Natural number

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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, and so on, possibly excluding 0.<ref name="Enderton"/> Some start counting with 0, defining the natural numbers as the non-negative integers Template:Math, while others start with 1, defining them as the positive integers Template:Nobr Some authors acknowledge both definitions whenever convenient.<ref name=":1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Sometimes, the whole numbers are the natural numbers as well as zero. In other cases, the whole numbers refer to all of the integers, including negative integers.<ref>Template:Cite dictionary</ref> The counting numbers are another term for the natural numbers, particularly in primary education, and are ambiguous as well although typically start at 1.<ref name=MathWorld_CountingNumber>{{#invoke:Template wrapper|{{#if:|list|wrap}}|_template=cite web |_exclude=urlname, _debug, id |url = https://mathworld.wolfram.com/{{#if:CountingNumber%7CCountingNumber.html}} |title = Counting Number |author = Weisstein, Eric W. |website = MathWorld |access-date = |ref = Template:SfnRef }}</ref>

The natural numbers are used for counting things, like "there are six coins on the table", in which case they are called cardinal numbers. They are also used to put things in order, like "this is the third largest city in the country", which are called ordinal numbers. Natural numbers are also used as labels, like jersey numbers on a sports team, where they serve as nominal numbers and do not have mathematical properties.<ref>Template:Cite journal</ref>

The natural numbers form a set, commonly symbolized as a bold Template:Math or blackboard bold Template:Tmath. Many other number sets are built from the natural numbers. For example, the integers are made by adding 0 and negative numbers. The rational numbers add fractions, and the real numbers add all infinite decimals. Complex numbers add the [[Imaginary unit|square root of Template:Math]]. This chain of extensions canonically embeds the natural numbers in the other number systems.<ref>Template:Harvtxt says: "The whole fantastic hierarchy of number systems is built up by purely set-theoretic means from a few simple assumptions about natural numbers."</ref><ref>Template:Harvtxt: "Numbers make up the foundation of mathematics."</ref>

Natural numbers are studied in different areas of math. Number theory looks at things like how numbers divide evenly (divisibility), or how prime numbers are spread out. Combinatorics studies counting and arranging numbered objects, such as partitions and enumerations.

HistoryEdit

Ancient rootsEdit

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CitationClass=web }}, on permanent display at the Royal Belgian Institute of Natural Sciences, Brussels, Belgium.</ref> is believed to have been used 20,000 years ago for natural number arithmetic.

The 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.<ref>Template:Cite book</ref>

A much later advance was the development of the idea that Template:Num 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.Template:Efn The Olmec and Maya civilizations used 0 as a separate number as early as the Template:Nowrap, but this usage did not spread beyond Mesoamerica.<ref>Template:Cite book</ref><ref>Template:Cite book</ref> 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 {{#invoke:Lang|lang}}, the Latin word for "none", was employed to denote a 0 value.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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.Template:Efn 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).<ref name="Mueller 2006 p. 58">Template:Cite book</ref> However, in the definition of perfect number which comes shortly afterward, Euclid treats 1 as a number like any other.<ref>Template:Cite book In definition VII.3 a "part" was defined as a number, but here 1 is considered to be a part, so that for example Template:Math is a perfect number.</ref>

Independent studies on numbers also occurred at around the same time in India, China, and Mesoamerica.<ref>Template:Cite book</ref>

Emergence as a termEdit

Nicolas Chuquet used the term progression naturelle (natural progression) in 1484.<ref>Template:Cite book</ref> The earliest known use of "natural number" as a complete English phrase is in 1763.<ref>Template:Cite book</ref><ref name="MacTutor"/> The 1771 Encyclopaedia Britannica defines natural numbers in the logarithm article.<ref name="MacTutor">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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.<ref>Template:Cite book</ref> In 1889, Giuseppe Peano used N for the positive integers and started at 1,<ref>Template:Cite book</ref> but he later changed to using N₀ and N₁.<ref>Template:Cite book</ref> Historically, most definitions have excluded 0,<ref name="MacTutor"/><ref>Template:Cite book</ref><ref>Template:Cite book</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="MacTutor"/>

