${\rm cl}$-prereductions, ${\rm i}$-postexpansions, and related structures

Abstract

Expanding on the work of Kemp, Ratliff and Shah, for any closure ${\rm cl}$ defined on a class of modules over a Noetherian ring, we develop the theory of ${\rm cl}$-prereductions of submodules. For any interior ${\rm i}$ on a class of $R$-modules, we also develop the theory of {\rm i}-postexpansions. Using the duality of Epstein, R.G. and Vassilev, we show that if ${\rm i}$ is the interior dual to ${\rm cl}$, then these notions are in fact dual to each other. We consider the ${\rm cl}$-precore (${\rm i}$-postcore), the intersection of all ${\rm cl}$-prereductions ${\rm i}$-postexpansions) of a submodule and the ${\rm cl}$-prehull (${\rm i}$-posthull), the sum of all ${\rm cl}$-prereductions (${\rm i}$-postexpansions) of a submodule and give comparisons with the ${\rm cl}$-core (${\rm i}$-hull). We further give a classification of ${\rm cl}$-prereductions of ${\rm cl}$-closed ideals of a Noetherian ring where ${\rm cl}$ is a closure with a special part.