If $G$ is an infinite abelian group such that $G \cong H$ for every non trivial subgroup $H$ of $G$ , then is $G$ cyclic , or equivalently asking , then is $[G:H]$ finite for every non trivial subgroup $H$ of $G$ ?
I know that the assumption holds for $G=\mathbb Z$ , so I am asking is this the only group upto isomorphism for which the stated assumption holds . Also I know that if for an infinite group $G$ , $[G:H]$ is finite for every non trivial subgroup of $H$ then $G$ is cyclic .
Please help . Thanks in advance
Let $x\in G$, $x\ne0$; then $\langle x\rangle$ is a (non trivial) cyclic subgroup of $G$. Since $\langle x\rangle\cong G$, we have that $G$ is cyclic.