Nike Sun

Factor models on locally tree-like graphs

We consider homogeneous factor models on uniformly sparse graph sequences converging locally to a (unimodular) random tree T, and study the existence of the free energy density ϕ, the limit of the log-partition function divided by the number of vertices n as n tends to infinity. We provide a new interpolation scheme and use it to prove existence of, and to explicitly compute, the quantity ϕ subject to uniqueness of a relevant Gibbs measure for the factor model on T. By way of example we compute ϕ for the independent set (or hard-core) model at low fugacity, for the ferromagnetic Ising model at all parameter values, and for the ferromagnetic Potts model with both weak enough and strong enough interactions. Even beyond uniqueness regimes our interpolation provides useful explicit bounds on ϕ. In the regimes in which we establish existence of the limit, we show that it coincides with the Bethe free energy functional evaluated at a suitable fixed point of the belief propagation (Bethe) recursions on T. In the special case that T has a Galton–Watson law, this formula coincides with the nonrigorous “Bethe prediction” obtained by statistical physicists using the “replica” or “cavity” methods. Thus our work is a rigorous generalization of these heuristic calculations to the broader class of sparse graph sequences converging locally to trees. We also provide a variational characterization for the Bethe prediction in this general setting, which is of independent interest.

Maximum independent sets on random regular graphs

We determine the asymptotics of the independence number of the random d-regular graph for all

Conformally invariant scaling limits in planar critical percolation

{d\geq d_0}

Strong path convergence from Loewner driving function convergence

d≥d0. It is highly concentrated, with constant-order fluctuations around

The Computational Hardness of Counting in Two-Spin Models on d-Regular Graphs

{n\alpha_*-c_*\log n}

Central limit theorem for biased random walk on multi-type Galton-Watson trees

nα∗-c∗logn for explicit constants