Natural number
In mathematics, the natural numbers are those 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 language, words used for counting are "cardinal numbers" and words used for ordering are "ordinal numbers".
Some definitions, including the standard ISO 800002,^{[1]} 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, ….^{[2]}^{[3]}^{[4]}^{[5]} Texts that exclude zero from the natural numbers sometimes refer to the natural numbers together with zero as the whole numbers, but in other writings, that term is used instead for the integers (including negative integers).
The natural numbers are the basis from which many other number sets may be built by extension: the integers, by including (if not yet in) the neutral element and an additive inverse (−n) for each natural number n (and zero, if it is not there already, as its own additive inverse); the rational numbers, by including a multiplicative inverse (1/n) for each nonzero integer n; the real numbers by including with the rationals the (converging) Cauchy sequences of rationals; the complex numbers, by including with the real numbers the unresolved square root of minus one; and so on.^{[6]}^{[7]} These chains 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, for example in primary school, natural numbers may be called counting numbers^{[8]} to contrast the discreteness of counting to the continuity of measurement, established by the real numbers.
The natural numbers can, at times, appear as a convenient set of names (labels), that is, as what linguists call nominal numbers, foregoing many or all of the properties of being a number in a mathematical sense.
Contents
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 the powers of 10 up to over 1 million. A stone carving from Karnak, dating from around 1500 BC 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.^{[12]}
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 BC by the Babylonians, but they omitted such a digit when it would have been the last symbol in the number.^{[13]} The Olmec and Maya civilizations used 0 as a separate number as early as the 1st century BC, but this usage did not spread beyond Mesoamerica.^{[14]}^{[15]} The use of a numeral 0 in modern times originated with the Indian mathematician Brahmagupta in 628. 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, 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.^{[16]}
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.^{[17]}
Independent studies also occurred at around the same time in India, China, and Mesoamerica.^{[18]}
Modern definitions
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In 19th century Europe, there was mathematical and philosophical discussion about the exact nature of the natural numbers. A school 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 "God made the integers, all else is the work of man".
In opposition to the Naturalists, the constructivists saw a need to improve the logical rigor in the foundations of mathematics.^{[19]} 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, they were shown to be equivalent in most practical applications.
Settheoretical definitions of natural numbers were initiated by Frege and he initially defined a natural number as the class of all sets that are in onetoone correspondence with a particular set, but this definition turned out to lead to paradoxes including Russell's paradox. Therefore, this 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 Giuseppe Peano and 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^{[5]} although many have kept the older tradition and take 1 to be the first natural number.^{[24]} Computer scientists often start from zero when enumerating items like loop counters and string or array elements.^{[25]}^{[26]}
Notation
Mathematicians use N or ℕ (an N in blackboard bold) to refer to the set of all natural numbers. Older texts have also occasionally employed J as the symbol for this set.^{[27]} This set is countably infinite: it is infinite but countable by definition. This is also expressed by saying that the cardinal number of the set is alephnaught (ℵ_{0}).^{[28]}
To be unambiguous about whether 0 is included or not, sometimes an index (or superscript) "0" is added in the former case, and a superscript "*" or subscript ">0" is added in the latter case:^{[1]}
 ℕ^{0} = ℕ_{0} = {0, 1, 2, …}
 ℕ^{*} = ℕ^{+} = ℕ_{1} = ℕ_{>0} = {1, 2, …}.
Alternatively, natural numbers may be distinguished from positive integers with the index notation, but it must be understood by context that since both symbols are used, the natural numbers contain zero.^{[29]}
 ℕ = {0,1, 2, …}.
 ℤ^{+}= {1, 2, …}.
Properties
Addition
One can recursively define an addition operator on the natural numbers by setting a + 0 = a and a + S(b) = S(a + b) for all a, b. Here S should be read as "successor". This turns the natural numbers (ℕ, +) into a commutative monoid with identity element 0, the socalled free object with one generator. This monoid satisfies the cancellation property and can be embedded in a group (in the mathematical sense of the word 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 (i.e., 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, 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 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
 a = bq + r and r < b.
The number q is called the quotient and r is called the remainder of division of a by b. The numbers q and r are uniquely determined by a and b. This Euclidean division is key to 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.
 Associativity: for all natural numbers a, b, and c, a + (b + c) = (a + b) + c and a × (b × c) = (a × b) × c.
 Commutativity: for all natural numbers a and b, a + b = b + a and a × b = b × a.
 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).
Generalizations
Two generalizations of natural numbers arise from the two uses:
 A natural number can be used to express the size of a finite set; more generally a cardinal number is a measure for the size of a set also suitable for infinite sets; this refers to a concept of "size" such that if there is a bijection between two sets they have the same size. The set of natural numbers itself and any other countably infinite set has cardinality alephnull (ℵ_{0}).
