As part of my homework I was given a list of descriptions of groups and I need to determine which pairs are isomorphic. Here are two I am not sure about:
- The group $(\mathbb{Q}_+,\cdot)$ of positive rationals with multiplication
- The group $(\{\frac{n}{m}\mid m\text{ is odd}\},+)$ of rationals with odd denominators, with addition
Both groups are countable infinite. In both of them, only the identity is of finite order. I don't have other ideas how to distinguish them. As for trying to prove they're isomorphic, my intuition was to maybe look at prime factorizations of integers, but I couldn't make it work.
An answer or a hint would be appreciated.