The Ramsey number of mixed-parity cycles I

Denote by $R(G_1,G_2,G_3)$ the minimum integer $N$ such that any three-colouring of the edges of the complete graph on $N$ vertices contains a monochromatic copy of a graph $G_i$ coloured with colour $i$ for some $i\in 1,2,3$. In a series of three papers of which this is the first, we consider the case where $G_1,G_2$ and $G_3$ are cycles of mixed parity. Specifically, in this and the subsequent paper, we consider $R(C_n,C_m,C_{\mathrm{\ell }})$, where $n$ and $m$ are even and $\mathrm{\ell }$ is odd. Figaj and \L uczak determined an asymptotic result for this case, which we improve upon to give an exact result. We prove that for $n,m$ and $\mathrm{\ell }$ sufficiently large $R(C_n,C_m,C_\ell)=\max{2n+m-3, n+2m-3, \tfrac{1}{2} n +\tfrac{1}{2} m + \ell

• 2}$. In the case that the longest cycle is of even length, the proof in this paper is self-contained. However, in the case that the longest cycle is of odd length, we require an additional technical result, the proof of which makes up the majority of the subsequent paper.