Symbol often used to denote the set of integers

The integers (from the Latin integer, literally "untouched," hence "whole": the word entire comes from the same origin, but via French^[1]) are formed by the natural numbers (including 0) (0, 1, 2, 3, ...) together with the negatives of the non-zero natural numbers (−1, −2, −3, ...). Viewed as a subset of the real numbers, 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, 21, 4, and −2048 are integers; 9.75, 5½, and √2 are not integers.

The set of all integers is often denoted by a boldface Z (or blackboard bold Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbb{Z} , Unicode U+2124 ℤ), which stands for Zahlen (German for numbers, pronounced [ˈtsaːlən]).^[2]

The integers (with addition as operation) form the smallest group containing the additive monoid of the natural numbers. Like the natural numbers, the integers form a countably infinite set.

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.

Integers can be thought of as discrete, equally spaced points on an infinitely long number line. Nonnegative integers (purple) and negative integers (red).

Algebraic properties[link]

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 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.^{[citation needed]}

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.^{[citation needed]}

The first four properties listed above for multiplication say that Z under multiplication is a commutative monoid. However not every integer has a multiplicative inverse; e.g. there is no integer x such that 2x = 1, because the left hand side is even, while the right hand side is odd. This means that Z under multiplication is not a group.^{[citation needed]}

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.^{[citation needed]}

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. The process of constructing the rationals from the integers can be mimicked to form the field of fractions of any integral domain.^{[citation needed]}

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 a = q × b + r and 0 ≤ r < | b |, 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.^{[citation needed]}

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.^{[citation needed]}

Order-theoretic properties[link]

Z is a totally ordered set without upper or lower bound. The ordering of Z is given by:^{[citation needed]}

... −3 < −2 < −1 < 0 < 1 < 2 < 3 < ...

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.

It follows that Z together with the above ordering is an ordered ring.^{[citation needed]}

The integers are the only integral domain whose positive elements are well-ordered, and in which order is preserved by addition.^{[citation needed]}

Construction[link]

Red Points represent ordered pairs of natural numbers. Linked red points are equivalence classes representing the blue integers at the end of the line.

The integers can be formally constructed as the equivalence classes of ordered pairs of natural numbers (a, b).^[3]

The intuition is that (a, b) stands for the result of subtracting b from a.^[3] To confirm our expectation that 1 − 2 and 4 − 5 denote the same number, we define an equivalence relation ~ on these pairs with the following rule:

Failed to parse (Missing texvc executable; please see math/README to configure.): (a,b) \sim (c,d) \,\!

precisely when

Failed to parse (Missing texvc executable; please see math/README to configure.): a+d = b+c. \,\!

Addition and multiplication of integers can be defined in terms of the equivalent operations on the natural numbers^[3]; denoting by [(a,b)] the equivalence class having (a,b) as a member, one has:

Failed to parse (Missing texvc executable; please see math/README to configure.): [(a,b)]+[(c,d)] := [(a+c,b+d)].\,

Failed to parse (Missing texvc executable; please see math/README to configure.): [(a,b)]\cdot[(c,d)] := [(ac+bd,ad+bc)].\,

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

Failed to parse (Missing texvc executable; please see math/README to configure.): -[(a,b)] := [(b,a)].\,

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

Failed to parse (Missing texvc executable; please see math/README to configure.): [(a,b)]-[(c,d)] := [(a+d,b+c)].\,

The standard ordering on the integers is given by:

Failed to parse (Missing texvc executable; please see math/README to configure.): [(a,b)]<[(c,d)]\,

iff Failed to parse (Missing texvc executable; please see math/README to configure.): a+d < b+c.\,

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

Every equivalence class has a unique member that is of the form (n,0) or (0,n) (or both at once). The natural number n is identified with the class [(n,0)] (in other words the natural numbers are embedded into the integers by map sending n to [(n,0)]), and the class [(0,n)] is denoted −n (this covers all remaining classes, and gives the class [(0,0)] a second time since −0 = 0.^{[citation needed]}

Thus, [(a,b)] is denoted by^{[citation needed]}

Failed to parse (Missing texvc executable; please see math/README to configure.): \begin{cases} a-b, & \mbox{if } a \ge b \\ -(b-a), & \mbox{if } a < b. \end{cases}

If the natural numbers are identified with the corresponding integers (using the embedding mentioned above), this convention creates no ambiguity.^{[citation needed]}

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

Some examples are:

Failed to parse (Missing texvc executable; please see math/README to configure.): \begin{align} 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[link]

An integer 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.) Fixed length integer approximation datatypes (or subsets) are denoted int or Integer in several programming languages (such as Algol68, C, Java, Delphi, etc.).^{[citation needed]}

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).^{[citation needed]}

Cardinality[link]

The cardinality of the set of integers is equal to Failed to parse (Missing texvc executable; please see math/README to configure.): \aleph_0

(aleph-null). This is readily demonstrated by the construction of a bijection, that is, a function that is injective and surjective from Z to N.[citation needed]

If N = {0, 1, 2, ...} then consider the function:

Failed to parse (Missing texvc executable; please see math/README to configure.): f(x) = \begin{cases} 2|x|, & \mbox{if } x < 0 \\ 0, & \mbox{if } x = 0 \\ 2x-1, & \mbox{if } x > 0. \end{cases}

{ ... (-4,8) (-3,6) (-2,4) (-1,2) (0,0) (1,1) (2,3) (3,5) ... }

If N = {1,2,3,...} then consider the function:

Failed to parse (Missing texvc executable; please see math/README to configure.): g(x) = \begin{cases} 2|x|, & \mbox{if } x < 0 \\ 2x+1, & \mbox{if } x \ge 0. \end{cases}

{ ... (-4,8) (-3,6) (-2,4) (-1,2) (0,1) (1,3) (2,5) (3,7) ... }

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.^{[citation needed]}

See also[link]

Mathematics portal

• 0.999...
• Algebraic integer
• Canonical representation of a positive integer
• Hyperinteger
• Integer (computer science)
• Integer lattice
• Integer part
• Integer sequence

Notes[link]

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2012)

References[link]

• Bell, E. T., Men of Mathematics. 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.
• Weisstein, Eric W., "Integer" from MathWorld.

External links[link]

Look up integer in Wiktionary, the free dictionary.

• The Positive Integers - divisor tables and numeral representation tools
• On-Line Encyclopedia of Integer Sequences cf OEIS

This article incorporates material from Integer on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.