Variants of the Erdos-Szekeres and Erdos-Hajnal Ramsey problems

Abstract

Given integers $\ell,n$, the $\ell$th power of the path $P_n$ is the ordered graph $P_n^{\ell}$ with vertex set $v_1<v_2<\cdots < v_n$, and all edges of the form $v_iv_j$ where $|i-j|\le \ell$. The ramsey number $r(P_n^{\ell}, P_n^{\ell})$ is the minimum $N$ such that every 2-coloring of ${[N] \choose 2}$ results in a monochromatic copy of $P_n^{\ell}$. It is well-known that $r(P_n^1, P_n^1)=(n-1)^2+1$. For $\ell>1$, Balko-Cibulka-Kr\'al-Kyn\v{c}l proved that $r(P_n^{\ell}, P_n^{\ell})< c_{\ell}n^{128 \ell}$ and asked for the growth rate for fixed $\ell$. When $\ell=2$, we improve this upper bound by proving $r(P_n^{2}, P_n^{2})< cn^{19.5}$. Using this result, we determine the correct tower growth rate of the $k$-uniform hypergraph ramsey number of a $(k+1)$-clique versus an ordered tight path. Finally, we consider an ordered version of the classical Erd Hos-Hajnal hypergraph ramsey problem, improve the tower height given by the trivial upper bound, and conjecture that this tower height is optimal.