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{{Short description|Number used for counting}}{{About|“positive integers” and “non-negative integers“|all the numbers ..., −2, −1, 0, 1, 2, ...|Integer}}{{Use dmy dates|date=May 2021}}(File:Three Baskets with Apples.svg|right|thumb|upright|Natural numbers can be used for counting: one apple; two apples are one apple added to another apple, three apples are one apple added to two apples, ...)In mathematics, the natural numbers are the numbers 0, 1, 2, 3, etc., possibly excluding 0.{{under discussion inline|Is 0 a natural number?}} Some define the natural numbers as the non-negative integers {{math|1=0, 1, 2, 3, ...}}, while others define them as the positive integers {{math|1, 2, 3, ...}}.{{efn|See {{section link|#Emergence as a term}}}} Some authors acknowledge both definitions whenever convenient. Some texts define the whole numbers as the natural numbers together with zero, excluding zero from the natural numbers, while in other writings, the whole numbers refer to all of the integers (including negative integers).DICTIONARY, Jack G., Ganssle, Michael, Barr, amp, 2003, Embedded Systems Dictionary, 978-1-57820-120-4, integer, 138 (integer), 247 (signed integer), & 276 (unsigned integer), Taylor & Francis, Google Books,books.google.com/books?id=zePGx82d_fwC, 28 March 2017, live,web.archive.org/web/20170329150719/https://books.google.com/books?id=zePGx82d_fwC, 29 March 2017, The counting numbers refer to the natural numbers in common language, particularly in primary school education, and are similarly ambiguous although typically exclude zero.{{MathWorld|title=Counting Number|id=CountingNumber}}The natural numbers can be used for counting (as in “there are six coins on the table“), in which case they serve as cardinal numbers. They may also be used for ordering (as in “this is the third largest city in the country“), in which case they serve as ordinal numbers. Natural numbers are sometimes used as labels{{mdash}}also known as nominal numbers, (e.g. jersey numbers in sports){{mdash}}which do not have the properties of numbers in a mathematical sense.WEB, Weisstein, Eric W., Natural Number,mathworld.wolfram.com/NaturalNumber.html, 11 August 2020, mathworld.wolfram.com, en, WEB, Natural Numbers, Brilliant Math & Science Wiki,brilliant.org/wiki/natural-numbers/, 11 August 2020, en-us, The natural numbers form a set, often symbolized as mathbb{N}. Many other number sets are built by successively extending the set of natural numbers: the integers, by including an additive identity 0 (if not yet in) and an additive inverse {{math|−n}} for each nonzero natural number {{mvar|n}}; the rational numbers, by including a multiplicative inverse 1/n for each nonzero integer {{mvar|n}} (and also the product of these inverses by integers); the real numbers by including the limits of Cauchy sequences{{efn|Any Cauchy sequence in the Reals converges,}} of rationals; the complex numbers, by adjoining to the real numbers a square root of {{math|−1}} (and also the sums and products thereof); and so on.{{efn|{{harvtxt|Mendelson|2008|page=x}} says: “The whole fantastic hierarchy of number systems is built up by purely set-theoretic means from a few simple assumptions about natural numbers.“}}{{efn|{{harvtxt|Bluman|2010|page=1}}: “Numbers make up the foundation of mathematics.“}} This chain of extensions canonically embeds the natural numbers 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.