Mathematicians have noted tendencies in which definition is used, such as algebra texts including 0,<ref name="MacTutor"/>Template:Efn number theory and analysis texts excluding 0,<ref name="MacTutor"/><ref name="Křížek">Template:Cite book</ref><ref>See, for example, Template:Harvtxt or Template:Harvtxt</ref> logic and set theory texts including 0,<ref>Template:Cite book</ref><ref>Template:Cite book</ref><ref>Template:Cite book</ref> dictionaries excluding 0,<ref name="MacTutor"/><ref>Template:Cite dictionary</ref> school books (through high-school level) excluding 0, and upper-division college-level books including 0.<ref name="Enderton">Template:Cite book</ref> There are exceptions to each of these tendencies and as of 2023 no formal survey has been conducted. Arguments raised include division by zero<ref name="Křížek"/> and the size of the empty set. Computer languages often start from zero when enumerating items like loop counters and string- or array-elements.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Including 0 began to rise in popularity in the 1960s.<ref name="MacTutor"/> 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.<ref name=ISO80000/>

Formal constructionEdit

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.<ref>Template:Cite book</ref> Leopold Kronecker summarized his belief as "God made the integers, all else is the work of man".Template:Efn

The constructivists saw a need to improve upon the logical rigor in the foundations of mathematics.Template:Efn 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 emerged, using set theory and Peano's axioms respectively. 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.<ref>Template:Harvnb</ref>

In 1881, Charles Sanders Peirce provided the first axiomatization of natural-number arithmetic.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> In 1888, Richard Dedekind proposed another axiomatization of natural-number arithmetic,<ref>Template:Cite book</ref> and in 1889, Peano published a simplified version of Dedekind's axioms in his book The principles of arithmetic presented by a new method (Template:Langx). 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.<ref>Template:Cite journal</ref> Theorems that can be proved in ZFC but cannot be proved using the Peano Axioms include Goodstein's theorem.<ref>Template:Cite journal</ref>

NotationEdit

The set of all natural numbers is standardly denoted Template:Math or <math>\mathbb N.</math><ref name=":1"/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Older texts have occasionally employed Template:Math as the symbol for this set.<ref>Template:Cite book</ref>

Since natural numbers may contain Template:Math 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:<ref name="ISO80000"/><ref name="Grimaldi">Template:Cite book</ref>

Naturals without zero: <math>\{1,2,...\}=\mathbb{N}^*= \mathbb N^+=\mathbb{N}_0\smallsetminus\{0\} = \mathbb{N}_1</math>
Naturals with zero: <math>\;\{0,1,2,...\}=\mathbb{N}_0=\mathbb N^0=\mathbb{N}^*\cup\{0\}</math>

Alternatively, since the natural numbers naturally form a subset of the integers (often Template:Nowrap they may be referred to as the positive, or the non-negative integers, respectively.<ref>Template:Cite book</ref> To be unambiguous about whether 0 is included or not, sometimes a superscript "<math>*</math>" or "+" is added in the former case, and a subscript (or superscript) "0" is added in the latter case:<ref name=ISO80000 >Template:Cite book</ref>

<math>\{1, 2, 3,\dots\} = \{x \in \mathbb Z : x > 0\}=\mathbb Z^+= \mathbb{Z}_{>0}</math>

<math>\{0, 1, 2,\dots\} = \{x \in \mathbb Z : x \ge 0\}=\mathbb Z^{+}_{0}=\mathbb{Z}_ {\ge 0}</math>

PropertiesEdit

This section uses the convention <math>\mathbb{N}=\mathbb{N}_0=\mathbb{N}^*\cup\{0\}</math>.

AdditionEdit

Given the set <math>\mathbb{N}</math> of natural numbers and the successor function <math>S \colon \mathbb{N} \to \mathbb{N}</math> sending each natural number to the next one, one can define addition of natural numbers recursively by setting Template:Math and Template:Math for all Template:Math, Template:Math. Thus, Template:Math, Template:Math, and so on. The algebraic structure <math>(\mathbb{N}, +)</math> 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 Template:Math, then Template:Math. That is, Template:Math is simply the successor of Template:Math.

MultiplicationEdit

Analogously, given that addition has been defined, a multiplication operator <math>\times</math> can be defined via Template:Math and Template:Math. This turns <math>(\mathbb{N}^*, \times)</math> 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 multiplicationEdit

Addition and multiplication are compatible, which is expressed in the distribution law: Template:Math. 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 <math>\mathbb{N}</math> is not closed under subtraction (that is, subtracting one natural from another does not always result in another natural), means that <math>\mathbb{N}</math> 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 Template:Math and Template:Math. Furthermore, <math>(\mathbb{N^*}, +)</math> has no identity element.

