In an extremely influential paper Mézard and Parisi put forward an analytic but non-rigorous approach called the cavity method for studying spin systems on the Bethe lattice, i.e., the random d -regular graph [Eur. Phys. J. B 20 (2001) 217-233]. Their technique was based on certain hypotheses; most importantly, that the phase space decomposes into a number of Bethe states that are free from long-range correlations and whose marginals are given by a recurrence called Belief Propagation. In this paper we establish this decomposition rigorously for a very general family of spin systems. In addition, we show that the free energy can be computed from this decomposition. We also derive a variational formula for the free energy. The general results have interesting ramifications on several special cases.
MSC: 05C80The following theorem establishes these conjectures rigorously. We say that G enjoys a property with high probability ('w.h.p.') if the probability that the property holds tends to one as n → ∞.Theorem 1.1. For any d ≥ 3, β > 0 the following is true. Let L = L(n) → ∞ be any integer sequence that tends to infinity. Then there exists a decomposition S 0 = S 0 (G), S 1 = S 1 (G), . . . , S ℓ = S ℓ (G), ℓ = ℓ(G) ≤ L, of the phase space {±1} n into non-empty sets such that µ G (S 0 ) = o(1) and such that with high probability (1.3)-(1.5) are satisfied for h = 1, . . . , ℓ.