Existence of special primary decompositions in multigraded modules

Abstract

Let $R=\bigoplus_{\underline{n} \in \mathbb{N}^t}R_{\underline{n}}$ be a commutative Noetherian $\mathbb{N}^t$-graded ring, and $L = \bigoplus_{\underline{n}\in\mathbb{N}^t}L_{\underline{n}}$ be a finitely generated $\mathbb{N}^t$-graded $R$-module. We prove that there exists a positive integer $k$ such that for any $\underline{n} \in \mathbb{N}^t$ with $L_{\underline{n}} \neq 0$, there exists a primary decomposition of the zero submodule $O_{\underline{n}}$ of $L_{\underline{n}}$ such that for any $P \in {\rm Ass}_{R_0}(L_{\underline{n}})$, the $P$-primary component $Q$ in that primary decomposition contains $P^k L_{\underline{n}}$. We also give an example which shows that not all primary decompositions of $O_{\underline{n}}$ in $L_{\underline{n}}$ have this property. As an application of our result, we prove that there exists a fixed positive integer $l$ such that the $0^{\rm th}$ local cohomology $H_I^0(L_{\underline{n}}) = \big(0 :_{L_{\underline{n}}} I^l\big)$ for all ideals $I$ of $R_0$ and for all $\underline{n} \in \mathbb{N}^t$.