OrderEdit

In this section, juxtaposed variables such as Template:Math indicate the product Template:Math,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and the standard order of operations is assumed.

A total order on the natural numbers is defined by letting Template:Math if and only if there exists another natural number Template:Math where Template:Math. This order is compatible with the arithmetical operations in the following sense: if Template:Math, Template:Math and Template:Math are natural numbers and Template:Math, then Template:Math and Template:Math.

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 Template:Math (omega).

DivisionEdit

In this section, juxtaposed variables such as Template:Math indicate the product Template:Math, 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 Template:Math and Template:Math with Template:Math there are natural numbers Template:Math and Template:Math such that

The number Template:Math is called the quotient and Template:Math is called the remainder of the division of Template:Math by Template:Math. The numbers Template:Math and Template:Math are uniquely determined by Template:Math and Template:Math. 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 numbersEdit

The addition (+) and multiplication (×) operations on natural numbers as defined above have several algebraic properties:

Closure under addition and multiplication: for all natural numbers Template:Math and Template:Math, both Template:Math and Template:Math are natural numbers.<ref>Template:Cite book</ref>
Associativity: for all natural numbers Template:Math, Template:Math, and Template:Math, Template:Math and Template:Math.<ref>Template:Cite book</ref>
Commutativity: for all natural numbers Template:Math and Template:Math, Template:Math and Template:Math.<ref>Template:Cite book</ref>
Existence of identity elements: for every natural number Template:Math, Template:Math and Template:Math.
- If the natural numbers are taken as "excluding 0", and "starting at 1", then for every natural number Template:Math, Template:Math. However, the "existence of additive identity element" property is not satisfied
Distributivity of multiplication over addition for all natural numbers Template:Math, Template:Math, and Template:Math, Template:Math.
No nonzero zero divisors: if Template:Math and Template:Math are natural numbers such that Template:Math, then Template:Math or Template:Math (or both).

GeneralizationsEdit

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 <math>\emptyset</math>. 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 (Template:Math).
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 <math>\emptyset</math>. 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 Template:Math; this is also the ordinal number of the set of natural numbers itself.

The least ordinal of cardinality Template:Math (that is, the initial ordinal of Template:Math) is Template:Math but many well-ordered sets with cardinal number Template:Math have an ordinal number greater than Template:Math.

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 Template:Section link.

Georges Reeb used to claim provocatively that "The naïve integers don't fill up <math>\mathbb{N}</math>".<ref>Template:Cite journal</ref>

Formal definitionsEdit

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 Template:Mvar is defined as an explicitly defined set, whose elements allow counting the elements of other sets, in the sense that the sentence "a set Template:Mvar has Template:Mvar elements" means that there exists a one to one correspondence between the two sets Template:Mvar and Template:Mvar.

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 axiomsEdit

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The five Peano axioms are the following:<ref>Template:Cite encyclopedia</ref>Template:Efn

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 <math> x </math> equals the successor of <math> y </math>, then <math> x</math> equals <math> y</math>.
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 <math> x</math> is <math> x + 1</math>.

Set-theoretic definitionEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Intuitively, the natural number Template:Mvar is the common property of all sets that have Template:Mvar elements. So, it seems natural to define Template:Mvar as an equivalence class under the relation "can be made in one to one correspondence". This does not work in all set theories, as such an equivalence class would not be a setTemplate:Efn (because of Russell's paradox). The standard solution is to define a particular set with Template:Mvar elements that will be called the natural number Template:Mvar.

The following definition was first published by John von Neumann,<ref name="vonNeumann1923pp199-208">Template:Harvp</ref> although Levy attributes the idea to unpublished work of Zermelo in 1916.<ref name="Levy">Template:Harvp</ref> 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 Template:Math, the empty set.
Define the successor Template:Math of any set Template:Mvar by Template:Math.
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 Template:Math, the sentence "a set Template:Mvar has Template:Mvar elements" can be formally defined as "there exists a bijection from Template:Mvar to Template:Mvar." This formalizes the operation of counting the elements of Template:Mvar. Also, Template:Math if and only if Template:Math is a subset of Template:Math. 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 Template:Vanchor.<ref name="Levy"/> It consists in defining Template:Math as the empty set, and Template:Math.

With this definition each nonzero 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 Template:Mvarth element of a sequence) is immediate. Unlike von Neumann's construction, the Zermelo ordinals do not extend to infinite ordinals.