Lagmental Vicfred

A Noetherian Ring Need Not Be Artinian by Vicfred

Last updated: Wed 17 May 2023

The polynomial ring in one variable is Noetherian but has an infinite descending chain of ideals. I will separate the object being defined from the consequence being claimed.

Notation

A ring \(A\) is Noetherian when ascending chains of ideals stabilise. Equivalently, every ideal \(I\triangleleft A\) is finitely generated, so finite data controls all later ideal growth.

$$ k[x]\ \text{is Noetherian} $$

The formulas should not be merged too early. The datum \(\mathsf D\), the conclusion \(\mathsf C\), and the bridge \(\Longrightarrow\) have three different logical jobs.

$$ (x)\supsetneq(x^2)\supsetneq(x^3)\supsetneq\cdots $$

Stress the formula

An explicit case prevents the notation from becoming ceremonial. Every subscript and superscript in the display contributes to the value.

$$ \begin{array}{c|c|c}A&\text{ACC on ideals}&\text{DCC on ideals}\\\hline k[x]&\checkmark&\times\\k[x]/(x^n)&\checkmark&\checkmark\end{array} $$

Interpretation

The two-row display is also a debugging tool: if the conclusion changes when only notation changes, some hidden choice has entered the argument.

$$ \begin{aligned} \mathsf{D}\;&:\quad k[x]\ \text{is Noetherian},\\[5pt] \mathsf{C}\;&:\quad (x)\supsetneq(x^2)\supsetneq(x^3)\supsetneq\cdots. \end{aligned} $$

Limit of the argument

Noetherian does not mean finite, Artinian, or a domain. Each additional adjective imposes a different chain condition or multiplicative property.

$$ \boxed{\begin{gathered} \text{compact conclusion}\\[-2pt] (x)\supsetneq(x^2)\supsetneq(x^3)\supsetneq\cdots \end{gathered}} $$

A symbolic summary is trustworthy only because the example and limitation remain visible beside it. The box compresses the conclusion without hiding its origin.

This article was posted on Wed 18 May 2022. Facts and circumstances may have changed since publication.
Please contact me before jumping to conclusions if something seems wrong or unclear.