History

Ancient roots

{{further|Prehistoric counting}}File:Ishango bone (cropped).jpg|thumb|The Ishango bone (on exhibition at the Royal Belgian Institute of Natural Sciences)WEB, Introduction, Ishango bone, Royal Belgian Institute of Natural Sciences, Brussels, Belgium,www.naturalsciences.be/expo/old_ishango/en/ishango/introduction.html,web.archive.org/web/20160304051733/https://www.naturalsciences.be/expo/old_ishango/en/ishango/introduction.html, 4 March 2016, WEB, Flash presentation, Ishango bone, Royal Belgian Institute of Natural Sciences, Brussels, Belgium,ishango.naturalsciences.be/Flash/flash_local/Ishango-02-EN.html,ishango.naturalsciences.be/Flash/flash_local/Ishango-02-EN.html," title="web.archive.org/web/20160527164619ishango.naturalsciences.be/Flash/flash_local/Ishango-02-EN.html,">web.archive.org/web/20160527164619ishango.naturalsciences.be/Flash/flash_local/Ishango-02-EN.html, 27 May 2016, WEB, The Ishango Bone, Democratic Republic of the Congo, UNESCO’s Portal to the Heritage of Astronomy,www2.astronomicalheritage.net/index.php/show-entity?identity=4&idsubentity=1,www2.astronomicalheritage.net/index.php/show-entity?identity=4&idsubentity=1," title="web.archive.org/web/20141110195426www2.astronomicalheritage.net/index.php/show-entity?identity=4&idsubentity=1,">web.archive.org/web/20141110195426www2.astronomicalheritage.net/index.php/show-entity?identity=4&idsubentity=1, 10 November 2014, , on permanent display at the Royal Belgian Institute of Natural SciencesRoyal Belgian Institute of Natural SciencesThe most primitive method of representing a natural number is to use one’s fingers, as in finger counting. Putting down a tally mark for each object is another primitive method. 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 place-value 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.BOOK, Georges, Ifrah, 2000, The Universal History of Numbers, Wiley, 0-471-37568-3, A much later advance was the development of the idea that {{num|0}} can be considered as a number, with its own numeral. The use of a 0 digit in place-value 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.{{efn| 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.WEB, A history of Zero, MacTutor History of Mathematics,www-history.mcs.st-and.ac.uk/history/HistTopics/Zero.html, live, 23 January 2013,www-history.mcs.st-and.ac.uk/history/HistTopics/Zero.html," title="web.archive.org/web/20130119083234www-history.mcs.st-and.ac.uk/history/HistTopics/Zero.html,">web.archive.org/web/20130119083234www-history.mcs.st-and.ac.uk/history/HistTopics/Zero.html, 19 January 2013, }} The Olmec and Maya civilizations used 0 as a separate number as early as the {{nowrap|1st century BCE}}, but this usage did not spread beyond Mesoamerica.BOOK, Charles C., Mann, 2005, 1491: New Revelations of the Americas before Columbus, 19, Knopf, 978-1-4000-4006-3,books.google.com/books?id=Jw2TE_UNHJYC&pg=PA19, live, Google Books, 3 February 2015,web.archive.org/web/20150514105855/https://books.google.com/books?id=Jw2TE_UNHJYC&pg=PA19, 14 May 2015, BOOK, Brian, Evans, 2014, The Development of Mathematics Throughout the Centuries: A brief history in a cultural context, John Wiley & Sons, 978-1-118-85397-9, Chapter 10. Pre-Columbian Mathematics: The Olmec, Maya, and Inca Civilizations, Google Books,books.google.com/books?id=3CPwAgAAQBAJ&pg=PT73, 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 , the Latin word for “none”, was employed to denote a 0 value.WEB, Michael, Deckers, Cyclus Decemnovennalis Dionysii – Nineteen year cycle of Dionysius,hbar.phys.msu.ru/gorm/chrono/paschata.htm, Hbar.phys.msu.ru, 25 August 2003, 13 February 2012,hbar.phys.msu.ru/gorm/chrono/paschata.htm," title="web.archive.org/web/20190115083618hbar.phys.msu.ru/gorm/chrono/paschata.htm,">web.archive.org/web/20190115083618hbar.phys.msu.ru/gorm/chrono/paschata.htm, 15 January 2019, live, 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.{{efn|This convention is used, for example, in Euclid’s Elements, see D. Joyce’s web edition of Book VII.BOOK, Euclid, Euclid, D., Joyce, Book VII, definitions 1 and 2, Euclid’s Elements, Elements, Clark University,aleph0.clarku.edu/~djoyce/java/elements/bookVII/defVII1.html, }} 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).BOOK, Mueller, Ian, 2006, Philosophy of mathematics and deductive structure in Euclid’s Elements, 58, Dover Publications, Mineola, New York, 978-0-486-45300-2, 69792712, However, in the definition of perfect number which comes shortly afterward, Euclid treats 1 as a number like any other.BOOK, Euclid, Euclid, D., Joyce, Book VII, definition 22, Euclid’s Elements, Elements, Clark University,aleph0.clarku.edu/~djoyce/java/elements/bookVII/defVII22.html, A perfect number is that which is equal to the sum of its own parts., In definition VII.3 a “part” was defined as a number, but here 1 is considered to be a part, so that for example {{math|1=6 = 1 + 2 + 3}} is a perfect number.Independent studies on numbers also occurred at around the same time in India, China, and Mesoamerica.BOOK, Morris, Kline, 1990, 1972, Mathematical Thought from Ancient to Modern Times, Oxford University Press, 0-19-506135-7,

