Permutons, meanders, and SLE-decorated Liouville quantum gravity

TL;DR

This paper introduces a class of random permutons derived from SLE curves on Liouville quantum gravity surfaces, analyzing their properties, connections to pattern-avoiding permutations, and conjecturing their relation to meandric permutations.

Contribution

It constructs and studies a new class of random permutons from SLE and LQG, proving sublinear longest increasing subsequence length and Hausdorff dimension results, and establishing properties of the meandric permuton.

Findings

Longest increasing subsequence is sublinear for these permutons.

The support of each permuton has Hausdorff dimension one.

Re-rooting invariance and explicit pattern density formulas are established.

Abstract

We study a class of random permutons which can be constructed from a pair of space-filling Schramm-Loewner evolution (SLE) curves on a Liouville quantum gravity (LQG) surface. This class includes the skew Brownian permutons introduced by Borga (2021), which describe the scaling limit of various types of random pattern-avoiding permutations. Another interesting permuton in our class is the meandric permuton, which corresponds to two independent SLE$_8$ curves on a $\gamma$-LQG surface with $\gamma = \sqrt{\frac13 \left( 17 - \sqrt{145} \right)}$. Building on work by Di Francesco, Golinelli, and Guitter (2000), we conjecture that the meandric permuton describes the scaling limit of uniform meandric permutations, i.e., the permutations induced by a simple loop in the plane which crosses a line a specified number of times. We show that for any sequence of random permutations which converges to one of the above random permutons, the length of the longest increasing subsequence is sublinear. This proves that the length of the longest increasing subsequence is sublinear for Baxter, strong-Baxter, and semi-Baxter permutations and leads to the conjecture that the same is true for meandric permutations. We also prove that the closed support of each of the random permutons in our class has Hausdorff dimension one. Finally, we prove a re-rooting invariance property for the meandric permuton and write down a formula for its expected pattern densities in terms of LQG correlation functions (which are known explicitly) and the probability that an SLE$_8$ hits a given set of points in numerical order (which is not known explicitly). We conclude with a list of open problems.