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for commutative rings with identity
Authors:John Lawrence   Boza Tasic
Affiliation:University of Waterloo, Department of Pure Mathematics, Waterloo, Ontario, Canada N2L 3G1 ; University of Waterloo, Department of Pure Mathematics, Waterloo, Ontario, Canada N2L 3G1
Abstract:Let $I$, $H$, $S$, $P$, $P_s$ be the usual operators on classes of rings: $I$ and $H$ for isomorphic and homomorphic images of rings and $S$, $P$, $P_s$ respectively for subrings, direct, and subdirect products of rings. If $mathcal K$ is a class of commutative rings with identity (and in general of any kind of algebraic structures), then the class $HSP({mathcal K})$ is known to be the variety generated by the class $mathcal K$. Although the class $SHPS({mathcal K})$ is in general a proper subclass of the class $HSP({mathcal K})$ for many familiar varieties $HSP({mathcal K})= SHPS({mathcal K})$. Our goal is to give an example of a class $mathcal K$ of commutative rings with identity such that $HSP({mathcal K})not = SHPS({mathcal K})$. As a consequence we will describe the structure of two partially ordered monoids of operators.

Keywords:Class operators   commutative rings with identity   partially ordered monoid
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