Definition:Negative

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Standard Number System

The concept of negative can be applied to the following sets of numbers:

$(1): \quad$ The integers $\Z$
$(2): \quad$ The rational numbers $\Q$
$(3): \quad$ The real numbers $\R$

The Complex Numbers cannot be Totally Ordered, so there is no such concept as a negative complex number.


Integers

The negative integers comprise the set:

$\set {0, -1, -2, -3, \ldots}$

As the set of integers is the Inverse Completion of Natural Numbers, it follows that elements of $\Z$ are the isomorphic images of the elements of equivalence classes of $\N \times \N$ where two tuples are equivalent if the difference between the two elements of each tuple is the same.

Thus negative can be formally defined on $\Z$ as a relation induced on those equivalence classes as specified in the definition of integers.

That is, the integers being defined as all the difference congruence classes, negative can be defined directly as the relation specified as follows:


The integer $z \in \Z: z = \eqclass {\tuple {a, b} } \boxminus$ is negative if and only if $b > a$.

The set of negative integers is denoted $\Z_{\le 0}$.


An element of $\Z$ can be specifically indicated as being negative by prepending a $-$ sign:

$-x \in \Z_{\le 0} \iff x \in \Z_{\ge 0}$


Rational Numbers

The negative rational numbers are the set defined as:

$\Q_{\le 0} := \left\{{x \in \Q: x \le 0}\right\}$

That is, all the rational numbers that are less than or equal to zero.


Real Numbers

The negative real numbers are the set defined as:

$\R_{\le 0} := \set {x \in \R: x \le 0}$

That is, all the real numbers that are less than or equal to zero.


Complex Numbers

As the Complex Numbers cannot be Totally Ordered, the concept of a negative complex number, relative to a specified zero, is not defined.


However, the negative of a complex number is defined as follows:

Let $z = a + i b$ be a complex number.

Then the negative of $z$ is defined as:

$-z = -a - i b$


Ordered Ring

Let $\struct {R, +, \circ, \le}$ be an ordered ring whose zero is $0_R$.


Then $x \in R$ is negative if and only if $x \le 0_R$.


The set of all negative elements of $R$ is denoted:

$R_{\le 0_R} := \set {x \in R: x \le 0_R}$


Also known as

The notations $R_-$ and $R^-$ are also frequently seen for $\set {x \in R: x \le 0_R}$.

However, these notations are also used to denote $\set {x \in R: x < 0_R}$, that is $R_{< 0_R}$, and hence are ambiguous.


Some treatments of this subject use the term define non-positive to define $x \in R$ where $0_R \le x$, reserving the term negative for what is defined on this website as strictly negative.

With the conveniently unambiguous notation that has been adopted on this site, the distinction between the terms loses its importance, as the symbology removes the confusion.


Also see