Emergence as a term

Nicolas Chuquet used the term progression naturelle (natural progression) in 1484.BOOK, Chuquet, Nicolas, Nicolas Chuquet, Le Triparty en la science des nombres, 1881, 1484,gallica.bnf.fr/ark:/12148/bpt6k62599266/f75.image, fr, The earliest known use of “natural number” as a complete English phrase is in 1763.BOOK, Emerson, William, The method of increments, 1763, 113,archive.org/details/bim_eighteenth-century_the-method-of-increments_emerson-william_1763/page/112/mode/2up, The 1771 Encyclopaedia Britannica defines natural numbers in the logarithm article.WEB, Earliest Known Uses of Some of the Words of Mathematics (N),mathshistory.st-andrews.ac.uk/Miller/mathword/n/, Maths History, en, Starting at 0 or 1 has long been a matter of definition. In 1727, Bernard Le Bovier de Fontenelle wrote that his notions of distance and element led to defining the natural numbers as including or excluding 0.BOOK, Fontenelle, Bernard de, Eléments de la géométrie de l’infini, 1727, 3,gallica.bnf.fr/ark:/12148/bpt6k64762n/f31.item, fr, In 1889, Giuseppe Peano used N for the positive integers and started at 1,BOOK, Arithmetices principia: nova methodo, 1889, Fratres Bocca,archive.org/details/arithmeticespri00peangoog/page/n12/mode/2up, 12, Latin, but he later changed to using N0 and N1.BOOK, Peano, Giuseppe, Formulaire des mathematiques, 1901, Paris, Gauthier-Villars, 39,archive.org/details/formulairedesmat00pean/page/38/mode/2up, fr, Historically, most definitions have excluded 0,BOOK, Fine, Henry Burchard, A College Algebra, 1904, Ginn, 6,www.google.com/books/edition/A_College_Algebra/RR4PAAAAIAAJ?hl=en&gbpv=1&pg=PA6&printsec=frontcover&dq=%22natural%20number%22, en, BOOK, Advanced Algebra: A Study Guide to be Used with USAFI Course MC 166 Or CC166, 1958, United States Armed Forces Institute, 12,www.google.com/books/edition/Advanced_Algebra/184i06Py1ZYC?hl=en&gbpv=1&dq=%22natural%20number%22%201&pg=PA12&printsec=frontcover, en, but many mathematicians such as George A. Wentworth, Bertrand Russell, Nicolas Bourbaki, Paul Halmos, Stephen Cole Kleene, and John Horton Conway have preferred to include 0.WEB, Natural Number,archive.lib.msu.edu/crcmath/math/math/n/n035.htm, archive.lib.msu.edu, Mathematicians have noted tendencies in which definition is used, such as algebra texts including 0,{{efn|name=MacLaneBirkhoff1999p15|{{harvtxt|Mac Lane|Birkhoff|1999|page=15}} include zero in the natural numbers: ‘Intuitively, the set N={0,1,2,ldots} of all natural numbers may be described as follows: N contains an “initial” number {{math|0}}; ...’. They follow that with their version of the Peano’s axioms.