English

Permutations with arithmetic constraints

Number Theory 2022-06-07 v2 Combinatorics

Abstract

Let Slcm(n)S_{\rm lcm}(n) denote the set of permutations π\pi of [n]={1,2,,n}[n]=\{1,2,\dots,n\} such that lcm[j,π(j)]n{\rm lcm}[j,\pi(j)]\le n for each j[n]j\in[n]. Further, let Sdiv(n)S_{\rm div}(n) denote the number of permutations π\pi of [n][n] such that jπ(j)j\mid\pi(j) or π(j)j\pi(j)\mid j for each j[n]j\in[n]. Clearly Sdiv(n)Slcm(n)S_{\rm div}(n)\subset S_{\rm lcm}(n). We get upper and lower bounds for the counts of these sets, showing they grow geometrically. We also prove a conjecture from a recent paper on the number of "anti-coprime" permutations of [n][n], meaning that each gcd(j,π(j))>1\gcd(j,\pi(j))>1 except when j=1j=1.

Keywords

Cite

@article{arxiv.2206.01699,
  title  = {Permutations with arithmetic constraints},
  author = {Carl Pomerance},
  journal= {arXiv preprint arXiv:2206.01699},
  year   = {2022}
}