Natural number
In mathematics, the natural numbers are those numbers used for counting (as in "there are six coins on the table") and ordering (as in "this is the third largest city in the country"). In common mathematical terminology, words colloquially used for counting are "cardinal numbers", and words used for ordering are "ordinal numbers". The natural numbers can, at times, appear as a convenient set of codes (labels or "names"), that is, as what linguists call nominal numbers, forgoing many or all of the properties of being a number in a mathematical sense.^{[1]}^{[2]}
Some definitions, including the standard ISO 800002,^{[3]}^{[a]} begin the natural numbers with 0, corresponding to the nonnegative integers 0, 1, 2, 3, ..., whereas others start with 1, corresponding to the positive integers 1, 2, 3, ...^{[4]}^{[b]} Texts that exclude zero from the natural numbers sometimes refer to the natural numbers together with zero as the whole numbers, while in other writings, that term is used instead for the integers (including negative integers).^{[5]}
The natural numbers are a basis from which many other number sets may be built by extension: the integers, by including (if not yet in) the neutral element 0 and an additive inverse (−n) for each nonzero natural number n; the rational numbers, by including a multiplicative inverse () for each nonzero integer n (and also the product of these inverses by integers); the real numbers by including with the rationals the limits of (converging) Cauchy sequences of rationals; the complex numbers, by including with the real numbers the unresolved square root of minus one (and also the sums and products thereof); and so on.^{[c]}^{[d]} This chain of extensions make the natural numbers canonically embedded (identified) in the other number systems.
Properties of the natural numbers, such as divisibility and the distribution of prime numbers, are studied in number theory. Problems concerning counting and ordering, such as partitioning and enumerations, are studied in combinatorics.
In common language, particularly in primary school education, natural numbers may be called counting numbers^{[6]} to intuitively exclude the negative integers and zero, and also to contrast the discreteness of counting to the continuity of measurement—a hallmark characteristic of real numbers.
History
Ancient roots
The most primitive method of representing a natural number is to put down a mark for each object. Later, a set of objects could be tested for equality, excess or shortage—by striking out a mark and removing an object from the set.
The first major advance in abstraction was the use of numerals to represent numbers. This allowed systems to be developed for recording large numbers. The ancient Egyptians developed a powerful system of numerals with distinct hieroglyphs for 1, 10, and all powers of 10 up to over 1 million. A stone carving from Karnak, dating back from around 1500 BCE and now at the Louvre in Paris, depicts 276 as 2 hundreds, 7 tens, and 6 ones; and similarly for the number 4,622. The Babylonians had a placevalue system based essentially on the numerals for 1 and 10, using base sixty, so that the symbol for sixty was the same as the symbol for one—its value being determined from context.^{[10]}
A much later advance was the development of the idea that 0 can be considered as a number, with its own numeral. The use of a 0 digit in placevalue notation (within other numbers) dates back as early as 700 BCE by the Babylonians, who omitted such a digit when it would have been the last symbol in the number.^{[e]} The Olmec and Maya civilizations used 0 as a separate number as early as the 1st century BCE, but this usage did not spread beyond Mesoamerica.^{[12]}^{[13]} The use of a numeral 0 in modern times originated with the Indian mathematician Brahmagupta in 628 CE. However, 0 had been used as a number in the medieval computus (the calculation of the date of Easter), beginning with Dionysius Exiguus in 525 CE, without being denoted by a numeral (standard Roman numerals do not have a symbol for 0). Instead, nulla (or the genitive form nullae) from nullus, the Latin word for "none", was employed to denote a 0 value.^{[14]}
The first systematic study of numbers as abstractions is usually credited to the Greek philosophers Pythagoras and Archimedes. Some Greek mathematicians treated the number 1 differently than larger numbers, sometimes even not as a number at all.^{[f]} Euclid, for example, defined a unit first and then a number as a multitude of units, thus by his definition, a unit is not a number and there are no unique numbers (e.g., any two units from indefinitely many units is a 2).^{[16]}
Independent studies on numbers also occurred at around the same time in India, China, and Mesoamerica.^{[17]}
Modern definitions
In 19th century Europe, there was mathematical and philosophical discussion about the exact nature of the natural numbers. A school^{[which?]} of Naturalism stated that the natural numbers were a direct consequence of the human psyche. Henri Poincaré was one of its advocates, as was Leopold Kronecker, who summarized his belief as "God made the integers, all else is the work of man".^{[g]}
In opposition to the Naturalists, the constructivists saw a need to improve upon the logical rigor in the foundations of mathematics.^{[h]} In the 1860s, Hermann Grassmann suggested a recursive definition for natural numbers, thus stating they were not really natural—but a consequence of definitions. Later, two classes of such formal definitions were constructed; later still, they were shown to be equivalent in most practical applications.
