Sifted degrees of the equations of the Rees module and their connection with the Artin-Rees numbers

Abstract

Let $A$ be a noetherian ring, $I$ an ideal of $A$ and $N\subset M$ finitely generated $A$-modules. The relation type of $I$ with respect to $M$, denoted by ${\bf rt}\,(I;M)$, is the maximal degree in a minimal generating set of relations of the Rees module ${\bf R}(I;M)=\oplus_{n\geq 0}I^nM$. It is a well-known invariant that gives a first measure of the complexity of ${\bf R}(I;M)$. To help to measure this complexity, we introduce the sifted type of ${\bf R}(I;M)$, denoted by ${\bf st}\,(I;M)$, a new invariant which counts the non-zero degrees appearing in a minimal generating set of relations of ${\bf R}(I;M)$. Just as the relation type ${\bf rt}\,(I;M/N)$ is closely related to the strong Artin-Rees number ${\bf s}\,(N,M;I)$, it turns out that the sifted type ${\bf st}\,(I;M/N)$ is closely related to the medium Artin-Rees number ${\bf m}\,(N,M;I)$, a new invariant which lies in between the weak and the strong Artin-Rees numbers of $(N,M;I)$. We illustrate the meaning, interest and mutual connection of ${\bf m}\,(N,M;I)$ and ${\bf st}\,(I;M)$ with some examples.