# nLab weak local ring

### Context

#### Algebra

higher algebra

universal algebra

# Contents

## Idea

A weak notion of local ring in the context of constructive mathematics.

## Definition

A weak local ring is a commutative ring such that

• $0 \neq 1$; and

• the sum of two non-invertible elements is non-invertible

These are the same as local rings in classical mathematics, but are a weaker (and thus more general) notion than local rings in constructive mathematics.

## Properties

The non-invertible elements in a weak local ring form an ideal. Thus, the quotient of a weak local ring by its ideal of non-invertible elements form a residue field in the sense of Johnstone 1977.

Every weak local ring has an equivalence relation $\approx$, defined as $x \approx y$ if and only if $x - y$ is non-invertible. Then Johnstone’s residue fields are precisely the weak local rings for which $\approx$ implies equality.

## Examples

• Every Johnstone residue field is an weak local ring where every non-invertible element is equal to zero.

• The dual algebra $\mathbb{R}[\epsilon]/\epsilon^2$ of the MacNeille real numbers $\mathbb{R}$ is a weak local ring where the nilpotent infinitesimal $\epsilon \in \mathbb{R}[\epsilon]/\epsilon^2$ is a non-zero non-invertible element.

• For any prime number $p$ and any positive natural number $n$, the prime power local ring $\mathbb{Z}/p^n\mathbb{Z}$ is an weak local ring, whose ideal of non-invertible elements is the ideal $p(\mathbb{Z}/p^n\mathbb{Z})$. The quotient of $\mathbb{Z}/p^n\mathbb{Z}$ by its ideal of non-invertible elements is the finite field $\mathbb{Z}/p\mathbb{Z}$.

• Every local ring is a weak local ring with an apartness relation $\#$ such that for all $a \in R$ and $b \in R$, $a \# b$ if and only if $a - b$ is invertible. The negation of $a \# b$ is an equivalence relation which holds if and only if $a - b$ is non-invertible, making every local ring a weak local ring.

## Weakly ordered local rings

A weakly ordered local ring is a weak local ring $R$ with a preorder $\leq$ such that

• for all $a \in R$ and $b \in R$, $a \approx b$ if and only if $a \leq b$ and $b \leq a$
• for all $a \in R$, $b \in R$, and $c \in R$, if $a \leq b$, then $a + c \leq b + c$
• for all $a \in R$ and $b \in R$, if $0 \leq a$ and $0 \leq b$, then $0 \leq a \cdot b$

If additionally, for all $a \in R$ and $b \in R$, $a \approx b$ implies that $a = b$, then a weakly ordered local ring becomes a weakly ordered residue field, and the preorder becomes a partial order.

Every ordered local ring and thus every ordered field is a weakly ordered local ring.