## The Annals of Probability

- Ann. Probab.
- Volume 46, Number 2 (2018), 1114-1169.

### On the cycle structure of Mallows permutations

#### Abstract

We study the length of cycles of random permutations drawn from the Mallows distribution. Under this distribution, the probability of a permutation $\pi\in\mathbb{S}_{n}$ is proportional to $q^{\operatorname{inv}(\pi)}$ where $q>0$ and $\operatorname{inv}(\pi)$ is the number of inversions in $\pi$.

We focus on the case that $q<1$ and show that the expected length of the cycle containing a given point is of order $\min\{(1-q)^{-2},n\}$. This marks the existence of two asymptotic regimes: with high probability, when $n$ tends to infinity with $(1-q)^{-2}\ll n$ then all cycles have size $o(n)$ whereas when $n$ tends to infinity with $(1-q)^{-2}\gg n$ then macroscopic cycles, of size proportional to $n$, emerge. In the second regime, we prove that the distribution of normalized cycle lengths follows the Poisson–Dirichlet law, as in a uniformly random permutation. The results bear formal similarity with a conjectured localization transition for random band matrices.

Further results are presented for the variance of the cycle lengths, the expected diameter of cycles and the expected number of cycles. The proofs rely on the exact sampling algorithm for the Mallows distribution and make use of a special diagonal exposure process for the graph of the permutation.

#### Article information

**Source**

Ann. Probab., Volume 46, Number 2 (2018), 1114-1169.

**Dates**

Received: January 2016

Revised: May 2017

First available in Project Euclid: 9 March 2018

**Permanent link to this document**

https://projecteuclid.org/euclid.aop/1520586277

**Digital Object Identifier**

doi:10.1214/17-AOP1202

**Mathematical Reviews number (MathSciNet)**

MR3773382

**Zentralblatt MATH identifier**

06864081

**Subjects**

Primary: 60C05: Combinatorial probability 05A05: Permutations, words, matrices

Secondary: 60F05: Central limit and other weak theorems 60K35: Interacting random processes; statistical mechanics type models; percolation theory [See also 82B43, 82C43] 82B23: Exactly solvable models; Bethe ansatz 82B26: Phase transitions (general) 60B99: None of the above, but in this section 60B20: Random matrices (probabilistic aspects; for algebraic aspects see 15B52)

**Keywords**

Mallows permutations cycle structure Poisson–Dirichlet law phase transition macroscopic cycles localization delocalization random band matrices

#### Citation

Gladkich, Alexey; Peled, Ron. On the cycle structure of Mallows permutations. Ann. Probab. 46 (2018), no. 2, 1114--1169. doi:10.1214/17-AOP1202. https://projecteuclid.org/euclid.aop/1520586277