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On some multicolour Ramsey properties of random graphs

The size-Ramsey number $\hat{R}(F)$ of a graph $F$ is the smallest integer $m$ such that there exists a graph $G$ on $m$ edges with the property that any colouring of the edges of $G$ with two colours yields a monochromatic copy of $F$. In this paper, first we focus on the size-Ramsey number of a path $P_n$ on $n$ vertices. In particular, we show that $5n/2-15/2 \le \hat{R}(P_n) \le 74n$ for $n$ sufficiently large. (The upper bound uses expansion properties of random $d$-regular graphs.) This improves the previous lower bound, $\hat{R}(P_n) \ge (1+\sqrt{2})n-O(1)$, due to Bollobás, and the upper bound, $\hat{R}(P_n) \le 91n$, due to Letzter. Next we study long monochromatic paths in edge-coloured random graph $G(n,p)$ with $pn \to \infty$. Let $α> 0$ be an arbitrarily small constant. Recently, Letzter showed that a.a.s.\ any $2$-edge colouring of $G(n,p)$ yields a monochromatic path of length $(2/3-α)n$, which is optimal. Extending this result, we show that a.a.s.\ any $3$-edge colouring of $G(n,p)$ yields a monochromatic path of length $(1/2-α)n$, which is also optimal. In general, we prove that for $r\ge 4$ a.a.s.\ any $r$-edge colouring of $G(n,p)$ yields a monochromatic path of length $(1/r-α)n$. We also consider a related problem and show that for any $r \ge 2$, a.a.s.\ any $r$-edge colouring of $G(n,p)$ yields a monochromatic connected subgraph on $(1/(r-1)-α)n$ vertices, which is also tight.