The study of rings over which the direct sums of modules have
certain properties is a well recognized topic for research in Ring
Theory and Homological Algebra. In this dissertation, the class of
rings over which every essential extension of a direct sum of
simple right modules is a direct sum of quasi-injective right
modules is studied. It is shown that under this condition on a ring
R, (i) R must be directly finite, (ii) R has bounded index of
nilpotence if R is also right non-singular, and (iii) R is
right noetherian when R is semi-regular in the sense that
R/J(R) is a von-Neumann regular ring.
This dissertation initiates the study of rings having the
property that each right ideal is a finite direct sum of
quasi-injective right ideals. These rings have been named as right
Nakayama-Fuller rings (in short,
NF-rings). Prime right self-injective right
NF- rings are shown to be simple artinian. Right
artinian right non-singular right NF- rings are shown
to be upper triangular block matrix rings over rings which are
either zero rings or division rings. Examples are provided to show
that the NF- rings are not left-right symmetric nor
they are Morita invariant.
Carl Faith, Cailleau, Megibben and others have studied Σ-injective module M in the sense that
every direct sum of copies of M is injective. In this
dissertation, a new characterization for an injective module to be
Σ-injective has been provided. This leads to a new
characterization of right noetherian rings which generalizes
results of Cartan-Eilenberg, Bass and Beidar et al.
Zelinsky proved that every element in the ring of linear
transformations of a vector space V over a division ring D is a
sum of two units unless dim V = 1 and D = ℤ2.
Zelinsky's result has been extended to include all the previous
known results by proving that every element of a right
self-injective ring R is a sum of two units if and only if R
has no factor ring isomorphic to ℤ2, thus answering a
long-standing question on a characterization of right
self-injective ring generated by units.