An almost completely decomposable abelian (acd) group is an extension of a finite direct sum of subgroups of the additive group of rational numbers by a finite abelian group. Examples are easy to write and are frequently used but have been notoriously difficult to study and classify because of their computational nature. However, a general theory of acd groups has been developed and a suitable weakening of isomorphism, Lady's near-isomorphism, has been established as the rightconcept for studying acd groups. A number of important classes of acd groups has been successfully classified. Direct sum decompositions of acd groups are preserved under near-isomorphism and the well-known pathological decompositions can actually be surveyed in special cases.
Table of Contents
1. Notation and Background 2. Basics and Completely Decomposable Groups 3. Cyclic Essential Extensions 4. Regulating Subgroups and Regulators 5. Local-Global Relationships 6. Groups with Cyclic Regulating 7. Completely Decomposable Summands 8. Anti-Representations 8. Near-Isomorphism and Type-Isomorphism 9. Fundamental Decomposition Theorems 10. Finite Essential Extensions 11. Representing Matrices 12. Classification 13. Direct Decompositions of Block-Rigid crq-Groups 14. In Search for Good Categories 15. Associated Structures 16. Literature