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On the Classification Problem for Rank 2 Torsion‐Free Abelian Groups

We study here some foundational aspects of the classification problem for torsion‐free abelian groups of finite rank. These are, up to isomorphism, the subgroups of the additive groups (Qn, +), for some n = 1, 2, 3,…. The torsion‐free abelian groups of rank ⩽ n are the subgroups of (Qn, +).