Strong Ramsey game on two boards

Abstract

The strong Ramsey game $R(\mathcal{B},H)$ is a two-player game played on a graph $\mathcal{B}$, referred to as the board, with a target graph $H$. In this game, two players, $P_1$ and $P_2$, alternately claim unclaimed edges of $\mathcal{B}$, starting with $P_1$. The goal is to claim a subgraph isomorphic to $H$, with the first player achieving this declared the winner. A fundamental open question, persisting for over three decades, asks whether there exists a graph $H$ such that in the game $R(K_n,H)$, $P_1$ does not have a winning strategy in a bounded number of moves as $n\to \infty$. In this paper, we shift the focus to the variant $R(K_n\bigsqcup K_n,H)$, introduced by David, Hartarsky, and Tiba, where the board $K_n\bigsqcup K_n$ consists of two disjoint copies of $K_n$. We prove that there exist infinitely many graphs $H$ such that $P_1$ cannot win in $R(K_n\bigsqcup K_n,H)$ within a bounded number of moves through a concise proof. This perhaps provides evidence for the existence of examples to the above longstanding open problem.

12 pages