Settheoretical definitions of natural numbers were initiated by Frege. He initially defined a natural number as the class of all sets that are in onetoone correspondence with a particular set. However, this definition turned out to lead to paradoxes, including Russell's paradox. To avoid such paradoxes, the formalism was modified so that a natural number is defined as a particular set, and any set that can be put into onetoone correspondence with that set is said to have that number of elements.^{[20]}
The second class of definitions was introduced by Charles Sanders Peirce, refined by Richard Dedekind, and further explored by Giuseppe Peano; this approach is now called Peano arithmetic. It is based on an axiomatization of the properties of ordinal numbers: each natural number has a successor and every nonzero natural number has a unique predecessor. Peano arithmetic is equiconsistent with several weak systems of set theory. One such system is ZFC with the axiom of infinity replaced by its negation. Theorems that can be proved in ZFC but cannot be proved using the Peano Axioms include Goodstein's theorem.^{[21]}
With all these definitions, it is convenient to include 0 (corresponding to the empty set) as a natural number. Including 0 is now the common convention among set theorists^{[22]} and logicians.^{[23]} Other mathematicians also include 0,^{[a]} and computer languages often start from zero when enumerating items like loop counters and string or arrayelements.^{[24]}^{[25]} On the other hand, many mathematicians have kept the older tradition to take 1 to be the first natural number.^{[26]}
Notation
Mathematicians use N or to refer to the set of all natural numbers.^{[1]}^{[27]} The existence of such a set is established in set theory. Older texts have also occasionally employed J as the symbol for this set.^{[28]}
Since different properties are customarily associated to the tokens 0 and 1 (e.g., neutral elements for addition and multiplications, respectively), it is important to know which version of natural numbers is employed in the case under consideration. This can be done by explanation in prose, by explicitly writing down the set, or by qualifying the generic identifier with a super or subscript,^{[3]}^{[29]} for example, like this:
 Naturals without zero:
 Naturals with zero:
Alternatively, since the natural numbers naturally form a subset of the integers (often denoted ), they may be referred to as the positive, or the nonnegative integers, respectively.^{[30]} To be unambiguous about whether 0 is included or not, sometimes a subscript (or superscript) "0" is added in the former case, and a superscript "*" is added in the latter case:^{[3]}
Properties
Addition
Given the set of natural numbers and the successor function sending each natural number to the next one, one can define addition of natural numbers recursively by setting a + 0 = a and a + S(b) = S(a + b) for all a, b. Then (ℕ, +) is a commutative monoid with identity element 0. It is a free monoid on one generator. This commutative monoid satisfies the cancellation property, so it can be embedded in a group. The smallest group containing the natural numbers is the integers.
If 1 is defined as S(0), then b + 1 = b + S(0) = S(b + 0) = S(b). That is, b + 1 is simply the successor of b.
Multiplication
Analogously, given that addition has been defined, a multiplication operator can be defined via a × 0 = 0 and a × S(b) = (a × b) + a. This turns (ℕ^{*}, ×) into a free commutative monoid with identity element 1; a generator set for this monoid is the set of prime numbers.
