Ratliff-Rush Filtrations associated with ideals and modules over a Noetherian ring

Abstract

Let $R$ be a commutative Noetherian ring, $M$ a finitely generated $R$-module and $I$ a proper ideal of $R$. In this paper we introduce and analyze some properties of $r(I, M)=\bigcup_{k\geqslant 1} (I^{k+1}M: I^kM)$, {\it the Ratliff-Rush ideal associated with $I$ and $M$}. When $M= R$ (or more generally when $M$ is projective) then $r(I, M)= \widetilde{I}$, the usual Ratliff-Rush ideal associated with $I$. If $I$ is a regular ideal and $\ann M=0$ we show that $\{r(I^n,M) \}_{n\geqslant 0}$ is a stable $I$-filtration. If $M_{\p}$ is free for all ${\p}\in \spec R\setminus \mspec R,$ then under mild condition on $R$ we show that for a regular ideal $I$, $\ell(r(I,M)/{\widetilde I})$ is finite. Further $r(I,M)=\widetilde I $ if $A^*(I)\cap \mspec R =\emptyset $ (here $A^*(I)$ is the stable value of the sequence $\Ass (R/{I^n})$). Our generalization also helps to better understand the usual Ratliff-Rush filtration. When $I$ is a regular $\m$-primary ideal our techniques yield an easily computable bound for $k$ such that $\widetilde{I^n} = (I^{n+k} \colon I^k)$ for all $n \geqslant 1$. For any ideal $I$ we show that $\widetilde{I^nM}=I^nM+H^0_I(M)\quad\mbox{for all} n\gg 0.$ This yields that $\widetilde {\mathcal R}(I,M)=\bigoplus_{n\geqslant 0} \widetilde {I^nM}$ is Noetherian if and only if $\depth M>0$. Surprisingly if $\dim M=1$ then $\widetilde G_I(M)=\bigoplus_{n\geqslant 0} \widetilde{I^nM}/{\widetilde{I^{n+1}M}}$ is always a Noetherian and a Cohen-Macaulay $G_I(R)$-module. Application to Hilbert coefficients is also discussed.