Gallai-Ramsey numbers of $C_{10}$ and $C_{12}$

TL;DR

This paper proves a conjecture about Gallai-Ramsey numbers for certain even cycles and paths, establishing exact values for specific cases and confirming the conjecture for n=5,6.

Contribution

It confirms Song's conjecture for n=5,6 and determines exact Gallai-Ramsey numbers for cycles and paths of lengths 10 and 12.

Findings

Gallai-Ramsey numbers for C_{10} and C_{12} are exactly determined.

The conjecture holds for n=5,6 and all k ≥ 1.

Exact formulas for GR_k(C_{2n}) and GR_k(P_{2n}) for n=5,6.

Abstract

A Gallai coloring is a coloring of the edges of a complete graph without rainbow triangles, and a Gallai $k$-coloring is a Gallai coloring that uses $k$ colors. Given an integer $k\ge1$ and graphs $H_1, \ldots, H_k$, the Gallai-Ramsey number $GR(H_1, \ldots, H_k)$ is the least integer $n$ such that every Gallai $k$-coloring of the complete graph $K_n$ contains a monochromatic copy of $H_i$ in color $i$ for some $i \in \{1, \ldots, k\}$. When $H = H_1 = \cdots = H_k$, we simply write $GR_k(H)$. We continue to study Gallai-Ramsey numbers of even cycles and paths. For all $n\ge3$ and $k\ge1$, let $G_i=P_{2i+3}$ be a path on $2i+3$ vertices for all $i\in\{0,1, \ldots, n-2\}$ and $G_{n-1}\in\{C_{2n}, P_{2n+1}\}$. Let $ i_j\in\{0,1,\ldots, n-1\}$ for all $j\in\{1, \ldots, k\}$ with $ i_1\ge i_2\ge\cdots\ge i_k $. Song recently conjectured that $GR(G_{i_1}, \ldots, G_{i_k}) = 3+\min\{i_1, n^*-2\}+\sum_{j=1}^k i_j$, where $n^* =n$ when $G_{i_1}\ne P_{2n+1}$ and $n^* =n+1$ when $G_{i_1}= P_{2n+1}$. This conjecture has been verified to be true for $n\in\{3,4\}$ and all $k\ge1$. In this paper, we prove that the aforementioned conjecture holds for $n \in\{5, 6\}$ and all $k \ge1$. Our result implies that for all $k \ge 1$, $GR_k(C_{2n}) = GR_k(P_{2n}) = (n-1)k+n+1$ for $n\in\{5,6\}$ and $GR_k(P_{2n+1})= (n-1)k+n+2$ for $1\le n \le6 $.