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Defining Integers

The Set of Whole Numbers

An integer is formally defined as the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). These negations of positive natural numbers are specifically referred to as negative integers. The comprehensive set of all integers is commonly symbolized by the blackboard bold ℤ (from the German "Zahlen" meaning "numbers").

Hierarchical Relationships

The set of natural numbers (ℕ) constitutes a subset of ℤ. In turn, ℤ is a subset of the set of all rational numbers (ℤ), which itself is a subset of the real numbers (ℝ). This hierarchical embedding means that each number system builds upon the previous, with integers representing a crucial expansion beyond just positive counting numbers. For instance, 21, 4, 0, and −2048 are integers, whereas 9.75, 5⁄2, 5⁄4, and √2 are not, as they possess fractional components.

Countable Infinity

Similar to the set of natural numbers, the set of integers ℤ is considered countably infinite. This means that despite being infinite, its elements can be put into a one-to-one correspondence (a bijection) with the natural numbers. This property highlights a fundamental concept in set theory, demonstrating that not all infinities are of the same "size."

Historical Evolution

Etymological Roots

The term "integer" originates from the Latin word integer, meaning "whole" or, more literally, "untouched" (from in- "not" + tangere "to touch"). This etymology underscores the concept of a number without fractional parts. Historically, the term was applied to numbers that were multiples of 1, often synonymous with what we now call natural numbers. Early usage also referred to the whole part of a mixed number.

Inclusion of Negatives

The definition of integers expanded over time to incorporate negative numbers as their practical utility became increasingly recognized. For example, Leonhard Euler, in his 1765 work Elements of Algebra, explicitly defined integers to encompass both positive and negative numbers. This marked a significant conceptual shift, broadening the scope of "whole" numbers to include values less than zero.

The Symbol ℤ

The notation ℤ for the set of integers gained prominence towards the end of the 19th century, particularly with Georg Cantor's pioneering work on infinite sets and set theory. This symbol is derived from the German word Zahlen, meaning "numbers." Its adoption is often attributed to David Hilbert, and its earliest known appearance in a textbook dates to 1947, in the "Algรจbre" by the collective Nicolas Bourbaki. While not immediately universal, ℤ became the standard notation in modern algebra texts by 1961.

Algebraic Properties

Closure Under Operations

The set of integers ℤ exhibits closure under addition and multiplication, meaning that the sum and product of any two integers will always result in another integer. Crucially, unlike the natural numbers, ℤ is also closed under subtraction. This property is fundamental to its algebraic structure. However, ℤ is not closed under division, as the quotient of two integers (e.g., 1 divided by 2) is not necessarily an integer.

Group and Ring Structure

The integers form an abelian group under addition. This means addition is associative, commutative, has an identity element (0), and every integer has an additive inverse (e.g., for `a`, its inverse is `-a`). Furthermore, ℤ is a cyclic group, as every non-zero integer can be expressed as a finite sum of 1s or -1s. In fact, ℤ under addition is the unique infinite cyclic group up to isomorphism.

When considering both addition and multiplication, ℤ forms a commutative ring with unity. This means it satisfies all the properties of an abelian group under addition, plus multiplication is associative, commutative, has a multiplicative identity (1), and multiplication distributes over addition. The integers serve as the most basic example of such an algebraic structure.

No Zero Divisors & Euclidean Domain

A significant property of integers is the absence of zero divisors: if the product of two integers `a` and `b` is 0, then at least one of `a` or `b` must be 0. This makes ℤ an integral domain.

While standard division is not always defined, Euclidean division (division with remainder) is. For any integers `a` and `b` (where `b ≠ 0`), there exist unique integers `q` (quotient) and `r` (remainder) such that `a = q × b + r` and `0 ≤ r < |b|`. This property makes ℤ a Euclidean domain, which implies it is also a principal ideal domain. Consequently, every positive integer can be uniquely factored into prime numbers, a principle known as the Fundamental Theorem of Arithmetic.

Properties of Integer Operations

Property 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`
Identity Element: `a + 0 = a` `a × 1 = a`
Inverse Elements: `a + (−a) = 0` Only −1 and 1 are invertible (units).
Distributivity: `a × (b + c) = (a × b) + (a × c)` and `(a + b) × c = (a × c) + (b × c)`
No Zero Divisors: If `a × b = 0`, then `a = 0` or `b = 0` (or both)

Order-Theoretic Properties

Total Ordering

The set of integers ℤ is a totally ordered set, meaning that for any two distinct integers, one is always greater than the other. This ordering extends infinitely in both positive and negative directions, lacking any upper or lower bound. The familiar arrangement is: ... −3 < −2 < −1 < 0 < 1 < 2 < 3 < ...

Compatibility with Operations

The ordering of integers is compatible with their algebraic operations in specific ways:

  1. If `a < b` and `c < d`, then `a + c < b + d`. This means that adding inequalities preserves their direction.
  2. If `a < b` and `0 < c` (i.e., `c` is a positive integer), then `ac < bc`. This indicates that multiplying an inequality by a positive integer preserves its direction. If `c` were negative, the inequality would reverse.

