The Ramsey numbers of squares of paths and cycles

TL;DR

This paper determines exact Ramsey numbers for the squares of paths and cycles for large n, and establishes bounds for the Ramsey number of graphs with bounded bandwidth, degree, and 3-colourability.

Contribution

It provides exact values of Ramsey numbers for squares of paths and cycles, and bounds for graphs with certain structural properties.

Findings

Exact Ramsey numbers for squares of paths and cycles are established.

Bounded bandwidth and degree graphs have Ramsey numbers at most approximately three times their size.

Results apply to large graphs with specific coloring and structural constraints.

Abstract

The square $G^2$ of a graph $G$ is the graph on $V(G)$ with a pair of vertices $uv$ an edge whenever $u$ and $v$ have distance $1$ or $2$ in $G$. Given graphs $G$ and $H$, the Ramsey number $R(G,H)$ is the minimum $N$ such that whenever the edges of the complete graph $K_N$ are coloured with red and blue, there exists either a red copy of $G$ or a blue copy of $H$. We prove that for all sufficiently large $n$ we have \[R(P_{3n}^2,P_{3n}^2)=R(P_{3n+1}^2,P_{3n+1}^2)=R(C_{3n}^2,C_{3n}^2)=9n-3\mbox{ and } R(P_{3n+2}^2,P_{3n+2}^2)=9n+1.\] We also show that for any $\gamma>0$ and $\Delta$ there exists $\beta>0$ such that the following holds. If $G$ can be coloured with three colours such that all colour classes have size at most $n$, the maximum degree $\Delta(G)$ of $G$ is at most $\Delta$, and $G$ has bandwidth at most $\beta n$, then $R(G,G)\le (3+\gamma)n$.