Random generation and scaling limits of fixed genus factorizations into transpositions

TL;DR

This paper investigates the asymptotic properties of random factorizations of large cycles into transpositions with fixed genus, introducing efficient sampling algorithms and analyzing their limiting behavior through probabilistic processes linked to Brownian excursions.

Contribution

It provides a linear-time sampling algorithm for genus g factorizations and characterizes their asymptotic behavior via convergence to processes derived from Brownian excursions.

Findings

Efficient linear-time sampling algorithm for fixed genus factorizations.

Convergence of the factorization process to a limit process from Brownian excursions.

Establishment of a genus process tracking the appearance of genera in factorizations.

Abstract

We study the asymptotic behaviour of random factorizations of the $n$-cycle into transpositions of fixed genus $g>0$. They have a geometric interpretation as branched covers of the sphere and their enumeration as Hurwitz numbers was extensively studied in algebraic combinatorics and enumerative geometry. On the probabilistic side, several models and properties of permutation factorizations were studied in previous works, in particular minimal factorizations of cycles into transpositions (which corresponds to the case $g=0$ of this work). Using the representation of factorizations via unicellular maps, we first exhibit an algorithm which samples an asymptotically uniform factorization of genus $g$ in linear time. In a second time, we code a factorization as a process of chords appearing one by one in the unit disk, and we prove the convergence (as $n\to\infty$) of the process associated with a uniform genus $g$ factorization of the $n$-cycle. The limit process can be explicitly constructed from a Brownian excursion. Finally, we establish the convergence of a natural genus process, coding the appearance of the successive genera in the factorization.