}} number theory and analysis texts excluding 0,BOOK, Křížek, Michal, Somer, Lawrence, Å olcová, Alena, From Great Discoveries in Number Theory to Applications, 21 September 2021, Springer Nature, 978-3-030-83899-7, 6,www.google.com/books/edition/From_Great_Discoveries_in_Number_Theory/tklEEAAAQBAJ?hl=en&gbpv=1&dq=natural%20numbers%20zero&pg=PA6&printsec=frontcover, en, See, for example, {{harvtxt|Carothers|2000|p=3}} or {{harvtxt|Thomson|Bruckner|Bruckner|2008|p=2}} logic and set theory texts including 0,BOOK, Gowers, Timothy, The Princeton companion to mathematics, 2008, Princeton university press, Princeton, 978-0-691-11880-2, 17, BOOK, Bagaria, Joan, Set Theory,plato.stanford.edu/entries/set-theory/, The Stanford Encyclopedia of Philosophy, Winter 2014, 2017, 13 February 2015,plato.stanford.edu/entries/set-theory/," title="web.archive.org/web/20150314173026plato.stanford.edu/entries/set-theory/,">web.archive.org/web/20150314173026plato.stanford.edu/entries/set-theory/, 14 March 2015, live, BOOK, Goldrei, Derek, Classic Set Theory: A guided independent study,archive.org/details/classicsettheory00gold, limited, 1998, Chapman & Hall/CRC, Boca Raton, Fla. [u.a.], 978-0-412-60610-6, 33, 1. ed., 1. print, 3, dictionaries excluding 0,DICTIONARY,www.merriam-webster.com/dictionary/natural%20number, natural number, Merriam-Webster.com, Merriam-Webster, 4 October 2014,web.archive.org/web/20191213133201/https://www.merriam-webster.com/dictionary/natural%20number, 13 December 2019, live, school books (through high-school level) excluding 0, and upper-division college-level books including 0.BOOK, Enderton, Herbert B., Elements of set theory, 1977, Academic Press, New York, 0122384407, 66, There are exceptions to each of these tendencies and as of 2023 no formal survey has been conducted. Arguments raised include division by zero and the size of the empty set. Computer languages often start from zero when enumerating items like loop counters and string- or array-elements.JOURNAL, Brown, Jim, In defense of index origin 0, ACM SIGAPL APL Quote Quad, 1978, 9, 2, 7, 10.1145/586050.586053, 40187000, WEB, Hui, Roger, Is index origin 0 a hindrance?,www.jsoftware.com/papers/indexorigin.htm, jsoftware.com, 19 January 2015,www.jsoftware.com/papers/indexorigin.htm," title="web.archive.org/web/20151020195547www.jsoftware.com/papers/indexorigin.htm,">web.archive.org/web/20151020195547www.jsoftware.com/papers/indexorigin.htm, 20 October 2015, live, Including 0 began to rise in popularity in the 1960s. The ISO 31-11 standard included 0 in the natural numbers in its first edition in 1978 and this has continued through its present edition as ISO 80000-2.

