Ordered Size Ramsey Number of Paths

Abstract

An ordered graph is a simple graph with an ordering on its vertices. Define the ordered path $P_n$ to be the monotone increasing path with $n$ edges. The ordered size Ramsey number $\tilde{r}(P_r,P_s)$ is the minimum number $m$ for which there exists an ordered graph $H$ with $m$ edges such that every two-coloring of the edges of $H$ contains a red copy of $P_r$ or a blue copy of $P_s$. For $2\le r\le s$, we show $\frac{1}{8}{r}^{2}s\le \tilde{r}(P_r,P_s)\le C{r}^{2}s(\log s{)}^3$, where $C>0$ is an absolute constant. This problem is motivated by the recent results of Bucić-Letzter-Sudakov and Letzter-Sudakov for oriented graphs.

11 pages; the new version includes (as Theorem 1.3) an extension of the main result to more than 2 colors