Ramsey-type results for Maker-Breaker games

Abstract

A graph $G$ is minimal Ramsey for a graph $H$ if every $2$-colouring of the edges of $G$ contains a monochromatic copy of $H$, but for every proper subgraph of $G$, there is a $2$-colouring that does not contain such a monochromatic copy. Characterizing minimal Ramsey graphs is a widely studied problem. Recent research in this field includes the characterization of the size of the set $\mathcal{M}_2(H)$ of all minimal Ramsey graphs for $H$, or finding the smallest minimum degree among all graphs in $\mathcal{M}_2(H)$. In this paper, we introduce a game theoretic analogue of the above concept by considering the Maker-Breaker $H$-game on a graph $G$. In this game, two players, Maker and Breaker, alternately claim unclaimed edges of $G$, and Maker wins if in the end of the game the graph spanned by Maker's edges contains a copy of $H$. Otherwise, Breaker wins the game. We call a graph $G$ winnable for $H$ if Maker has a winning strategy for the Maker-Breaker $H$-game on $G$, and we call it minimal winnable if additionally Breaker wins the $H$-game on every proper subgraph of $G$. Along the lines of minimal-Ramsey theory, we characterize all graphs $H$ for which there exist infinitely many minimal winnable graphs, and we prove tight bounds for the smallest minimum degree among all these minimal winnable graphs. Amongst others, we obtain precise results for trees, cycles, cliques, and complete bipartite graphs. In general, we find many similarities between the Ramsey setting and the Maker-Breaker setting, but we also show substantial differences.