nLab strictly weakly ordered ring

Idea

A notion of ordered ring for strict weak orders.

Definition

With an order relation

A strictly weakly ordered ring is an ring $R$ with a strict weak order $\lt$ such that

• $0 \lt 1$

• for all $a \in R$ and $b \in R$, $0 \lt a$ and $0 \lt b$ implies that $0 \lt a + b$; alternatively, $0 \lt a + b$ implies that $0 \lt a$ or $0 \lt b$.

• for all $a \in R$ and $b \in R$, if $0 \lt a$ and $0 \lt b$, then $0 \lt a \cdot b$

We define the predicate $\mathrm{isPositive}$ as

$$\mathrm{isPositive}(a) \coloneqq 0 \lt a$$

With a positivity predicate

Let $R$ be a commutative ring. $R$ is an pseudo-ordered ring if there is a predicate $\mathrm{isPositive}$ stating an element $a$ is positive, such that

• 0 is not positive
• given element $a \in R$, if $a$ is positive, then $-a$ is not positive
• given elements $a \in R$ and $b \in R$; if $a$ is positive, then either $b$ is positive or $a - b$ is positive
• 1 is positive
• given elements $a \in R$ and $b \in R$, if $a$ is positive and $b$ is positive, $a + b$ is positive; alternatively, $a + b$ being positive implies that $a$ is positive or $b$ is positive.
• given elements $a \in R$ and $b \in R$, if $a$ is positive and $b$ is positive, $a \cdot b$ is positive

We define the order relation $\lt$ as

$$a \lt b \coloneqq \mathrm{isPositive}(b - a)$$

Properties

Every strictly weakly ordered ring is a preordered ring given by the negation of the strict weak order. In the presence of excluded middle, every strictly weakly ordered ring is a totally preordered ring.

Examples

• Every pseudo-ordered ring is a strictly weakly ordered ring.

• Every ordered local ring is a strictly weakly ordered ring where every element greater than zero or less than zero is invertible.

• In particular, the set of Cauchy sequences of rational numbers is a strictly weakly ordered ring.

• Every ordered Kock field is a example of a strictly weakly ordered ring which in the presence of excluded middle is a pseudo-ordered ring.

See also

• ordered local ring

• ordered Kock field