English

How permutations displace points and stretch intervals

Combinatorics 2015-09-21 v1

Abstract

Let SnS_n be the set of permutations on {1,,n}\{1,\,\dots,\,n\} and πSn\pi\in S_n. Let d(π)\mathrm{d}(\pi) be the arithmetic average of {iπ(i);  1in}\{|i-\pi(i)|;\;1\le i\le n\}. Then d(π)/n[0,1/2]\mathrm{d}(\pi)/n\in[0,\,1/2], the expected value of d(π)/n\mathrm{d}(\pi)/n approaches 1/31/3 as nn approaches infinity, and d(π)/n\mathrm{d}(\pi)/n is close to 1/31/3 for most permutations. We describe all permutations π\pi with maximal d(π)\mathrm{d}(\pi). Let s+(π)\mathrm{s}^+(\pi) and s(π)\mathrm{s}^*(\pi) be the arithmetic and geometric averages of {π(i)π(i+1);  1i<n}\{|\pi(i)-\pi(i+1)|;\;1\le i<n\}, and let M+M^+, MM^* be the maxima of s+\mathrm{s}^+ and s\mathrm{s}^* over SnS_n, respectively. Then M+=(2m21)/(2m1)M^+=(2m^2-1)/(2m-1) when n=2mn=2m, M+=(2m2+2m1)/(2m)M^+ = (2m^2+2m-1)/(2m) when n=2m+1n=2m+1, M=(mm(m+1)m1)1/(n1)M^* = (m^m(m+1)^{m-1})^{1/(n-1)} when n=2mn=2m, and, interestingly, M=(mm(m+1)(m+2)m1)1/(n1)M^* = (m^m(m+1)(m+2)^{m-1})^{1/(n-1)} when n=2m+1>1n=2m+1>1. We describe all permutations π\pi, σ\sigma with maximal s+(π)\mathrm{s}^+(\pi) and s(σ)\mathrm{s}^*(\sigma).

Keywords

Cite

@article{arxiv.1509.05649,
  title  = {How permutations displace points and stretch intervals},
  author = {Daniel Daly and Petr Vojtěchovský},
  journal= {arXiv preprint arXiv:1509.05649},
  year   = {2015}
}
R2 v1 2026-06-22T10:59:53.085Z