Definitions
Nearby Words

integer

[in-ti-jer]
integer: see number; number theory.

The integers (from the Latin integer, literally "untouched", hence "whole": the word entire comes from the same origin, but via French) are the set of numbers consisting of the natural numbers including 0 (0, 1, 2, 3, ...) and their negatives (0, −1, −2, −3, ...). They are numbers that can be written without a fractional or decimal component, and fall within the set {... −2, −1, 0, 1, 2, ...}. For example, 65, 7, and −756 are integers; 1.6 and 1½ are not integers. In other terms, integers are the numbers one can count with items such as apples or fingers, and their negatives, including 0.

More formally, the integers are the only integral domain whose positive elements are well-ordered, and in which order is preserved by addition. Like the natural numbers, the integers form a countably infinite set. The set of all integers is often denoted by a boldface Z (or blackboard bold $mathbb\left\{Z\right\}$, Unicode U+2124 ℤ), which stands for Zahlen (German for numbers, pronounced "tsAH-len").

In algebraic number theory, these commonly understood integers, embedded in the field of rational numbers, are referred to as rational integers to distinguish them from the more broadly defined algebraic integers.

Algebraic properties

Like the natural numbers, Z is closed under the operations of addition and multiplication, that is, the sum and product of any two integers is an integer. However, with the inclusion of the negative natural numbers, and, importantly, zero, Z (unlike the natural numbers) is also closed under subtraction. Z is not closed under the operation of division, since the quotient of two integers (e.g., 1 divided by 2), need not be an integer. Although the natural numbers are closed under exponentiation, the integers are not (since the result can be a fraction when the exponent is negative).

The following lists some of the basic properties of addition and multiplication for any integers a, b and c.

 addition multiplication closure: a + b   is an integer a × b   is an integer associativity: a + (b + c)  =  (a + b) + c a × (b × c)  =  (a × b) × c commutativity: a + b  =  b + a a × b  =  b × a existence of an identity element: a + 0  =  a a × 1  =  a existence of inverse elements: a + (−a)  =  0 distributivity: a × (b + c)  =  (a × b) + (a × c) No zero divisors: if a × b = 0, then either a = 0 or b = 0 (or both)

In the language of abstract algebra, the first five properties listed above for addition say that Z under addition is an abelian group. As a group under addition, Z is a cyclic group, since every nonzero integer can be written as a finite sum 1 + 1 + ... 1 or (−1) + (−1) + ... + (−1). In fact, Z under addition is the only infinite cyclic group, in the sense that any infinite cyclic group is isomorphic to Z.

The first four properties listed above for multiplication say that Z under multiplication is a commutative monoid. However, note that not every integer has a multiplicative inverse; e.g. there is no integer x such that because the left hand side is even, while the right hand side is odd. This means that Z under multiplication is not a group.

All the rules from the above property table, except for the last, taken together say that Z together with addition and multiplication is a commutative ring with unity. Adding the last property says that Z is an integral domain. In fact, Z provides the motivation for defining such a structure.

The lack of multiplicative inverses, which is equivalent to the fact that Z is not closed under division, means that Z is not a field. The smallest field containing the integers is the field of rational numbers. This process can be mimicked to form the field of fractions of any integral domain.

Although ordinary division is not defined on Z, it does possess an important property called the division algorithm: that is, given two integers a and b with b ≠ 0, there exist unique integers q and r such that and where | b | denotes the absolute value of b. The integer q is called the quotient and r is called the remainder, resulting from division of a by b. This is the basis for the Euclidean algorithm for computing greatest common divisors.

Again, in the language of abstract algebra, the above says that Z is a Euclidean domain. This implies that Z is a principal ideal domain and any positive integer can be written as the products of primes in an essentially unique way. This is the fundamental theorem of arithmetic.

Order-theoretic properties

Z is a totally ordered set without upper or lower bound. The ordering of Z is given by
... < −2 < −1 < 0 < 1 < 2 < ...
An integer is positive if it is greater than zero and negative if it is less than zero. Zero is defined as neither negative nor positive.

The ordering of integers is compatible with the algebraic operations in the following way:

1. if a < b and c < d, then a + c < b + d
2. if a < b and 0 < c, then ac < bc. (From this fact, one can show that if c < 0, then ac > bc.)

It follows that Z together with the above ordering is an ordered ring.

