3-uniform monotone paths and multicolor Ramsey numbers

Abstract

The monotone path $P_{n+2}$ is an ordered 3-uniform hypergraph whose vertex set has size $n+2$ and edge set consists of all consecutive triples. In this note, we consider the collection ${\mathcal{J}}_n$ of ordered 3-uniform hypergraphs named monotone paths with $n$ jumps, and we prove the following relation \begin{equation*} r(3;n) \leq R(P_{n+2},\mathcal{J}_n) \leq 4^n \cdot r(3;n), \end{equation*} where $r(3;n)$ is the multicolor Ramsey number for triangles and $R(P_{n+2},{\mathcal{J}}_n)$ is the hypergraph Ramsey number for $P_{n+2}$ versus any member of ${\mathcal{J}}_n$. In particular, whether $r(3;n)$ is exponential, which is a very old problem of Erdős, is equivalent to whether $R(P_{n+2},{\mathcal{J}}_n)$ is exponential.