Negative and nonnegative numbers
From Academic Kids

A negative number is a number that is less than zero, such as −3. A positive number is a number that is greater than zero, such as 3. Zero itself is neither negative nor positive, though in computing zero is sometimes treated as though it were a positive number. The nonnegative numbers are the real numbers that are not negative (positive or zero). The nonpositive numbers are the real numbers that are not positive (negative or zero).
In the context of complex numbers positive implies real, but for clarity one may say "positive real number".
Contents 
Negative numbers
Negative integers can be regarded as an extension of the natural numbers, such that the equation x − y = z has a meaningful solution for all values of x and y. The other sets of numbers are then derived as progressively more elaborate extensions and generalizations from the integers.
Negative numbers are useful to describe values on a scale that goes below zero, such as temperature, and also in bookkeeping where they can be used to represent debts. In bookkeeping, debts are often represented by red numbers, or a number in parentheses.
Nonnegative numbers
A number is nonnegative if and only if it is greater than or equal to zero, i.e. positive or zero. Thus the nonnegative integers are all the integers from zero on upwards, and the nonnegative reals are all the real numbers from zero on upwards.
A real matrix A is called nonnegative if every entry of A is nonnegative.
A real matrix A is called totally nonnegative by matrix theorists or totally positive by computer scientists if the determinant of every square submatrix of A is nonnegative.
Sign function
It is possible to define a function sgn(x) on the real numbers which is 1 for positive numbers, −1 for negative numbers and 0 for zero (sometimes called the signum function):
 <math>\sgn(x)=\left\{\begin{matrix} 1 & : x < 0 \\ \;0 & : x = 0 \\ \;1 & : x > 0 \end{matrix}\right. <math>
We then have (except for x=0):
 <math>\sgn(x) = \frac{x}{x} = \frac{x}{x} = \frac{d{x}}{d{x}} = 2H(x)1. <math>
where x is the absolute value of x and H(x) is the Heaviside step function. See also derivative.
Arithmetic involving signed numbers
Addition and subtraction
For purposes of addition and subtraction, one can think of negative numbers as debts.
Adding a negative number is the same as subtracting the corresponding positive number:
 <math> 5 + (3) = 5  3 = 2 \,<math>
 (if you have $5 and acquire a debt of $3, then you have a net worth of $2)
 <math> 2 + (5) = 2  5 = 7 \,<math>
Subtracting a positive number from a smaller positive number yields a negative result:
 <math> 4  6 = 2 \,<math>
 (if you have $4 and spend $6 then you have a debt of $2).
Subtracting a positive number from any negative number yields a negative result:
 <math> 3  6 = 9 \,<math>
 (if you have a debt of $3 and spend another $6, you have a debt of $9).
Subtracting a negative is equivalent to adding the corresponding positive:
 <math> 5  (2) = 5 + 2 = 7 \,<math>
 (if you have a net worth of $5 and you get rid of a debt of $2, then your new net worth is $7).
Also:
 <math> (8)  (3) = 5 \,<math>
 (if you have a debt of $8 and get rid of a debt of $3, then you still have a debt of $5).
Multiplication
Multiplication of a negative number by a positive number yields a negative result: (−2) × 3 = −6. The reason is that this multiplication can be understood as repeated addition: (−2) × 3 = (−2) + (−2) + (−2) = −6. Alternatively: if you have a debt of $2, and then your debt is tripled, you end up with a debt of $6.
Multiplication of two negative numbers yields a positive result: (−3) × (−4) = 12. This situation cannot be understood as repeated addition, and the analogy to debts doesn't help either. The ultimate reason for this rule is that we want the distributive law to work:
 <math> (3 + (3)) \times (4) = 3 \times (4) + (3) \times (4). \,<math>
The left hand side of this equation equals 0 × (−4) = 0. The right hand side is a sum of −12 + (−3) × (−4); for the two to be equal, we need (−3) × (−4) = 12.
Division
Division is similar to multiplication. If both the dividend and the divisor have different signs, the result is negative:
 <math> \; 8 \;/\; (2) = (4) \,<math>
 <math> (10) \;/\; 2 = (5) \,<math>
If both numbers are of the same sign, the result is positive (even if they are both negative):
 <math> (12) \;/\; (3) = 4 \,<math>
Formal construction of negative and nonnegative integers
In a similar manner to rational numbers, we can extend the natural numbers N to the integers Z by defining integers as an ordered pair of natural numbers (a, b). We can extend addition and multiplication to these pairs with the following rules:
 <math> ( a , b ) + ( c , d ) = ( a + c , b + d ) \,<math>
 <math> ( a , b ) \times ( c , d ) = ( a \times c + b \times d , a \times d + b \times c ) \,<math>
We define an equivalence relation ~ upon these pairs with the following rule:
 <math> (a, b)\sim(c, d)<math> if and only if <math> a + d = b + c . \,<math>
This equivalence relation is compatible with the addition and multiplication defined above, and we may define Z to be the quotient set N^{2}/~, i.e. we identify two pairs (a, b) and (c, d) if they are equivalent in the above sense.
We can also define a total order on Z by writing
 <math> ( a , b ) \leq ( c , d ) \,<math> if and only if <math> a + d \leq b + c . \,<math>
This will lead to an additive zero of the form (a, a), an additive inverse of (a, b) of the form (b, a), a multiplicative unit of the form (a+1, a), and a definition of subtraction
 <math> ( a , b )  ( c , d ) = ( a + d , b + c ). \,<math>
History
The Indus Valley Civilization peoples (c. 2600 BC) demonstrate the earliest known use of negative numbers (see 0 (number): History). But the Indus Valley Civilization deteriorated sometime around 1900 BC (see Indus Valley Civilization: Decline and collapse).
Nevertheless, Ancient India is then credited with the earliest known use and legitimization of negative numbers in mathematics after the era of the Indus Valley peoples, in Brahmagupta's BrahmaSphutaSidd'hanta [628], an Indian mathematical text. In Brahmagupta's work, it appears as if negative numbers evolved from a need to represent negative asset or debts. European mathematicians, for the most part, resisted the concept of negative numbers until the 17th century.
See also
 hyperreal number
 integer
 negative and nonnegative in binary
 rational number
 real number
 surreal number
de:Positive und negative Zahlen fr:Nombre négatif ko:음의 정수 he:מספר חיובי nl:Negatief (wiskunde) ja:負の整数 sl:Negativno število sv:Positiva tal th:จำนวนลบและจำนวนไม่เป็นลบ zh:负数