English

Bad places for the approximation property for finite groups

Number Theory 2021-01-12 v2

Abstract

Given a number field kk and a finite kk-group GG, the Tame Approximation Problem for GG asks whether the restriction map H1(k,G)vΣH1(kv,G)H^1(k,G)\to\prod_{v\in\Sigma}H^1(k_v,G) is surjective for every finite set of places ΣΩk\Sigma\subseteq\Omega_k disjoint from BadG\text{Bad}_G, where BadG\text{Bad}_G is the finite set of places that either divides the order of GG or ramifies in the minimal extension splitting GG. In this paper we prove that the set BadG\text{Bad}_G is "sharp". To achieve this we prove that there are finite abelian kk-groups AA where the map H1(k,A)vΣ0H1(kv,A)H^1(k,A)\to\prod_{v\in\Sigma_0}H^1(k_v,A) is not surjective in a set Σ0BadA\Sigma_0\subseteq\text{Bad}_A with particular properties, namely Σ0\Sigma_0 is the set of places that do not divide the order of AA and ramify in the minimal extension splitting AA.

Keywords

Cite

@article{arxiv.2009.00691,
  title  = {Bad places for the approximation property for finite groups},
  author = {Felipe Rivera-Mesas},
  journal= {arXiv preprint arXiv:2009.00691},
  year   = {2021}
}

Comments

9 pages

R2 v1 2026-06-23T18:15:05.426Z