 Linguistic ordinal numbers "first", "second", "third" can be assigned to the elements of a totally ordered finite set, and also to the elements of wellordered countably infinite sets like the set of natural numbers itself. This can be generalized to ordinal numbers which describe the position of an element in a wellordered set in general. An ordinal number is also used to describe the "size" of a wellordered set, in a sense different from cardinality: if there is an order isomorphism 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.
Many wellordered sets with cardinal number ℵ_{0} have an ordinal number greater than ω (the latter is the lowest possible). The least ordinal of cardinality ℵ_{0} (i.e., the initial ordinal) is ω.
For finite wellordered sets, there is 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 (i.e., 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 Peano axioms.^{[30]}^{[31]}
 Axiom One: 0 is a natural number.
 Axiom Two: Every natural number has a successor.
 Axiom Three: 0 is not the successor of any natural number.
 Axiom Four: If the successor of x equals the successor of y, then x equals y.
 Axiom Five (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 x is x + 1. Replacing Axiom Five by an axiom schema one obtains a (weaker) firstorder theory called Peano Arithmetic.
Constructions based on set theory
Von Neumann construction
In the area of mathematics called set theory, a special case of the von Neumann ordinal construction ^{[32]} 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.
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.
Other constructions
Although the standard construction is useful, it is not the only possible construction. Zermelo's construction goes as follows:
 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 of the natural number preceding it.
See also
 Integer
 Settheoretic definition of natural numbers
 Peano axioms
 Canonical representation of a positive integer
 Countable set
 Number#Classification for other number systems (rational, real, complex etc.)
Notes
 ^ ^{a} ^{b} "Standard number sets and intervals". ISO 800002:2009. International Organization for Standardization. p. 6.
 ^ Weisstein, Eric W. "Natural Number". MathWorld.
 ^ "natural number", MerriamWebster.com, MerriamWebster, retrieved 4 October 2014
 ^ Carothers (2000) says: "ℕ is the set of natural numbers (positive integers)" (p. 3)
 ^ ^{a} ^{b} Mac Lane & Birkhoff (1999) include zero in the natural numbers: 'Intuitively, the set ℕ = {0, 1, 2, ...} of all natural numbers may be described as follows: ℕ contains an "initial" number 0; ...'. They follow that with their version of the Peano Postulates. (p. 15)
 ^ Mendelson (2008) says: "The whole fantastic hierarchy of number systems is built up by purely settheoretic means from a few simple assumptions about natural numbers." (Preface, p. x)
 ^ Bluman (2010): "Numbers make up the foundation of mathematics." (p. 1)
 ^ Weisstein, Eric W. "Counting Number". MathWorld.
 ^ Introduction, Royal Belgian Institute of Natural Sciences, Brussels, Belgium.
 ^ Flash presentation, Royal Belgian Institute of Natural Sciences, Brussels, Belgium.
 ^ The Ishango Bone, Democratic Republic of the Congo, on permanent display at the Royal Belgian Institute of Natural Sciences, Brussels, Belgium. UNESCO's Portal to the Heritage of Astronomy
 ^ Georges Ifrah, The Universal History of Numbers, Wiley, 2000, ISBN 0471375683

^ "A history of Zero". MacTutor History of Mathematics. Retrieved 20130123.
… a tablet found at Kish … thought to date from around 700 BC, uses three hooks to denote an empty place in the positional notation. Other tablets dated from around the same time use a single hook for an empty place
 ^ Mann, Charles C. (2005), 1491: New Revelations Of The Americas Before Columbus, Knopf, p. 19, ISBN 9781400040063.
 ^ Evans, Brian (2014), "Chapter 10. PreColumbian Mathematics: The Olmec, Maya, and Inca Civilizations", The Development of Mathematics Throughout the Centuries: A Brief History in a Cultural Context, John Wiley & Sons, ISBN 9781118853979.
 ^ Michael L. Gorodetsky (20030825). "Cyclus Decemnovennalis Dionysii – Nineteen year cycle of Dionysius". Hbar.phys.msu.ru. Retrieved 20120213.
 ^ This convention is used, for example, in Euclid's Elements, see Book VII, definitions 1 and 2.
 ^ Morris Kline, Mathematical Thought From Ancient to Modern Times, Oxford University Press, 1990 [1972], ISBN 0195061357
 ^ "Much of the mathematical work of the twentieth century has been devoted to examining the logical foundations and structure of the subject." (Eves 1990, p. 606)
 ^ Eves 1990, Chapter 15
 ^ L. Kirby; J. Paris, Accessible Independence Results for Peano Arithmetic, Bulletin of the London Mathematical Society 14 (4): 285. doi:10.1112/blms/14.4.285, 1982.
 ^ Bagaria, Joan. "Set Theory". The Stanford Encyclopedia of Philosophy (Winter 2014 Edition).