Formal construction

In 19th century Europe, there was mathematical and philosophical discussion about the exact nature of the natural numbers. Henri Poincaré stated that axioms can only be demonstrated in their finite application, and concluded that it is “the power of the mind” which allows conceiving of the indefinite repetition of the same act.BOOK, Poincaré, Henri, William John, Greenstreet, La Science et l’hypothèse, Science and Hypothesis, 1902, 1905, On the nature of mathematical reasoning,en.wikisource.org/wiki/Science_and_Hypothesis/Chapter_1, VI, Leopold Kronecker summarized his belief as “God made the integers, all else is the work of man”.{{efn|The English translation is from Gray. In a footnote, Gray attributes the German quote to: “Weber 1891–1892, 19, quoting from a lecture of Kronecker’s of 1886.“BOOK , BOOK , WEB , }}The constructivists saw a need to improve upon the logical rigor in the foundations of mathematics.{{efn|“Much of the mathematical work of the twentieth century has been devoted to examining the logical foundations and structure of the subject.” {{harv|Eves|1990|p=606}} }} 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.Set-theoretical definitions of natural numbers were initiated by Frege. He initially defined a natural number as the class of all sets that are in one-to-one 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 one-to-one correspondence with that set is said to have that number of elements.{{harvnb|Eves|1990|loc=Chapter 15}}In 1881, Charles Sanders Peirce provided the first axiomatization of natural-number arithmetic within this second class of definitions.JOURNAL, Peirce, C. S., Charles Sanders Peirce, 1881, On the Logic of Number,archive.org/details/jstor-2369151, American Journal of Mathematics, 4, 1, 85–95, 10.2307/2369151, 1507856, 2369151, BOOK, Shields, Paul, 1997, Studies in the Logic of Charles Sanders Peirce,archive.org/details/studiesinlogicof00nath, registration, 3. Peirce’s Axiomatization of Arithmetic,books.google.com/books?id=pWjOg-zbtMAC&pg=PA43, Houser< b. The number {{math|q}} is called the quotient and {{math|r}} is called the remainder of the division of {{math|a}} by {{math|b}}. The numbers {{math|q}} and {{math|r}} are uniquely determined by {{math|a}} and {{math|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 {{math|a}} and {{math|b}}, both {{math|a + b}} and {{math|a × b}} are natural numbers.BOOK, Fletcher, Harold, Howell, Arnold A., 9 May 2014, Mathematics with Understanding, Elsevier, 978-1-4832-8079-0, 116, en,books.google.com/books?id=7cPSBQAAQBAJ&q=Natural+numbers+closed&pg=PA116, ...the set of natural numbers is closed under addition... set of natural numbers is closed under multiplication,
• Associativity: for all natural numbers {{math|a}}, {{math|b}}, and {{math|c}}, {{math|a + (b + c) {{=}} (a + b) + c}} and {{math|a × (b × c) {{=}} (a × b) × c}}.BOOK, Davisson, Schuyler Colfax, College Algebra, 1910, Macmillian Company, 2, en,books.google.com/books?id=E7oZAAAAYAAJ&q=Natural+numbers+associative&pg=PA2, Addition of natural numbers is associative.,
• Commutativity: for all natural numbers {{math|a}} and {{math|b}}, {{math|a + b {{=}} b + a}} and {{math|a × b {{=}} b × a}}.BOOK, Brandon, Bertha (M.), Brown, Kenneth E., Gundlach, Bernard H., Cooke, Ralph J., 1962, Laidlaw mathematics series, Laidlaw Bros., 8, 25, en,books.google.com/books?id=xERMAQAAIAAJ&q=Natural+numbers+commutative,
• Existence of identity elements: for every natural number {{Math|a}}, {{math|a + 0 {{=}} a}} and {{math|a × 1 {{=}} a}}.
  • If the natural numbers are taken as “excluding 0”, and “starting at 1”, then for every natural number {{Math|a}}, {{math|a × 1 {{=}} a}}. However, the “existence of additive identity element” property is not satisfied
• Distributivity of multiplication over addition for all natural numbers {{math|a}}, {{math|b}}, and {{math|c}}, {{math|a × (b + c) {{=}} (a × b) + (a × c)}}.
• No nonzero zero divisors: if {{math|a}} and {{math|b}} are natural numbers such that {{math|a × b {{=}} 0}}, then {{math|a {{=}} 0}} or {{math|b {{=}} 0}} (or both).

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. The numbering of cardinals usually begins at zero, to accommodate the empty set emptyset. 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 aleph-null ({{math|{{not a typo|ℵ}}0}}).
• Natural numbers are also used as linguistic ordinal numbers: “first”, “second”, “third”, and so forth. The numbering of ordinals usually begins at zero, to accommodate the order type of the empty set emptyset. This way they can be assigned to the elements of a totally ordered finite set, and also to the elements of any well-ordered countably infinite set without limit points. This assignment can be generalized to general well-orderings 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 well-ordered set, in a sense different from cardinality: if there is an order isomorphism (more than a bijection) between two well-ordered sets, they have the same ordinal number. The first ordinal number that is not a natural number is expressed as {{math|ω}}; this is also the ordinal number of the set of natural numbers itself.

