Collares Neto and Morris [Random Structures and Algorithms, 2016]. Additionally, we make a general conjecture that, roughly speaking, asserts that subfamilies of $2^{[n]}$ of size sufficiently above $La(n,P)$ robustly contain $P$, for any poset $P$ whose Hasse diagram is a tree.