In mathematics, the natural numbers are the numbers 0, 1, 2, 3, etc., possibly excluding 0. Some define the natural numbers as the non-negative integers, while others define them as the positive integers 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).[1] The counting numbers refer to the natural numbers in common language, particularly in primary school education, and are similarly ambiguous although typically exclude zero.
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 labelsalso known as nominal numbers, (e.g. jersey numbers in sports)which do not have the properties of numbers in a mathematical sense.[2] [3]
1/n
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.
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.[4]
A much later advance was the development of the idea that 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. The Olmec and Maya civilizations used 0 as a separate number as early as the, but this usage did not spread beyond Mesoamerica.[5] [6] 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.[7]
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. 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).[8] However, in the definition of perfect number which comes shortly afterward, Euclid treats 1 as a number like any other.[9]
Independent studies on numbers also occurred at around the same time in India, China, and Mesoamerica.[10]
Nicolas Chuquet used the term progression naturelle (natural progression) in 1484.[11] The earliest known use of "natural number" as a complete English phrase is in 1763.[12] The 1771 Encyclopaedia Britannica defines natural numbers in the logarithm article.[13]
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.[14] In 1889, Giuseppe Peano used N for the positive integers and started at 1,[15] but he later changed to using N0 and N1.[16] Historically, most definitions have excluded 0,[13] [17] [18] 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.[19] [13]
Mathematicians have noted tendencies in which definition is used, such as algebra texts including 0,[13] number theory and analysis texts excluding 0,[13] [20] [21] logic and set theory texts including 0,[22] [23] [24] dictionaries excluding 0,[13] [25] school books (through high-school level) excluding 0, and upper-division college-level books including 0.[26] There are exceptions to each of these tendencies and as of 2023 no formal survey has been conducted. Arguments raised include division by zero[20] and the size of the empty set. Computer languages often start from zero when enumerating items like loop counters and string- or array-elements.[27] [28] Including 0 began to rise in popularity in the 1960s.[13] 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.
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.[29] Leopold Kronecker summarized his belief as "God made the integers, all else is the work of man".
The constructivists saw a need to improve upon the logical rigor in the foundations of mathematics. 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.
In 1881, Charles Sanders Peirce provided the first axiomatization of natural-number arithmetic within this second class of definitions.[30] [31] In 1888, Richard Dedekind proposed another axiomatization of natural-number arithmetic,[32] and in 1889, Peano published a simplified version of Dedekind's axioms in his book The principles of arithmetic presented by a new method (Latin: [[Arithmetices principia, nova methodo exposita]]). 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.[33] Theorems that can be proved in ZFC but cannot be proved using the Peano Axioms include Goodstein's theorem.[34]
The set of all natural numbers is standardly denoted or
N.
Since natural numbers may contain 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:[37]
\{1,2,...\}=N*=
| +=N | |
| N | |
| 0\smallsetminus\{0\} |
=N1
| 0=N | |
| \{0,1,2,...\}=N | |
| 0=N |
*\cup\{0\}
Alternatively, since the natural numbers naturally form a subset of the integers (often they may be referred to as the positive, or the non-negative integers, respectively.[38] To be unambiguous about whether 0 is included or not, sometimes a superscript "
*
\{1,2,3,...\}=\{x\inZ:x>0\}=Z+=Z>0
\{0,1,2,...\}=\{x\inZ:x\ge
| + | |
| 0\}=Z | |
| 0 |
=Z\ge
This section uses the convention
| *\cup\{0\} | |
| N=N | |
| 0=N |
Given the set
N
S\colonN\toN
(N,+)
If 1 is defined as, then . That is, is simply the successor of .
Analogously, given that addition has been defined, a multiplication operator
x
(N*, x )
Addition and multiplication are compatible, which is expressed in the distribution law: . 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
N
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 and . Furthermore,
| (N*, |
+)
In this section, juxtaposed variables such as indicate the product,[40] and the standard order of operations is assumed.
A total order on the natural numbers is defined by letting if and only if there exists another natural number where . This order is compatible with the arithmetical operations in the following sense: if, and are natural numbers and, then and .
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 (omega).
In this section, juxtaposed variables such as indicate the product, 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 and with there are natural numbers and such that
a=bq+randr<b.
The number is called the quotient and is called the remainder of the division of by . The numbers and are uniquely determined by and . This Euclidean division is key to the several other properties (divisibility), algorithms (such as the Euclidean algorithm), and ideas in number theory.
The addition (+) and multiplication (×) operations on natural numbers as defined above have several algebraic properties:
Two important generalizations of natural numbers arise from the two uses of counting and ordering: cardinal numbers and ordinal numbers.
\emptyset
\emptyset
The least ordinal of cardinality (that is, the initial ordinal of) is but many well-ordered sets with cardinal number have an ordinal number greater than .
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 .
Georges Reeb used to claim provocatively that "The naïve integers don't fill up
N
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 is defined as an explicitly defined set, whose elements allow counting the elements of other sets, in the sense that the sentence "a set has elements" means that there exists a one to one correspondence between the two sets and .
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.
See main article: Peano axioms.
The five Peano axioms are the following:[45]
x
y
x
y
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
x+1
See main article: Set-theoretic definition of natural numbers.
Intuitively, the natural number is the common property of all sets that have elements. So, it seems natural to define 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 elements that will be called the natural number .
The following definition was first published by John von Neumann, although Levy attributes the idea to unpublished work of Zermelo in 1916. 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:
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, the sentence "a set has elements" can be formally defined as "there exists a bijection from to . This formalizes the operation of counting the elements of . Also, if and only if is a subset of . 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 . It consists in defining as the empty set, and .
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 th element of a sequence) is immediate. Unlike von Neumann's construction, the Zermelo ordinals do not extend to infinite ordinals.