How permutations displace points and stretch intervals
Combinatorics
2015-09-21 v1
Abstract
Let Sn be the set of permutations on {1,…,n} and π∈Sn. Let d(π) be the arithmetic average of {∣i−π(i)∣;1≤i≤n}. Then d(π)/n∈[0,1/2], the expected value of d(π)/n approaches 1/3 as n approaches infinity, and d(π)/n is close to 1/3 for most permutations. We describe all permutations π with maximal d(π). Let s+(π) and s∗(π) be the arithmetic and geometric averages of {∣π(i)−π(i+1)∣;1≤i<n}, and let M+, M∗ be the maxima of s+ and s∗ over Sn, respectively. Then M+=(2m2−1)/(2m−1) when n=2m, M+=(2m2+2m−1)/(2m) when n=2m+1, M∗=(mm(m+1)m−1)1/(n−1) when n=2m, and, interestingly, M∗=(mm(m+1)(m+2)m−1)1/(n−1) when n=2m+1>1. We describe all permutations π, σ with maximal s+(π) and s∗(σ).
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}
}