Clustering of consecutive numbers in permutations under a Mallows distribution

Ross G. Pinsky

Published 2020-08-17Version 1

Let $A^{(n)}_{l;k}\subset S_n$ denote the set of permutations of $[n]$ for which the set of $l$ consecutive numbers $\{k, k+1,\cdots, k+l-1\}$ appears in a set of consecutive positions. Under the uniformly random measure $P_n$ on $S_n$, one has $P_n(A^{(n)}_{l;k})\sim\frac{l!}{n^{l-1}}$ as $n\to\infty$. In this paper we consider the probability of the clustering of consecutive numbers under Mallows distributions $P_n^q$, $q>0$. Because of a duality, it suffices to consider $q\in(0,1)$. In particular, we show that for fixed $q$, $\lim_{l\to\infty}\lim_{n\to\infty}P_n^q(A^{(n)}_{l;k_n})=\big(\prod_{i=1}^\infty(1-q^i)\big)^2,\ \text{if}\ \lim_{n\to\infty}\min(k_n,n-k_n)=\infty$, and that for $q_n=1-\frac c{n^\alpha}$, with $c>0$ and $\alpha\in(0,1)$, $P_n^q(A^{(n)}_{l;k_n})$ is on the order $\frac1{n^{\alpha(l-1)}}$, uniformly over all sequences $\{k_n\}_{n=1}^\infty$. Thus, letting $N^{(n)}_l=\sum_{k=1}^{n-l+1}1_{A^{(n)}_{l;k}}$ denote the number of sets of $l$ consecutive numbers appearing in sets of consecutive positions, we have \begin{equation*} \lim_{n\to\infty}E_n^{q_n}N^{(n)}_l=\begin{cases}\infty,\ \text{if}\ l<\frac{1+\alpha}\alpha;\\ 0,\ \text{if} \ l>\frac{1+\alpha}\alpha. \end{cases}. \end{equation*}