Published online by Cambridge University Press: 20 November 2018
A U-ring is a ring in which every subring is a meta ideal. A meta ideal of a ring R is a subring I of R which lies in a chain of subrings,
with the properties:
(1) Iλ is an ideal of Iλ+1 for all λ < β;
(2) If α is a limit ordinal number, then Iα = ∪λ<αIλ.
Freidman [3] proved that every nil U-ring is a locally nilpotent ring. Since there are many locally nilpotent rings which are not U-rings, the class of locally nilpotent rings is not a very good bound for the class of nil U-rings. This paper establishes a new bound for nil U-rings based on a property of the multiplicative semigroup of the ring.