English

How often is $x\mapsto x^3$ one-to-one in $\mathbb{Z}/n\mathbb{Z}$?

Number Theory 2025-04-21 v1

Abstract

We characterize the integers n such that xx3x\mapsto x^3 describes a bijection from the set Z/nZ\mathbb{Z}/n\mathbb{Z} to itself and we determine the frequency of these integers. Precisely, denoting by WW the set of these integers, we prove that an integer belongs to WW if and only if it is square-free with no prime factor that is congruent to 1 modulo 3, and that there exists C>0C>0 such that W{1,,n}Cnlogn .|W\cap\{1,\dots,n\}|\sim C\frac{n}{\sqrt{\log n}}\ . These facts (or equivalent facts) are stated without proof on the OEIS website. We give the explicit value of CC, which did not seem to be known. Analogous results are also proved for families of integers for which congruence classes for prime factors are imposed. The proofs are based on a Tauberian Theorem by Delange.

Keywords

Cite

@article{arxiv.2504.13511,
  title  = {How often is $x\mapsto x^3$ one-to-one in $\mathbb{Z}/n\mathbb{Z}$?},
  author = {Olivier Garet},
  journal= {arXiv preprint arXiv:2504.13511},
  year   = {2025}
}