# Mixing of fast random walks on dynamic random permutations

## TL;DR

This paper studies how quickly a random walk on a dynamic random permutation reaches a uniform distribution, showing a sudden transition in mixing behavior.

## Contribution

The paper introduces a novel analysis of mixing times for random walks on dynamic permutations with coagulation and fragmentation dynamics.

## Key findings

- The total variation distance drops abruptly in a single jump after a random time.
- The post-jump distance follows a deterministic function related to Erdős–Rényi graph component sizes.
- Both coagulation-only and coagulation-fragmentation dynamics exhibit similar mixing behavior.

## Abstract

We analyse the mixing profile of a random walk on a dynamic random permutation, focusing on the regime where the walk evolves much faster than the permutation. Two types of dynamics generated by random transpositions are considered: one allows for coagulation of permutation cycles only, the other allows for both coagulation and fragmentation. We show that for both types, after scaling time by the length of the permutation and letting this length tend to infinity, the total variation distance between the current distribution and the uniform distribution converges to a limit process that drops down in a single jump. This jump is similar to a one-sided cut-off, occurs after a random time whose law we identify, and goes from the value 1 to a value that is a strictly decreasing and deterministic function of the time of the jump, related to the size of the largest component in Erdős–Rényi random graphs. After the jump, the total variation distance follows this function down to 0.