A probabilistic cellular automaton that admits no successful basic i.i.d. coupling

In this paper, we revisit a classic example of probabilistic cellular automaton (PCA) on {0, 1} Z , namely, addition modulo 2 of the states of the left-and right-neighbouring cells, followed by either preserving the result of the addition, with probability p, or flipping it, with probability 1 -- p. It is well-known that, for any value of p $\in$]0, 1[, this PCA is ergodic. We show that, for p sufficiently close to 1, no coupling of the PCA dynamics based on the composition of i.i.d. random functions of nearest-neighbour states (we call this a basic i.i.d. coupling), can be successful, where successful means that, for any given cell, the probability that every possible initial condition leads to the same state after t time steps, goes to 1 as t goes to infinity. In particular, this precludes the possibility of a CFTP scheme being based on such a coupling. This property stands in sharp contrast with the case of monotone PCA, for which, as soon as ergodicity holds, there exists a successful basic i.i.d. coupling.

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