The least ordinal of cardinality {{math|{{not a typo|ℵ}}0}} (that is, the initial ordinal of {{math|{{not a typo|ℵ}}0}}) is {{math|ω}} but many well-ordered sets with cardinal number {{math|{{not a typo|ℵ}}0}} have an ordinal number greater than {{math|ω}}.For finite well-ordered sets, there is a one-to-one 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 non-standard model of arithmetic satisfying the Peano Arithmetic (that is, the first-order 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. Other generalizations are discussed in {{section link|Number#Extensions of the concept}}.Georges Reeb used to claim provocatively that “The naïve integers don’t fill up mathbb{N}”.JOURNAL, Approaches To Analysis With Infinitesimals Following Robinson, Nelson, And Others, Real Analysis Exchange, 2017, 42, 2, 193–253, 10.14321/realanalexch.42.2.0193, free, 1703.00425, Fletcher, Peter, Hrbacek, Karel, Kanovei, Vladimir, Katz, Mikhail G., Lobry, Claude, Sanders, Sam,

Formal definitions

There are two standard methods for formally defining natural numbers. The first one, named for Giuseppe Peano, consists of an autonomous axiomatic theory called Peano arithmetic, based on few axioms called Peano axioms.The second definition is based on set theory. It defines the natural numbers as specific sets. More precisely, each natural number {{mvar|n}} is defined as an explicitly defined set, whose elements allow counting the elements of other sets, in the sense that the sentence “a set {{mvar|S}} has {{mvar|n}} elements” means that there exists a one to one correspondence between the two sets {{mvar|n}} and {{mvar|S}}.The sets used to define natural numbers satisfy Peano axioms. It follows that every theorem that can be stated and proved in Peano arithmetic can also be proved in set theory. However, the two definitions are not equivalent, as there are theorems that can be stated in terms of Peano arithmetic and proved in set theory, which are not provable inside Peano arithmetic. A probable example is Fermat’s Last Theorem.The definition of the integers as sets satisfying Peano axioms provide a model of Peano arithmetic inside set theory. An important consequence is that, if set theory is consistent (as it is usually guessed), then Peano arithmetic is consistent. In other words, if a contradiction could be proved in Peano arithmetic, then set theory would be contradictory, and every theorem of set theory would be both true and wrong.

Peano axioms

The five Peano axioms are the following:ENCYCLOPEDIA , {{efn|{{harvtxt|Hamilton|1988|pages=117 ff}} calls them “Peano’s Postulates” and begins with “1.{{spaces|2}}0 is a natural number.“{{harvtxt|Halmos|1960|page=46}} uses the language of set theory instead of the language of arithmetic for his five axioms. He begins with “(I){{spaces|2}}{{math|0 ∈ ω}} (where, of course, {{math|0 {{=}} ∅}}” ({{math|ω}} is the set of all natural numbers).{{harvtxt|Morash|1991}} gives “a two-part axiom” in which the natural numbers begin with 1. (Section 10.1: An Axiomatization for the System of Positive Integers)}}

1. 0 is a natural number.
2. Every natural number has a successor which is also a natural number.
3. 0 is not the successor of any natural number.
4. If the successor of x equals the successor of y , then x equals y.
5. 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.

Set-theoretic definition

Intuitively, the natural number {{mvar|n}} is the common property of all sets that have {{mvar|n}} elements. So, it seems natural to define {{mvar|n}} as an equivalence class under the relation “can be made in one to one correspondence”. This does not work in set theory, as such an equivalence class would not be a set (because of Russell’s paradox). The standard solution is to define a particular set with {{mvar|n}} elements that will be called the natural number {{mvar|n}}.The following definition was first published by John von Neumann,{{Harvp|von Neumann|1923}} although Levy attributes the idea to unpublished work of Zermelo in 1916.{{harvp|Levy|1979|page=52}} As this definition extends to infinite set as a definition of ordinal number, the sets considered below are sometimes called von Neumann ordinals.The definition proceeds as follows:

• Call {{math|0 {{=}} {{mset| }}}}, the empty set.
• Define the successor {{math|S(a)}} of any set {{mvar|a}} by {{math|S(a) {{=}} a ∪ {{mset|a}}}}.
• By the axiom of infinity, there exist sets which contain 0 and are closed under the successor function. Such sets are said to be inductive. The intersection of all inductive sets is still an inductive set.
• This intersection is the set of the natural numbers.

