Percolation games, probabilistic cellular automata, and the hard-core model


Let each site of the square lattice \(\mathbb {Z}^2\) be independently assigned one of three states: a trap with probability p, a target with probability q, and open with probability \(1-p-q\), where \(0<p+q<1\). Consider the following game: a token starts at the origin, and two players take turns to move, where a move consists of moving the token from its current site x to either \(x+(0,1)\) or \(x+(1,0)\). A player who moves the token to a trap loses the game immediately, while a player who moves the token to a target wins the game immediately. Is there positive probability that the game is drawn with best play—i.e. that neither player can force a win? This is equivalent to the question of ergodicity of a certain family of elementary one-dimensional probabilistic cellular automata (PCA). These automata have been studied in the contexts of enumeration of directed lattice animals, the golden-mean subshift, and the hard-core model, and their ergodicity has been noted as an open problem by several authors. We prove that these PCA are ergodic, and correspondingly that the game on \(\mathbb {Z}^2\) has no draws. On the other hand, we prove that certain analogous games do exhibit draws for suitable parameter values on various directed graphs in higher dimensions, including an oriented version of the even sublattice of \(\mathbb {Z}^d\) in all \(d\ge 3\). This is proved via a dimension reduction to a hard-core lattice gas in dimension \(d-1\). We show that draws occur whenever the corresponding hard-core model has multiple Gibbs distributions. We conjecture that draws occur also on the standard oriented lattice \(\mathbb {Z}^d\) for \(d\ge 3\), but here our method encounters a fundamental obstacle.

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JBM was supported by EPSRC Fellowship EP/E060730/1. IM was supported by the Fondation Sciences Mathématiques de Paris. We are grateful to two referees for valuable comments. In particular, the proof of Theorem 2(ii) was prompted by a referee’s observation that such an approach gives a straightforward proof of the \(q=0\) case of Theorem 1.

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Correspondence to Irène Marcovici.

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Holroyd, A.E., Marcovici, I. & Martin, J.B. Percolation games, probabilistic cellular automata, and the hard-core model. Probab. Theory Relat. Fields 174, 1187–1217 (2019).

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  • Combinatorial game
  • Percolation
  • Probabilistic cellular automaton
  • Ergodicity
  • Hard-core model

Mathematics Subject Classification

  • 05C57
  • 60K35
  • 37B15