Every Abelian group can be related to an associative ring with an
identity element, the ring of all its endomorphisms. Recently the theory
of endomor- phism rings of Abelian groups has become a rapidly
developing area of algebra. On the one hand, it can be considered as a
part of the theory of Abelian groups; on the other hand, the theory can
be considered as a branch of the theory of endomorphism rings of modules
and the representation theory of rings. There are several reasons for
studying endomorphism rings of Abelian groups: first, it makes it
possible to acquire additional information about Abelian groups
themselves, to introduce new concepts and methods, and to find new
interesting classes of groups; second, it stimulates further develop-
ment of the theory of modules and their endomorphism rings. The theory
of endomorphism rings can also be useful for studies of the structure of
additive groups of rings, E-modules, and homological properties of
Abelian groups. The books of Baer [52] and Kaplansky [245] have
played an important role in the early development of the theory of
endomorphism rings of Abelian groups and modules. Endomorphism rings of
Abelian groups are much stu- died in monographs of Fuchs [170],
[172], and [173]. Endomorphism rings are also studied in the works
of Kurosh [287], Arnold [31], and Benabdallah [63].