Semigroup rings and semilattice sums of rings
Proceedings of the American Mathematical Society
A generalization of the concept of a decomposition of a ring into a direct sum of ideals is introduced. The question of semisimplicity of the ring in terms of the semisimplicity of its summands is investigated. The results are applied to semigroup rings. Introduction. Many substantial results in the theory of rings either concern themselves with or use a decomposition of the ring into a direct sum of ideals. In this paper we introduce a generalization of direct sum decompositions. A ring is a
... ions. A ring is a supplementary semilattice sum of subrings if as an additive abelian group it is a direct sum of the additive groups of the subrings and if the subrings multiply in a certain natural way. We begin the investigation of semilattice sum decompositions by showing that in certain cases the semisimplicity of the subrings implies the semisimplicity of the ring. The investigation is carried out in the context of 7r-semisimplicity where 77 is a hereditary, homomorphic invariant property of rings and thus the result holds for Jacobson semisimplicity, nil semisimplicity and nilpotent semisimplicity (semiprime). The results are then applied to semigroup rings. We prove that if R is a commutative ring with identity and D is a commutative semigroup such that a power of each element lies in a subgroup then the semigroup ring RD is semiprime if and only if D is a semilattice of groups Ga, a e Q, and RGa is semiprime for every a e O. This generalizes Theorem 5.21 of Clifford and Preston . In the paper subsequent to this one Janeski and the author study regularity of semilattice sum decompositions.