Ordered ring

Ordered ring
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ordered_ring".

In abstract algebra, an ordered ring is a commutative ring $R$ with a total order $\leq$ such that

if $a\leq b$ and $c\in R$ , then $a+c \leq b+c$

if $0 \leq a$ and $0\leq b$ , then $0 \leq ab.$

Ordered rings are familiar from arithmetic. Examples include the real numbers. (The rationals and reals in fact form ordered fields.) The complex numbers do not form an ordered ring (or ordered field).

In analogy with ordinary numbers, we call an element c of an ordered ring positive if $0\leq c, c\neq 0$ and negative if $c\leq 0, c\neq0$ . The set of positive (or, in some cases, nonnegative) elements in the ring $R$ is often denoted by $R +$ .

If $a$ is an element of an ordered ring $R$ , then the absolute value of $a$ , denoted $| a |$ , is defined thus:

$|a| := \begin{cases} a, & \mbox{if } 0 \leq a, \\ -a, & \mbox{otherwise}, \end{cases}$

where $- a$ is the additive inverse of $a$ and $0$ is the additive identity element.

A discrete ordered ring or discretely ordered ring is an ordered ring in which there is no element between 0 and 1. The integers are a discrete ordered ring, but the rational numbers are not.

Basic properties

If $a\leq b$ and $0\leq c$ , then $ac\leq bc.$ ^[1] This property is sometimes used to define ordered rings instead of the second property in the definition above.
If $a,b \in R$ , then $| a b | = | a | | b | .$ ^[2]
An ordered ring that is not trivial is infinite. ^[3]
If $a\in R$ , then either $a\in R_+$ , or $-a \in R_+$ , or $a=0\,.$ ^[4] This property follows from the fact that ordered rings are abelian, linearly ordered groups with respect to addition.
An ordered ring $R$ has no zero divisors if and only if $R +$ is closed under multiplication—that is, $a b$ is positive if both $a$ and $b$ are positive.^[5]
In an ordered ring, no negative element is a square.^[6] This is because if $a\neq 0$ and $a = b 2$ then $b\neq 0$ and $a = ( - b) 2$ ; as either $b$ or $- b$ is positive, $a$ must be positive.

Notes

The names below refer to theorems formally verified by the IsarMathLib project.

^ OrdRing_ZF_1_L9
^ OrdRing_ZF_2_L5
^ ord_ring_infinite
^ OrdRing_ZF_3_L2, see also OrdGroup_decomp
^ OrdRing_ZF_3_L3
^ OrdRing_ZF_1_L12

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