Improved bounds for the Ramsey number of tight cycles versus cliques

TL;DR

This paper establishes improved upper bounds for the Ramsey numbers of certain 3-uniform tight cycles versus cliques, showing they grow at most as a quasi-exponential function of n, which advances understanding of hypergraph Ramsey theory.

Contribution

The paper proves nearly tight upper bounds for the Ramsey numbers of specific 3-uniform tight cycles versus cliques, answering a longstanding open question.

Findings

Upper bounds of the form 2^{c_s n log n} for certain cycles

Nearly tight bounds matching known exponential lower bounds

Progress on hypergraph Ramsey number growth rates

Abstract

The 3-uniform tight cycle $C_s^3$ has vertex set $ Z_s$ and edge set $\{\{i, i+1, i+2\}: i \in Z_s\}$. We prove that for every $s \not\equiv 0$ (mod 3) and $s \ge 16$ or $s \in \{8,11,14\}$ there is a $c_s>0$ such that the 3-uniform hypergraph Ramsey number $$r(C_s^3, K_n^3)< 2^{c_s n \log n}$$ This answers in strong form a question of the author and R\"odl who asked for an upper bound of the form $2^{n^{1+\epsilon_s}}$ for each fixed $s \ge 4$, where $\epsilon_s \rightarrow 0$ as $s \rightarrow \infty$ and $n$ is sufficiently large. The result is nearly tight as the lower bound is known to be exponential in $n$.