The Energy of the Alphabet Model

We call Alphabet model a generalization to N types of particles of the classic ABC model. We have particles of different types stochastically evolving on a one-dimensional lattice with an exchange dynamics. The rates of exchange are local, but under suitable conditions the dynamics is reversible with a Gibbsian-like invariant measure with long-range interactions. We discuss geometrically the conditions of reversibility on a ring that correspond to a gradient condition on the graph of configurations or equivalently to a divergence-free condition on a graph structure associated with the types of particles. We show that much of the information on the interactions between particles can be encoded in associated Tournaments that are a special class of oriented directed graphs. In particular we show that the interactions of reversible models are corresponding to strongly connected tournaments. The possible minimizers of the energies are in correspondence with the Hamiltonian cycles of the tournaments. We can then determine how many and which are the possible minimizers of the energy looking at the structure of the associated tournament. As a by-product we obtain a probabilistic proof of a classic Theorem of Camion (C R Acad Sci Paris 249: 2151–2152, 1959) on the existence of Hamiltonian cycles for strongly connected tournaments. Using these results, we obtain in the case of an equal number of k types of particles new representations of the Hamiltonians in terms of translation invariant k-body long range interactions. We show that when k = 3, 4 the minimizer of the energy is always unique up to translations. Starting from the case k = 5, it is possible to have more than one minimizer. In particular, it is possible to have minimizers for which particles of the same type are not joined together in single clusters.