Relationship between addition and multiplication
Addition and multiplication are compatible, which is expressed in the distribution law: a × (b + c) = (a × b) + (a × c). These properties of addition and multiplication make the natural numbers an instance of a commutative semiring. Semirings are an algebraic generalization of the natural numbers where multiplication is not necessarily commutative. The lack of additive inverses, which is equivalent to the fact that ℕ is not closed under subtraction (that is, subtracting one natural from another does not always result in another natural), means that ℕ is not a ring; instead it is a semiring (also known as a rig).
If the natural numbers are taken as "excluding 0", and "starting at 1", the definitions of + and × are as above, except that they begin with a + 1 = S(a) and a × 1 = a.
Order
In this section, juxtaposed variables such as ab indicate the product a × b,^{[31]} and the standard order of operations is assumed.
A total order on the natural numbers is defined by letting a ≤ b if and only if there exists another natural number c where a + c = b. This order is compatible with the arithmetical operations in the following sense: if a, b and c are natural numbers and a ≤ b, then a + c ≤ b + c and ac ≤ bc.
An important property of the natural numbers is that they are wellordered: every nonempty set of natural numbers has a least element. The rank among wellordered sets is expressed by an ordinal number; for the natural numbers, this is denoted as ω (omega).
Division
In this section, juxtaposed variables such as ab indicate the product a × b, and the standard order of operations is assumed.
While it is in general not possible to divide one natural number by another and get a natural number as result, the procedure of division with remainder or Euclidean division is available as a substitute: for any two natural numbers a and b with b ≠ 0 there are natural numbers q and r such that
The number q is called the quotient and r is called the remainder of the division of a by b. The numbers q and r are uniquely determined by a and b. This Euclidean division is key to the several other properties (divisibility), algorithms (such as the Euclidean algorithm), and ideas in number theory.
Algebraic properties satisfied by the natural numbers
The addition (+) and multiplication (×) operations on natural numbers as defined above have several algebraic properties:
 Closure under addition and multiplication: for all natural numbers a and b, both a + b and a × b are natural numbers.^{[32]}
 Associativity: for all natural numbers a, b, and c, a + (b + c) = (a + b) + c and a × (b × c) = (a × b) × c.^{[33]}
 Commutativity: for all natural numbers a and b, a + b = b + a and a × b = b × a.^{[34]}
 Existence of identity elements: for every natural number a, a + 0 = a and a × 1 = a.
 Distributivity of multiplication over addition for all natural numbers a, b, and c, a × (b + c) = (a × b) + (a × c).
 No nonzero zero divisors: if a and b are natural numbers such that a × b = 0, then a = 0 or b = 0 (or both).
Infinity
The set of natural numbers is an infinite set. By definition, this kind of infinity is called countable infinity. All sets that can be put into a bijective relation to the natural numbers are said to have this kind of infinity. This is also expressed by saying that the cardinal number of the set is alephnought (ℵ_{0}).^{[35]}
Generalizations
Two important generalizations of natural numbers arise from the two uses of counting and ordering: cardinal numbers and ordinal numbers.
 A natural number can be used to express the size of a finite set; more precisely, a cardinal number is a measure for the size of a set, which is even suitable for infinite sets. This concept of "size" relies on maps between sets, such that two sets have the same size, exactly if there exists a bijection between them. The set of natural numbers itself, and any bijective image of it, is said to be countably infinite and to have cardinality alephnull (ℵ_{0}).
 Natural numbers are also used as linguistic ordinal numbers: "first", "second", "third", and so forth. This way they can be assigned to the elements of a totally ordered finite set, and also to the elements of any wellordered countably infinite set. This assignment can be generalized to general wellorderings with a cardinality beyond countability, to yield the ordinal numbers. An ordinal number may also be used to describe the notion of "size" for a wellordered set, in a sense different from cardinality: if there is an order isomorphism (more than a bijection!) between two wellordered sets, they have the same ordinal number. The first ordinal number that is not a natural number is expressed as ω; this is also the ordinal number of the set of natural numbers itself.