It follows that the natural numbers are defined iteratively as follows: It can be checked that the natural numbers satisfy the Peano axioms.With this definition, given a natural number {{math|n}}, the sentence “a set {{mvar|S}} has {{mvar|n}} elements” can be formally defined as “there exists a bijection from {{mvar|n}} to {{mvar|S}}. This formalizes the operation of counting the elements of {{mvar|S}}. Also, {{math|n ≤ m}} if and only if {{math|n}} is a subset of {{math|m}}. In other words, the set inclusion defines the usual total order on the natural numbers. This order is a well-order.It follows from the definition that each natural number is equal to the set of all natural numbers less than it. This definition, can be extended to the von Neumann definition of ordinals for defining all ordinal numbers, including the infinite ones: “each ordinal is the well-ordered set of all smaller ordinals.“If one does not accept the axiom of infinity, the natural numbers may not form a set. Nevertheless, the natural numbers can still be individually defined as above, and they still satisfy the Peano axioms.There are other set theoretical constructions. In particular, Ernst Zermelo provided a construction that is nowadays only of historical interest, and is sometimes referred to as {{vanchor|Zermelo ordinals}}. It consists in defining {{math|0}} as the empty set, and {{math|S(a) {{=}} {{mset|a}}}}.With this definition each natural number is a singleton set. So, the property of the natural numbers to represent cardinalities is not directly accessible; only the ordinal property (being the {{mvar|n}}th element of a sequence) is immediate. Unlike von Neumann’s construction, the Zermelo ordinals do not extend to infinite ordinals.

See also

• {{annotated link|Canonical representation of a positive integer}}
• {{annotated link|Countable set}}
• Sequence â€“ Function of the natural numbers in another set
• {{annotated link|Ordinal number}}
• {{annotated link|Cardinal number}}
• {{annotated link|Set-theoretic definition of natural numbers}}

{{Classification of numbers}}

Notes

{{Notelist}}

References

{{Reflist}}

Bibliography

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, – English translation of {{Harvnb|von Neumann|1923}}.

External links

{{Commons category|Natural numbers}}

• {{springer|title=Natural number|id=p/n066090}}
• WEB

, {{Number systems}}{{Classes of natural numbers}}{{Authority control}}

editor2-last= Roberts

editor3-last= Van Evra

publisher= Indiana University Press

pages= 43–52, In 1888, Richard Dedekind proposed another axiomatization of natural-number arithmetic,WAS SIND UND WAS SOLLEN DIE ZAHLEN? >DATE=1893

