Additive subgroup isomorphic to multiplicative subgroup

field-theory

One can show (see here https://www.imc-math.org.uk/?year=2018&section=problems&item=prob2q) that there is no field $F$ such that
its additive and multiplicative groups $A$ and $M$ are isomorphic.

Here is a more general question. Are there a field $F$ and subgroups of $A$ and $M$ which are isomorphic. It certainly cannot happen for finite ones, as any additive subgroup has cardinality a power of a prime $p$, while the cardinality of the multiplicative one (and hence of any of its subgroups) is not divisible by $p$.

Any help appreciated!

Best Answer

Of course.

There trivial groups $\{0\}, +$ and $\{1\}, \times$ but we could have general cyclic groups with the same order.

Example in $\mathbb R$ then $<1,+> = \mathbb Z$ is isomorphic to $\{q^n|n\in \mathbb Z\}, \times$ (assuming $q>0; q\ne 1$).

Related Question