The least ordinal of cardinality ℵ_{0} (that is, the initial ordinal of ℵ_{0}) is ω but many wellordered sets with cardinal number ℵ_{0} have an ordinal number greater than ω.
For finite wellordered sets, there is a onetoone correspondence between ordinal and cardinal numbers; therefore they can both be expressed by the same natural number, the number of elements of the set. This number can also be used to describe the position of an element in a larger finite, or an infinite, sequence.
A countable nonstandard model of arithmetic satisfying the Peano Arithmetic (that is, the firstorder Peano axioms) was developed by Skolem in 1933. The hypernatural numbers are an uncountable model that can be constructed from the ordinary natural numbers via the ultrapower construction.
Georges Reeb used to claim provocatively that The naïve integers don't fill up ℕ. Other generalizations are discussed in the article on numbers.
Formal definitions
Peano axioms
Many properties of the natural numbers can be derived from the five Peano axioms:^{[36]} ^{[i]}
 0 is a natural number.
 Every natural number has a successor which is also a natural number.
 0 is not the successor of any natural number.
 If the successor of equals the successor of , then equals .
 The axiom of induction: If a statement is true of 0, and if the truth of that statement for a number implies its truth for the successor of that number, then the statement is true for every natural number.
These are not the original axioms published by Peano, but are named in his honor. Some forms of the Peano axioms have 1 in place of 0. In ordinary arithmetic, the successor of is . Replacing axiom 5 by an axiom schema, one obtains a (weaker) firstorder theory called Peano arithmetic.
Constructions based on set theory
In the area of mathematics called set theory, a specific construction due to John von Neumann^{[37]}^{[38]} defines the natural numbers as follows:
 Set 0 = { }, the empty set,
 Define S(a) = a ∪ {a} for every set a. S(a) is the successor of a, and S is called the successor function.
 By the axiom of infinity, there exists a set which contains 0 and is closed under the successor function. Such sets are said to be inductive. The intersection of all such inductive sets is defined to be the set of natural numbers. It can be checked that the set of natural numbers satisfies the Peano axioms.
 It follows that each natural number is equal to the set of all natural numbers less than it:

 0 = { },
 1 = 0 ∪ {0} = {0} = {{ }},
 2 = 1 ∪ {1} = {0, 1} = {{ }, {{ }}},
 3 = 2 ∪ {2} = {0, 1, 2} = {{ }, {{ }}, {{ }, {{ }}}},
 n = n−1 ∪ {n−1} = {0, 1, ..., n−1} = {{ }, {{ }}, ..., {{ }, {{ }}, ...}}, etc.
With this definition, a natural number n is a particular set with n elements, and n ≤ m if and only if n is a subset of m. The standard definition, now called definition of von Neumann ordinals, is: "each ordinal is the wellordered set of all smaller ordinals."
Also, with this definition, different possible interpretations of notations like ℝ^{n} (ntuples versus mappings of n into ℝ) coincide.
Even if one does not accept the axiom of infinity and therefore cannot accept that the set of all natural numbers exists, it is still possible to define any one of these sets.
Although the standard construction is useful, it is not the only possible construction. Ernst Zermelo's construction goes as follows:^{[38]}
 Set 0 = { }
 Define S(a) = {a},
 It then follows that

 0 = { },
 1 = {0} = {{ }},
 2 = {1} = {{{ }}},
 n = {n−1} = {{{...}}}, etc.
 Each natural number is then equal to the set containing just the natural number preceding it. This is the definition of Zermelo ordinals. Unlike von Neumann's construction, the Zermelo ordinals do not account for infinite ordinals.
See also
 Benacerraf's identification problem
 Canonical representation of a positive integer
 Countable set
 Number#Classification for other number systems (rational, real, complex etc.)
 Ordinal number
 Settheoretic definition of natural numbers
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