Matching-star size Ramsey numbers under connectivity constraint

Abstract

Recently, Caro, Patk\'os, and Tuza (2022) introduced the concept of connected Tur\'an number. We study a similar parameter in Ramsey theory. Given two graphs $G_1$ and $G_2$, the size Ramsey number $\hat{r}(G_1,G_2)$ refers to the smallest number of edges in a graph $G$ such that for any red-blue edge-coloring of $G$, either a red subgraph $G_1$ or a blue subgraph $G_2$ is present in $G$. If we further restrict the host graph $G$ to be connected, we obtain the connected size Ramsey number, denoted as $\hat{r}_c(G_1,G_2)$. Erd\H{o}s and Faudree (1984) proved that $\hat r(nK_2,K_{1,m})=mn$ for all positive integers $m,n$. In this paper, we concentrate on the connected analog of this result. Rahadjeng, Baskoro, and Assiyatun (2016) provided the exact values of $\hat r_c(nK_2,K_{1,m})$ for $n=2,3$. We establish a more general result: for all positive integers $m$ and $n$ with $m\ge (n^2+2pn+n-3)/2$, we have $\hat r_c(nK_{1,p},K_{1,m})=n(m+p)-1$. As a corollary, $\hat r_c(nK_2,K_{1,m})=nm+n-1$ for $m\ge (n^2+3n-3)/2$. We also propose a conjecture for the interested reader.