Some Ordered Ramsey Numbers of Graphs on Four Vertices

TL;DR

This paper investigates ordered Ramsey numbers for small graphs on four vertices, providing new upper bounds, extending results to larger graphs, and computing exact values using a SAT solver.

Contribution

It offers novel upper bounds for ordered Ramsey numbers of four-vertex graphs and extends some results to larger graphs, employing computational methods.

Findings

Established non-trivial upper bounds for ordered Ramsey numbers of four-vertex graphs.

Extended results to n+1 vertex graphs with a pendant edge.

Computed exact ordered Ramsey numbers using a SAT solver.

Abstract

An ordered graph $H$ on $n$ vertices is a graph whose vertices have been labeled bijectively with $\{1,...,n\}$. The ordered Ramsey number $r_<(H)$ is the minimum $n$ such that every two-coloring of the edges of the complete graph $K_n$ contains a monochromatic copy of $H$ such that the vertices in the copy appear in the same order as in $H$. Although some bounds on the ordered Ramsey numbers of certain infinite families of graphs are known, very little is known about the ordered Ramsey numbers of specific small graphs compared to how much we know about the usual Ramsey numbers for these graphs. In this paper we tackle the problem of proving non-trivial upper bounds on orderings of graphs on four vertices. We also extend one of our results to $n+1$ vertex graphs that consist of a complete graph on $n$ vertices with a pendant edge to vertex 1. Finally, we use a SAT solver to compute some numbers exactly.