These compatibilities establish ℤ, along with its ordering, as an ordered ring.

Unique Ordered Group

The integers are uniquely characterized as the only non-trivial totally ordered abelian group whose positive elements are well-ordered. This means that every non-empty subset of positive integers has a least element. This property is deeply connected to concepts in abstract algebra, such as Noetherian valuation rings, which are either fields or discrete valuation rings.

Construction of Integers

Traditional Approach

In foundational mathematics education, integers are often introduced intuitively as the union of positive natural numbers, zero, and the negations of the natural numbers. This can be formalized by first constructing the natural numbers (e.g., using Peano axioms), then creating a disjoint set for their negations, and finally including zero as a distinct element. While straightforward for conceptual understanding, defining arithmetic operations under this approach requires numerous case distinctions (e.g., positive + positive, positive + negative, etc.), making formal proofs of arithmetic laws cumbersome.

Equivalence Classes of Pairs

A more abstract and elegant construction, prevalent in modern set theory, defines integers as equivalence classes of ordered pairs of natural numbers, (a,b). The intuition behind this is that the pair (a,b) represents the result of subtracting b from a. For example, (1,2) and (4,5) both represent −1. An equivalence relation `~` is defined such that `(a,b) ~ (c,d)` if and only if `a + d = b + c`.

Using `[(a,b)]` to denote the equivalence class containing `(a,b)`:

  • Addition: `[(a,b)] + [(c,d)] := [(a+c, b+d)]`
  • Multiplication: `[(a,b)] · [(c,d)] := [(ac+bd, ad+bc)]`
  • Negation (Additive Inverse): `-[ (a,b) ] := [(b,a)]`
  • Subtraction: `[(a,b)] - [(c,d)] := [(a+d, b+c)]`
  • Ordering: `[(a,b)] < [(c,d)]` if and only if `a+d < b+c`

This construction ensures that these operations are well-defined, independent of the chosen representatives for the equivalence classes. Natural numbers `n` are identified with classes `[(n,0)]`, and negative integers `−n` with `[(0,n)]`, thereby recovering the familiar set {..., −2, −1, 0, 1, 2, ...}.

Integers in Computing

Data Types

In computer science, an integer is frequently a primitive data type in programming languages (e.g., `int` in C, Java, Delphi). However, due to the finite capacity of practical computers, these data types can only represent a subset of all mathematical integers. They are typically implemented with a fixed size, often a number of bits that is a power of 2 (e.g., 4, 8, 16, 32, 64 bits).

Representation & Sign

Common representations like two's complement inherently distinguish between "negative" and "non-negative" values, rather than explicitly separating negative, positive, and zero. While a computer can certainly determine if an integer value is truly positive, this underlying representation impacts how arithmetic operations are performed at a low level. For handling integers beyond fixed-size limits, variable-length representations, such as "bignums," are employed, allowing storage of any integer that fits within the computer's available memory.

Automated Reasoning

In theoretical computer science, particularly in automated theorem proving and term rewrite engines, integers are constructed using algebraic terms with basic operations like `zero`, `succ` (successor), and `pred` (predecessor). These constructions vary in the number and types of arguments, and whether they are "free constructors" (meaning each integer has only one unique representation). The equivalence class approach discussed earlier, for instance, is not a free constructor method, as zero can be represented by multiple pairs like (0,0), (1,1), etc. Other methods, often simpler and more efficient for computer implementation, utilize free constructors.

Cardinality of ℤ

Countably Infinite

The set of integers ℤ is classified as countably infinite. This means that despite being an infinite set, its elements can be put into a one-to-one correspondence with the set of natural numbers (1, 2, 3, ...). This concept, introduced by Georg Cantor, demonstrates that not all infinite sets are of the same "size."

Establishing a Bijection

To prove that ℤ is countably infinite, one must demonstrate a bijection (a one-to-one and onto mapping) between ℤ and ℕ. A common example of such a pairing is:

(0,  1), (1,  2), (−1,  3), (2,  4), (−2,  5), (3,  6), ...

More generally, this mapping can be expressed as:

  • For positive integers `k`: `k` maps to `2k`
  • For non-positive integers `k` (i.e., `0` and negative integers): `k` maps to `1 - 2k`
This systematic pairing ensures that every integer is mapped to a unique natural number, and every natural number corresponds to a unique integer.

Aleph-Null Cardinality

More technically, the cardinality of ℤ is denoted by ℵ0 (aleph-null), which is the smallest infinite cardinal number. This is the same cardinality as the set of natural numbers, reinforcing the idea that these two sets, despite one containing the other and its negatives, are "equally infinite" in terms of their countability.

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References

References

  1.  Encyclopaedia Britannica 1771, p.ย 367
  2.  Encyclopaedia Britannica 1771, p.ย 83
  3.  Keith Pledger and Dave Wilkins, "Edexcel AS and A Level Modular Mathematics: Core Mathematics 1" Pearson 2008
A full list of references for this article are available at the Integer Wikipedia page

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