URL=HTTPS://ARCHIVE.ORG/DETAILS/WASSINDUNDWASSO00DEDEGOOG/PAGE/N42/MODE/2UP

AT=71-73, and in 1889, Peano published a simplified version of Dedekind’s axioms in his book The principles of arithmetic presented by a new method (). 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 non-zero 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.JOURNAL , Baratella, Stefano , Ferro, Ruggero , 10.1002/malq.19930390138 , 3 , Mathematical Logic Quarterly , 1270381 , 338–352 , A theory of sets with the negation of the axiom of infinity , 39 , 1993, Theorems that can be proved in ZFC but cannot be proved using the Peano Axioms include Goodstein’s theorem.JOURNAL, Kirby, Laurie, Paris, Jeff, Accessible Independence Results for Peano Arithmetic, Bulletin of the London Mathematical Society, Wiley, 14, 4, 1982, 0024-6093, 10.1112/blms/14.4.285, 285–293, Notation The set of all natural numbers is standardly denoted {{math|N}} or mathbb N.WEB, Listing of the Mathematical Notations used in the Mathematical Functions Website: Numbers, variables, and functions,functions.wolfram.com/Notations/1/, 27 July 2020, functions.wolfram.com, Older texts have occasionally employed {{math|J}} as the symbol for this set.BOOK,archive.org/details/1979RudinW, Principles of Mathematical Analysis, Rudin, W., McGraw-Hill, 1976, 978-0-07-054235-8, New York, 25, Since natural numbers may contain {{math|0}} or not, it may be important to know which version is referred to. This is often specified by the context, but may also be done by using a subscript or a superscript in the notation, such as:BOOK, Grimaldi, Ralph P., Discrete and Combinatorial Mathematics: An applied introduction, Pearson Addison Wesley, 978-0-201-72634-3, 5th, 2004, Naturals without zero: {1,2,...}=mathbb{N}^= mathbb N^+=mathbb{N}_0smallsetminus{0} = mathbb{N}_1 Naturals with zero: ;{0,1,2,...}=mathbb{N}_0=mathbb N^0=mathbb{N}^cup{0} Alternatively, since the natural numbers naturally form a subset of the integers (often {{nowrap|denoted mathbb Z),}} they may be referred to as the positive, or the non-negative integers, respectively.BOOK, Grimaldi, Ralph P., A review of discrete and combinatorial mathematics, 2003, Addison-Wesley, Boston, 978-0-201-72634-3, 133, 5th, To be unambiguous about whether 0 is included or not, sometimes a superscript “*” or “+” is added in the former case, and a subscript (or superscript) “0” is added in the latter case:BOOK, ISO 80000-2:2019,cdn.standards.iteh.ai/samples/64973/329519100abd447ea0d49747258d1094/ISO-80000-2-2019.pdf#page=10, International Organization for Standardization, Standard number sets and intervals, 19 May 2020, 4,www.iso.org/standard/64973.html, {1, 2, 3,dots} = {x in mathbb Z : x > 0}=mathbb Z^+= mathbb{Z}_{>0} {0, 1, 2,dots} = {x in mathbb Z : x ge 0}=mathbb Z^{+}_{0}=mathbb{Z}_ {ge 0} Properties This section uses the convention mathbb{N}=mathbb{N}_0=mathbb{N}^*cup{0}. Addition Given the set mathbb{N} of natural numbers and the successor function S colon mathbb{N} to mathbb{N} sending each natural number to the next one, one can define addition of natural numbers recursively by setting {{math|a + 0 {{=}} a}} and {{math|a + S(b) {{=}} S(a + b)}} for all {{math|a}}, {{math|b}}. Thus, {{math|a + 1 {{=}} a + S(0) {{=}} S(a+0) {{=}} S(a)}}, {{math|a + 2 {{=}} a + S(1) {{=}} S(a+1) {{=}} S(S(a))}}, and so on. The algebraic structure (mathbb{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 {{math|S(0)}}, then {{math|b + 1 {{=}} b + S(0) {{=}} S(b + 0) {{=}} S(b)}}. That is, {{math|b + 1}} is simply the successor of {{math|b}}. Multiplication Analogously, given that addition has been defined, a multiplication operator times can be defined via {{math|a × 0 {{=}} 0}} and {{math|a × S(b) {{=}} (a × b) + a}}. This turns (mathbb{N}^*, times) 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: {{math|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 mathbb{N} is not closed under subtraction (that is, subtracting one natural from another does not always result in another natural), means that mathbb{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 {{math|a + 1 {{=}} S(a)}} and {{math|a × 1 {{=}} a}}. Furthermore, (mathbb{N^*}, +) has no identity element. Order In this section, juxtaposed variables such as {{math|ab}} indicate the product {{math|a × b}},WEB, Weisstein, Eric W., Multiplication,mathworld.wolfram.com/Multiplication.html, 27 July 2020, mathworld.wolfram.com, en, and the standard order of operations is assumed.A total order on the natural numbers is defined by letting {{math|a ≤ b}} if and only if there exists another natural number {{math|c}} where {{math|a + c {{=}} b}}. This order is compatible with the arithmetical operations in the following sense: if {{math|a}}, {{math|b}} and {{math|c}} are natural numbers and {{math|a ≤ b}}, then {{math|a + c ≤ b + c}} and {{math|ac ≤ bc}}.An important property of the natural numbers is that they are well-ordered: every non-empty set of natural numbers has a least element. The rank among well-ordered sets is expressed by an ordinal number; for the natural numbers, this is denoted as {{math|ω}} (omega). Division In this section, juxtaposed variables such as {{math|ab}} indicate the product {{math|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 {{math|a}} and {{math|b}} with {{math|b ≠ 0}} there are natural numbers {{math|q}} and {{math|r}} such that a = bq + r text{ and } r