English

Reductions of points on algebraic groups

Number Theory 2023-06-22 v2

Abstract

Let AA be the product of an abelian variety and a torus defined over a number field KK. Fix some prime number \ell. If αA(K)\alpha \in A(K) is a point of infinite order, we consider the set of primes p\mathfrak p of KK such that the reduction (αmodp)(\alpha \bmod \mathfrak p) is well-defined and has order coprime to \ell. This set admits a natural density. By refining the method of R.~Jones and J.~Rouse (2010), we can express the density as an \ell-adic integral without requiring any assumption. We also prove that the density is always a rational number whose denominator (up to powers of \ell) is uniformly bounded in a very strong sense. For elliptic curves, we describe a strategy for computing the density which covers every possible case.

Keywords

Cite

@article{arxiv.1612.02847,
  title  = {Reductions of points on algebraic groups},
  author = {Davide Lombardo and Antonella Perucca},
  journal= {arXiv preprint arXiv:1612.02847},
  year   = {2023}
}

Comments

v2: we show that the densities we are interested in are rational numbers and prove results on the structure of their denominators. References updated to reflect small changes in https://arxiv.org/abs/1612.02845

R2 v1 2026-06-22T17:18:01.545Z