On the Multi-coloured Ramsey Numbers of Cycles

For a graph $L$ and an integer $k\geq 2$, $R_k(L)$ denotes the smallest integer $N$ for which for any edge-colouring of the complete graph $K_N$ by $k$ colours there exists a colour $i$ for which the corresponding colour class contains $L$ as a subgraph. Bondy and Erdős conjectured that for an odd cycle $C_n$ on $n$ vertices, $R_k(C_n) = 2^{k-1}(n-1)+1 \text{for $n>3$.}$ They proved the case when $k=2$ and also provided an upper bound $R_k(C_n)\leq (k+2)!n$. Recently, this conjecture has been verified for $k=3$ if $n$ is large. In this note, we prove that for every integer $k\geq 4$, $R_k(C_n)\leq k2^kn+o(n), \text{as $n\to\infty$.}$ When $n$ is even, Yongqi, Yuansheng, Feng, and Bingxi gave a construction, showing that $R_k(C_n)\geq (k-1)n-2k+4.$ Here we prove that if $n$ is even, then $R_k(C_n)\leq kn+o(n), \text{as $n\to\infty$.}$

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