Abstract

Diluted mean-field models are spin systems whose geometry of interactions is induced by a sparse random graph or hypergraph. Such models play an eminent role in the statistical mechanics of disordered systems as well as in combinatorics and computer science. In a path-breaking paper based on the non-rigorous `cavity method', physicists predicted not only the existence of a replica symmetry breaking phase transition in such models but also sketched a detailed picture of the evolution of the Gibbs measure within the replica symmetric phase and its impact on important problems in combinatorics, computer science and physics [Krzakala et al.: PNAS 2007]. In this paper we rigorise this picture completely for a broad class of models, encompassing the Potts antiferromagnet on the random graph, the $k$-XORSAT model and the diluted $k$-spin model for even $k$. We also prove a conjecture about the detection problem in the stochastic block model that has received considerable attention [Decelle et al.: Phys. Rev. E 2011].

Highlights

  • Models based on random graphs have come to play a role in combinatorics, probability, statistics and computer science that can hardly be overstated

  • The random k-SAT model is of fundamental interest in computer science [4], the stochastic block model has gained prominence in statistics [1, 24, 36], low-density parity check codes have become a

  • Very similar models have been studied in statistical physics as models of disordered systems [31] and over the past 20 years physicists developed an analytic but non-rigorous technique for the study of such models called the ‘cavity method’

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Summary

The cavity method

Models based on random graphs have come to play a role in combinatorics, probability, statistics and computer science that can hardly be overstated. Very similar models have been studied in statistical physics as models of disordered systems [31] and over the past 20 years physicists developed an analytic but non-rigorous technique for the study of such models called the ‘cavity method’ This nonrigorous approach has inspired numerous “predictions” with an impact on an astounding variety of problems (e.g., [15, 31, 33, 42]). While the cavity method can be applied almost mechanically to a wide variety of problems, most rigorous arguments still hinge on model-specific deliberations, a state of affairs that begs the questions of whether we can rigorise the physics calculations wholesale. This is the thrust of the present paper.

The Potts antiferromagnet
The stochastic block model
The diluted k-spin model
Definitions and assumptions
Results
Proof strategy
Two moments do not suffice
The condensation phase transition and the overlap
The Kesten-Stigum bound
Second moment redux
Virtuous cycles

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