Torsion-free Abelian groups G and H are called quasi-equal (G ≈ H) if λG is contained in H is contained in G for a certain natural number λ. It is known (see [3]) that the quasi-equality of torsion-free Abelian groups can be represented as the equality in an appropriate factor category. Thus while dealing with certain group properties it is usual to prove that the property under consideration is preserved under the transition to a quasi-equal group. This trick is especially frequently used when the author investigates module properties of Abelian groups; here a group is considered as a left module over its endomorphism ring. On the other hand, a topical problem in the Abelian group theory is the problem of investigation of pureness in the category of Abelian groups (see [4]). We consider the pureness introduced by P. Cohn for Abelian groups as modules over their endomorphism rings. Particularity of the investigation of the properties of pureness for the Abelian group G as the module _(E(G))G lies in the fact that this is a more general situation than the investigation of pureness for a unitary module over an arbitrary ring R with the identity element. Indeed, if _RM is an arbitrary unitary left module and M~+ is its Abelian group, then each element from R can be identified with an appropriate endomorphism from the ring E(M~+) under the canonical ring homomorphism R → E(M~+). Then it holds that if _(E(M~+))N is a pure submodule in E(M~+)M~+, then _RN is a pure submodule in _RM. In the present paper the interrelations between pureness, servantness, and quasi-decompositions for Abelian torsion-free groups of finite rank will be investigated.
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