The geometry of random minimal factorizations of a long cycle via biconditioned bitype random trees

TL;DR

This paper investigates the geometric structure of random minimal factorizations of long cycles into transpositions, revealing a phase transition and convergence to a family of random laminations linked to Lévy process excursions.

Contribution

It introduces a novel connection between minimal factorizations, laminations, and conditioned two-type BGW trees, establishing new limit theorems and phase transition phenomena.

Findings

Identification of a phase transition at order √n transpositions.

Convergence of laminations to a family constructed from Lévy excursions.

Development of limit theorems for conditioned two-type BGW trees.

Abstract

We study random typical minimal factorizations of the $n$-cycle into transpositions, which are factorizations of $(1, \ldots,n)$ as a product of $n-1$ transpositions. By viewing transpositions as chords of the unit disk and by reading them one after the other, one obtains a sequence of increasing laminations of the unit disk (i.e. compact subsets of the unit disk made of non-intersecting chords). When an order of $\sqrt{n}$ consecutive transpositions have been read, we establish, roughly speaking, that a phase transition occurs and that the associated laminations converge to a new one-parameter family of random laminations, constructed from excursions of specific L\'evy processes. Our main tools involve coding random minimal factorizations by conditioned two-type Bienaym\'e--Galton--Watson trees. We establish in particular limit theorems for two-type BGW trees conditioned on having given numbers of vertices of both types, and with an offspring distribution depending on the conditioning size. We believe that this could be of independent interest.