Multicolor Ramsey numbers of unbalanced double stars

Abstract

For integers $n\ge m\ge 1$, let $S(n,m)$ be the double star obtained by joining the centers of $K_{1,n}$ and $K_{1,m}$. Ruotolo and Song proved the upper bound $r(S(n,m);k)\le kn+m+2$ under the condition $(n+1)\lceil \frac{n+1}{k-1}\rceil >m((k-1)n+m)$, as well as the matching lower bound for odd $k$. Zhao, Mao and Zhou proved the matching lower bound for even $k$ and even $n$. We establish the remaining lower bound for even $k\ge 6$ and odd $n$ through a new edge-coloring construction. Combining these results, we obtain $r(S(n,m);k)=kn+m+2$ for all integers $k\ge 5$ and $n\ge m\ge 1$ satisfying $(n+1)\lceil \frac{n+1}{k-1}\rceil >m((k-1)n+m)$. We also determine the four-color Ramsey number for $m\ge 2$ and odd $n\ge 15m$, showing that $r(S(n,m);4)=4n+m+1$.