Upper Bounds for Ordered Ramsey Numbers of Small 1-Orderings

Abstract

A $k$-ordering of a graph $G$ assigns distinct order-labels from the set $\{1,\dots ,|G|\}$ to $k$ vertices in $G$. Given a $k$-ordering $H$, the ordered Ramsey number $R_{<}(H)$ is the minimum $n$ such that every edge-2-coloring of the complete graph on the vertex set $\{1,\dots ,n\}$ contains a copy of $H$, the $i$th smallest vertex of which either has order-label $i$ in $H$ or no order-label in $H$. This paper conducts the first systematic study of ordered Ramsey numbers for $1$-orderings of small graphs. We provide upper bounds for $R_{<}(H)$ for each connected $1$-ordering $H$ on $4$ vertices. Additionally, for every $1$-ordering $H$ of the $n$-vertex path $P_n$, we prove that $R_{<}(H)\in O(n)$. Finally, we provide an upper bound for the generalized ordered Ramsey number $R_{<}(K_n,H)$ which can be applied to any $k$-ordering $H$ containing some vertex with order-label $1$.