# Group of rational numbers with square-free denominators

## Definition

The group of rational numbers with square-free denominators is defined in the following equivalent ways:

1. It is the subgroup of the group of rational numbers comprising those rational numbers whose denominators are square-free numbers, i.e., no square of a prime should divide the denominator.
2. It is the join of the following subgroups of the group of rational numbers: for each prime number $p$, consider the cyclic subgroup generated by the element $1/p$.
3. As an abstract group, it is isomorphic to the quotient group of the restricted external direct product of countably many copies of the group of integers, with the copies indexed by prime numbers, by the identification of the prime times the group generator. Explicitly, if $p_1,p_2,\dots$ are the primes and $\mathbb{Z}_1,\mathbb{Z}_2,\dots$ are the direct factors for the respective $p_i$s, we want to set the $p_i \in \mathbb{Z}_i$ to equal the $p_j \in \mathbb{Z}_j$ for all $i,j$.