Somebody told me there exists a torsion-free abelian group $G$, such that $G$ is isomorphic to $G\oplus G\oplus G$, but $G$ is not isomorphic to $G\oplus G$. But he does not tell me such a concrete example, since he dose not know too.
I do not know if there is a well-known example about this. Could anyone give such an example? Or any references?