Construction

Mathematically we may construct the integers as equivalence classes of ordered pairs of natural numbers (a, b).

The intuition is that (a, b) stands for the result of subtracting b from a. To conform to our expectation that 1 − 2 and 4 − 5 denote the same number, we define an equivalence relation ~ on these pairs with the following rule:

$\left(a,b\right) sim \left(c,d\right) ,!$
precisely when
$a+d = b+c. ,!$

Addition and multiplication of integers can be defined in terms of the equivalent operations on the natural numbers:

$\left[\left(a,b\right)\right]+\left[\left(c,d\right)\right] := \left[\left(a+c,b+d\right)\right].,$
$\left[\left(a,b\right)\right]cdot\left[\left(c,d\right)\right] := \left[\left(ac+bd,ad+bc\right)\right].,$

The negation (or additive inverse) of an integer is obtained by reversing the order of the pair:

$-\left[\left(a,b\right)\right] := \left[\left(b,a\right)\right].,$

Hence subtraction can be defined as the addition of the additive inverse:

$\left[\left(a,b\right)\right]-\left[\left(c,d\right)\right] := \left[\left(a+d,b+c\right)\right].,$

The standard ordering on the integers is given by:

$\left[\left(a,b\right)\right]<\left[\left(c,d\right)\right],$ iff $a+d < b+c.,$

It is easily verified that these definitions are independent of the choice of representatives of the equivalence classes.

Taking 0 to be a natural number, the natural numbers may be considered to be integers by the embedding that maps n to [(n,0)], where [(a,b)] denotes the equivalence class having (a,b) as a member.

Typically, [(a,b)] is denoted by

$begin\left\{cases\right\} n, & mbox\left\{if \right\} a ge b -n, & mbox\left\{if \right\} a < b, end\left\{cases\right\}$
where
$n = |a-b|.,$
If the natural numbers are identified with the corresponding integers (using the embedding mentioned above), this convention creates no ambiguity.

This notation recovers the familiar representation of the integers as {...,−3,−2,−1,0,1,2,3,...}.

Some examples are:

begin\left\{align\right\}
`0 &= [(0,0)] &= [(1,1)] &= cdots & &= [(k,k)] `
`1 &= [(1,0)] &= [(2,1)] &= cdots & &= [(k+1,k)] `
-1 &= [(0,1)] &= [(1,2)] &= cdots & &= [(k,k+1)]
`2 &= [(2,0)] &= [(3,1)] &= cdots & &= [(k+2,k)] `
-2 &= [(0,2)] &= [(1,3)] &= cdots & &= [(k,k+2)] end{align}

Integers in computing

An integer (sometimes known as an "int", from the name of a datatype in the C programming language) is often a primitive datatype in computer languages. However, integer datatypes can only represent a subset of all integers, since practical computers are of finite capacity. Also, in the common two's complement representation, the inherent definition of sign distinguishes between "negative" and "non-negative" rather than "negative, positive, and 0". (It is, however, certainly possible for a computer to determine whether an integer value is truly positive.)

Variable-length representations of integers, such as bignums, can store any integer that fits in the computer's memory. Other integer datatypes are implemented with a fixed size, usually a number of bits which is a power of 2 (4, 8, 16, etc.) or a memorable number of decimal digits (e.g., 9 or 10).

In contrast, theoretical models of digital computers, such as Turing machines, typically do not have infinite (but only unbounded finite) capacity.

Cardinality

The cardinality of the set of integers is equal to ℵ0. This is readily demonstrated by the construction of a bijection, that is, a function that is injective and surjective from Z to N. Consider the function
$begin\left\{cases\right\} 2x+1, & mbox\left\{if \right\} x ge 0 2|x|, & mbox\left\{if \right\} x<0 end\left\{cases\right\}$.
If the domain is restricted to Z then each and every member of Z has one and only one corresponding member of N and by the definition of cardinal equality the two sets have equal cardinality.

References

• Bell, E. T., New York: Simon and Schuster, 1986. (Hardcover; ISBN 0-671-46400-0)/(Paperback; ISBN 0-671-62818-6)
• Herstein, I. N., Topics in Algebra, Wiley; 2 edition (June 20, 1975), ISBN 0-471-01090-1.
• Mac Lane, Saunders, and Garrett Birkhoff; Algebra, American Mathematical Society; 3rd edition (April 1999). ISBN 0-8218-1646-2.