Off-diagonal online size Ramsey numbers for paths

Abstract: Consider the following Ramsey game played on the edge set of $K_{\mathbb N}$. In every round, Builder selects an edge and Painter colours it red or blue. Builder's goal is to force Painter to create a red copy of a path $P_k$ on $k$ vertices or a blue copy of $P_n$ as soon as possible. The online (size) Ramsey number $\tilde{r}(P_k,P_n)$ is the number of rounds in the game provided Builder and Painter play optimally. We prove that $\tilde{r}(P_k,P_n)\le (5/3+o(1))n$ provided $k=o(n)$ and $n\to \infty$. We also show that $\tilde{r}(P_4,P_n)\le \lceil 7n/5\rceil -1$ for $n\ge 10$, which improves the upper bound obtained by J.~Cyman, T.~Dzido, J.~Lapinskas, and A.~Lo and implies their conjecture that $\tilde{r}(P_4,P_n)=\lceil 7n/5\rceil -1$.