
An Abelian group \(A\) is said to have self-cancellation if \(A\oplus A\cong A\oplus B\) implies \(A\cong B\). A very simple example of a rank 4 torsion-free Abelian group without the self-cancellation property is constructed. The construction is based on the author's criterion [Algebra Anal. 7, No. 6, 33-78 (1995); corrections ibid. 11, No. 4, 222-224 (1999; Zbl 0861.16011)] for an Abelian group \(A\) to have self-cancellation in terms of certain properties of a maximal order in the semi-simple algebra \(\mathbb{Q} R\), where \(R=\mathbb{E}(A)/N(\mathbb{E}(A))\), \(\mathbb{E}(A)\) is the endomorphism ring of \(A\) and \(N(\mathbb{E}(A))\) is the nil-radical of \(\mathbb{E}(A)\).
Torsion-free groups, finite rank, Separable algebras (e.g., quaternion algebras, Azumaya algebras, etc.), Direct sums, direct products, etc. for abelian groups, maximal orders in semi-simple algebras, self-cancellation, torsion-free Abelian groups of finite rank
Torsion-free groups, finite rank, Separable algebras (e.g., quaternion algebras, Azumaya algebras, etc.), Direct sums, direct products, etc. for abelian groups, maximal orders in semi-simple algebras, self-cancellation, torsion-free Abelian groups of finite rank
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