On the structure theorem of graded components of $\mathcal{F}$-finite, $\mathcal{F}$-modules over certain polynomial ring

TL;DR

This paper characterizes the structure of graded components of certain $ ext{F}$-finite modules over polynomial rings in positive characteristic, revealing their decomposition and constancy properties across specific index blocks.

Contribution

It establishes a structure theorem for graded $ ext{F}$-finite modules over polynomial rings in characteristic $p>0$, extending characteristic zero results to positive characteristic.

Findings

Decomposition of graded components into direct sums involving $E$, $Q$, and $A$ modules.

Constancy of the coefficients $a(b)$, $b(b)$, $c(b)$ on blocks $b$ in $a$-space.

Application to local cohomology modules, generalizing previous characteristic zero results.

Abstract

Let $K$ be a field of characteristic $p>0$, $A=K[[Y]]$ be a power series ring in one variable and $Q(A)$ be the field of fraction of $A$. Suppose that $R=A[X_1,\ldots,X_n]$ is a standard $\mathbb{N}^n$-graded polynomial ring over $A$, i.e., $\operatorname{deg} (A)=\underline{0}\in \mathbb{N}^n$ and $\operatorname{deg}(X_j)=e_j\in \mathbb{N}^n$. Assume that $M=\bigoplus_{\underline{u}\in \mathbb{Z}^n} M_{\underline{u}}$ is a $\mathbb{Z}^n$-graded $\mathcal{F}$-finite, $\mathcal{F}$-module over $R$. In this article we prove that, $\displaystyle M_{\underline{u}}\cong E(A/YA)^{a(\underline{u})}\oplus Q(A)^{b(\underline{u})}\oplus A^{c(\underline{u})}$ for some finite numbers $a(\underline{u}), b(\underline{u}), c(\underline{u})\geq 0$. Let for a subset of $U$ of $\mathcal{S}=\{1, \ldots, n\}$, define a block to be the set $\displaystyle\mathcal{B}(U)=\{\underline{u} \in \mathbb{Z}^n \mid u_i \geq 0 \mbox{ if } i \in U \mbox{ and } u_i \leq -1 \mbox{ if } i \notin U \}$. Note that $\bigcup_{U\subseteq \mathcal{S}}\mathcal{B}(U)=\mathbb{Z}^n$. We prove that the sets $\{a(\underline{u})\mid \underline{u}\in \mathbb{Z}^n\}$, $\{b(\underline{u})\mid \underline{u}\in \mathbb{Z}^n\}$ and $\{c(\underline{u})\mid \underline{u}\in \mathbb{Z}^n\}$ are constant on $\mathcal{B}(U)$ for each subset $U$ of $\{1,\ldots,n\}$. In particular, these results holds for composition of local cohomology modules of the form $ H^{i_1}_{I_1}(H^{i_2}_{I_2}(\dots H^{i_r}_{I_r}(R)\dots)$ where $I_1,\ldots,I_r$ are $\mathbb{N}^n$-graded ideals of $R$. This provides a positive characteristic analogue of the results proved in \cite{TS-23} by the authors in characteristic zero.