Ramsey numbers for partially-ordered sets

Abstract

We say that a poset $Q$ contains a copy (resp.~an induced copy) of a poset $P$ if there is an injection $f : P \to Q$ such that for any $x,y \in P$, $f(x)\leq f(y)$ in $Q$ if (resp.~if and only if) $x\leq y$ in $P$. Let $\mathcal{Q}=\{Q_{n} : n\geq 1\}$ be a family of posets such that $Q_n\subseteq Q_{n+1}$ and $|Q_n|<|Q_{n+1}|$ for each $n$. For given $k$ posets $P_1, P_2, \dots , P_k$, the \emph{weak (resp.~strong) poset Ramsey number for $t$-chains} is the smallest number $n$ such that for any coloring of $t$-chains in $Q_n\in \mathcal{Q}$ with $k$ colors, say $1,2, \dots, k$, there is a monochromatic (resp.~induced) copy of the poset $P_i$ in color $i$ for some $1\leq i\leq k$. In this paper, we give several lower and upper bounds on the weak and strong poset Ramsey number for $t$-chains.