Countable subgroups of Euclidean Space

TL;DR

This paper investigates properties of countable subgroups in Euclidean spaces and Polish groups, disproving a conjecture about their generation and revealing new dense subgroup constructions with unique intersection properties.

Contribution

It proves that not all countable subgroups of R^omega are compactly generated, and introduces dense subgroups with intersections in discrete sets across various dimensions.

Findings

Counterexample to Beros's conjecture about R^omega

Existence of dense subgroups meeting every line or subspace in a discrete set

Construction of dense subgroups with specific intersection properties

Abstract

In his PhD Thesis Konstantinos Beros proved a number of results about compactly generated subgroups of Polish groups. Such a group is K-sigma - the countable union of compact sets. He notes that the group of rationals under addition with the discrete topology is an example of a Polish group which is K-sigma (since it is countable) but not compactly generated. Beros showed that for any Polish group G, every K-sigma subgroup of G is compactly generated iff every countable subgroup of G is compactly generated. Beros showed that any K-sigma subgroup of Z^omega (infinite product of the integers) is compactly generated and more generally, for any Polish group G, if every countable subgroup of G is finitely generated, then every countable subgroup of G^omega is compactly generated. In unpublished work Beros asked whether finitely generated may be replaced by compactly generated in his theorem. He conjectured that the reals R under addition might be an example such that every countable subgroup of R is compactly generated but not every countable subgroup of R^omega is compactly generated. We prove that this is not true. The general question remains open. In the course of our proof we came up with some interesting countable subgroups. We show that there is a dense subgroup of the plane which meets every line in a discrete set. Furthermore, for each n there is a dense subgroup of Euclidean space R^n which meets every (n-1)-dimensional subspace in a discrete set. Similarly there is a dense subgroup of R^omega which meets every finite dimensional subspace of R^omega in a discrete set.