Thermodynamic Limit for the Mallows Model on $S_n$

The Mallows model on $S_n$ is a probability distribution on permutations, $q^{d(π,e)}/P_n(q)$, where $d(π,e)$ is the distance between $π$ and the identity element, relative to the Coxeter generators. Equivalently, it is the number of inversions: pairs $(i,j)$ where $1\leq i<j\leq n$, but $π_i>π_j$. Analyzing the normalization $P_n(q)$, Diaconis and Ram calculated the mean and variance of $d(π,e)$ in the Mallows model, which suggests the appropriate $n \to \infty$ limit has $q_n$ scaling as $1-β/n$. We calculate the distribution of the empirical measure in this limit, $u(x,y) dx dy = \lim_{n \to \infty} \frac{1}{n} \sum_{i=1}^{n} δ_{(i,π_i)}$. Treating it as a mean-field problem, analogous to the Curie-Weiss model, the self-consistent mean-field equations are $\frac{\partial^2}{\partial x \partial y} \ln u(x,y) = 2 βu(x,y)$, which is an integrable PDE, known as the hyperbolic Liouville equation. The explicit solution also gives a new proof of formulas for the blocking measures in the weakly asymmetric exclusion process, and the ground state of the $\mathcal{U}_q(\mathfrak{sl}_2)$-symmetric XXZ ferromagnet.

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