Ramsey numbers of Boolean lattices

TL;DR

This paper determines the asymptotic behavior of the Boolean lattice poset Ramsey number for two-dimensional cases and improves bounds for weak poset Ramsey numbers, providing explicit constructions and asymptotic results.

Contribution

It solves the asymptotic value of R(Q_2,Q_n) and improves bounds for weak poset Ramsey numbers with explicit constructions.

Findings

R(Q_2,Q_n)=n+O(n/ log n) asymptotically

R_w(Q_m,Q_n) ≥ n + m + 1 for all m ≥ 2 and large n

R_w(Q_2,Q_n)=n+3 explicitly

Abstract

The poset Ramsey number $R(Q_m,Q_n)$ is the smallest integer $N$ such that any blue-red coloring of the elements of the Boolean lattice $Q_N$ has a blue induced copy of $Q_m$ or a red induced copy of $Q_n$. The weak poset Ramsey number $R_w(Q_m,Q_n)$ is defined analogously, with weak copies instead of induced copies. It is easy to see that $R(Q_m,Q_n) \ge R_w(Q_m,Q_n)$. Axenovich and Walzer showed that $n+2 \le R(Q_2,Q_n) \le 2n+2$. Recently, Lu and Thompson improved the upper bound to $\frac{5}{3}n+2$. In this paper, we solve this problem asymptotically by showing that $R(Q_2,Q_n)=n+O(n/\log n)$. In the diagonal case, Cox and Stolee proved $R_w(Q_n,Q_n) \ge 2n+1$ using a probabilistic construction. In the induced case, Bohman and Peng showed $R(Q_n,Q_n) \ge 2n+1$ using an explicit construction. Improving these results, we show that $R_w(Q_m,Q_n) \ge n+m+1$ for all $m \ge 2$ and large $n$ by giving an explicit construction; in particular, we prove that $R_w(Q_2,Q_n)=n+3$.