Maker-Breaker is solved in polynomial time on hypergraphs of rank 3
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In the Maker-Breaker positional game, Maker and Breaker take turns picking vertices of a hypergraph H, and Maker wins if and only if he claims all the vertices of some edge of H. This paper provides a general framework to study Maker-Breaker games, centered on the notion of danger at a vertex x, which is a subhypergraph representing an urgent threat that Breaker must hit with his next pick if Maker picks x. We then apply this concept in hypergraphs of rank 3, providing a structural characterization of the winner with perfect play as well as optimal strategies for both players based on danger intersections. We construct a family ℱ of dangers such that a hypergraph H of rank 3 is a Breaker win if and only if the ℱ-dangers at x in H intersect for all x. By construction of ℱ, this will mean that H is a Maker win if and only if Maker can guarantee the appearance, within the first three rounds of play, of a very specific elementary subhypergraph (on which Maker easily wins). This last result has a consequence on the algorithmic complexity of deciding which player has a winning strategy on a given hypergraph: this problem, which is known to be PSPACE-complete on 6-uniform hypergraphs, is in polynomial time on hypergraphs of rank 3. This validates a conjecture by Rahman and Watson. Another corollary of our result is that, if Maker has a winning strategy on a hypergraph of rank 3, then he can ensure to claim an edge in a number of rounds that is logarithmic in the number of vertices. Note: The present updated version of this deposit provides a counterexample to a similar result which was incorrectly claimed recently (arXiv:2209.11202).
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