 ^ Goldrei, Derek (1998). "3". Classic set theory : a guided independent study (1. ed., 1. print ed.). Boca Raton, Fla. [u.a.]: Chapman & Hall/CRC. p. 33. ISBN 0412606100.
 ^ This is common in texts about Real analysis. See, for example, Carothers (2000, p. 3) or Thomson, Bruckner & Bruckner (2000, p. 2).
 ^ Brown, Jim (1978). "In Defense of Index Origin 0". ACM SIGAPL APL Quote Quad. 9 (2): 7. doi:10.1145/586050.586053. Retrieved 19 January 2015.
 ^ Hui, Roger. "Is Index Origin 0 a Hindrance?". jsoftware.com. Retrieved 19 January 2015.
 ^ Rudin, W. (1976). Principles of Mathematical Analysis (PDF). New York: McGrawHill. p. 25. ISBN 9780070542358.
 ^ Weisstein, Eric W. "Cardinal Number". MathWorld.
 ^ Grimaldi, Ralph P. (2003). A review of discrete and combinatorial mathematics (5th ed.). Boston, MA: AddisonWesley. p. 133. ISBN 9780201726343.
 ^ G.E. Mints (originator), "Peano axioms", Encyclopedia of Mathematics, Springer, in cooperation with the European Mathematical Society, retrieved 8 October 2014

^ Hamilton (1988) calls them "Peano's Postulates" and begins with "1. 0 is a natural number." (p. 117f)
Halmos (1960) uses the language of set theory instead of the language of arithmetic for his five axioms. He begins with "(I) 0 ∈ ω (where, of course, 0 = ∅" (ω is the set of all natural numbers). (p. 46)
Morash (1991) gives "a twopart axiom" in which the natural numbers begin with 1. (Section 10.1: An Axiomatization for the System of Positive Integers)  ^ Von Neumann 1923
References
 Bluman, Allan (2010), PreAlgebra DeMYSTiFieD (Second ed.), McGrawHill Professional
 Carothers, N.L. (2000), Real analysis, Cambridge University Press, ISBN 0521497566
 Clapham, Christopher; Nicholson, James (2014), The Concise Oxford Dictionary of Mathematics (Fifth ed.), Oxford University Press

Dedekind, Richard (1963), Essays on the Theory of Numbers, Dover, ISBN 0486210103
 Dedekind, Richard (2007), Essays on the Theory of Numbers, Kessinger Publishing, LLC, ISBN 054808985X
 Eves, Howard (1990), An Introduction to the History of Mathematics (6th ed.), Thomson, ISBN 9780030295584
 Halmos, Paul (1960), Naive Set Theory, Springer Science & Business Media
 Hamilton, A. G. (1988), Logic for Mathematicians (Revised ed.), Cambridge University Press
 James, Robert C.; James, Glenn (1992), Mathematics Dictionary (Fifth ed.), Chapman & Hall
 Landau, Edmund (1966), Foundations of Analysis (Third ed.), Chelsea Pub Co, ISBN 082182693X
 Mac Lane, Saunders; Birkhoff, Garrett (1999), Algebra (3rd ed.), American Mathematical Society
 Mendelson, Elliott (2008) [1973], Number Systems and the Foundations of Analysis, Dover Publications
 Morash, Ronald P. (1991), Bridge to Abstract Mathematics: Mathematical Proof and Structures (Second ed.), McgrawHill College
 Musser, Gary L.; Peterson, Blake E.; Burger, William F. (2013), Mathematics for Elementary Teachers: A Contemporary Approach (10th ed.), Wiley Global Education, ISBN 9781118457443
 Szczepanski, Amy F.; Kositsky, Andrew P. (2008), The Complete Idiot's Guide to Prealgebra, Penguin Group
 Thomson, Brian S.; Bruckner, Judith B.; Bruckner, Andrew M. (2008), Elementary Real Analysis (Second ed.), ClassicalRealAnalysis.com, ISBN 9781434843678

Von Neumann, Johann (1923), "Zur Einführung der transfiniten Zahlen", Acta litterarum ac scientiarum Ragiae Universitatis Hungaricae FranciscoJosephinae, Sectio scientiarum mathematicarum, 1: 199–208
 Von Neumann, John (January 2002) [1923], "On the introduction of transfinite numbers", in Jean van Heijenoort, From Frege to Gödel: A Source Book in Mathematical Logic, 18791931 (3rd ed.), Harvard University Press, pp. 346–354, ISBN 0674324498  English translation of von Neumann 1923.
External links
 Hazewinkel, Michiel, ed. (2001), "Natural number", Encyclopedia of Mathematics, Springer, ISBN 9781556080104
 Axioms and Construction of Natural Numbers
 Essays on the Theory of Numbers by Richard Dedekind at Project Gutenberg