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Countable set

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Mathematical set that can be enumerated

"Countable" redirects here. For the linguistic concept, seeCount noun. For the statistical concept, seeCount data. For the company, seeCountable (app).Not to be confused with(recursively) enumerable sets.

Inmathematics, aset iscountable if either it isfinite or it can be made inone to one correspondence with the set ofnatural numbers.^[a] Equivalently, a set iscountable if there exists aninjective function from it into the natural numbers; this means that each element in the set may be associated to a unique natural number, or that the elements of the set can be counted one at a time, although the counting may never finish due to an infinite number of elements.

In more technical terms, assuming theaxiom of countable choice, a set iscountable if itscardinality (the number of elements of the set) is not greater than that of the natural numbers. A countable set that is not finite is said to becountably infinite.

The concept is attributed toGeorg Cantor, who proved the existence ofuncountable sets, that is, sets that are not countable; for example the set of thereal numbers.

A note on terminology

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Although the terms "countable" and "countably infinite" as defined here are quite common, the terminology is not universal.^[1] An alternative style usescountable to mean what is here called countably infinite, andat most countable to mean what is here called countable.^[2]^[3]

The termsenumerable^[4] anddenumerable^[5]^[6] may also be used, e.g. referring to countable and countably infinite respectively,^[7] definitions vary and care is needed respecting the difference withrecursively enumerable.^[8]

Definition

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A set $S {\displaystyle S}$ iscountable if:

Itscardinality $|S|$ is less than or equal to $\aleph _{0}$ (aleph-null), the cardinality of the set ofnatural numbers $\mathbb {N}$ .^[9]
There exists aninjective function from $S {\displaystyle S}$ to $\mathbb {N}$ .^[10]^[11]
$S {\displaystyle S}$ is empty or there exists asurjective function from $\mathbb {N}$ to $S {\displaystyle S}$ .^[11]
There exists abijective mapping between $S {\displaystyle S}$ and a subset of $\mathbb {N}$ .^[12]
$S {\displaystyle S}$ is eitherfinite ( $|S|<\aleph _{0}$ ) or countably infinite.^[5]

All of these definitions are equivalent.

A set $S {\displaystyle S}$ iscountablyinfinite if:

Its cardinality $|S|$ is exactly $\aleph _{0}$ .^[9]
There is an injective and surjective (and thereforebijective) mapping between $S {\displaystyle S}$ and $\mathbb {N}$ .
$S {\displaystyle S}$ has aone-to-one correspondence with $\mathbb {N}$ .^[13]
The elements of $S {\displaystyle S}$ can be arranged in an infinite sequence $a_{0},a_{1},a_{2},\ldots$ , where $a_{i}$ is distinct from $a_{j}$ for $i\neq j$ and every element of $S {\displaystyle S}$ is listed.^[14]^[15]

A set isuncountable if it is not countable, i.e. its cardinality is greater than $\aleph _{0}$ .^[9]

History

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In 1874, inhis first set theory article, Cantor proved that the set ofreal numbers is uncountable, thus showing that not all infinite sets are countable.^[16] In 1878, he used one-to-one correspondences to define and compare cardinalities.^[17] In 1883, he extended the natural numbers with his infiniteordinals, and used sets of ordinals to produce an infinity of sets having different infinite cardinalities.^[18]

Introduction

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Aset is a collection ofelements, and may be described in many ways. One way is simply to list all of its elements; for example, the set consisting of the integers 3, 4, and 5 may be denoted $\{3,4,5\}$ , called roster form.^[19] This is only effective for small sets, however; for larger sets, this would be time-consuming and error-prone. Instead of listing every single element, sometimes an ellipsis ("...") is used to represent many elements between the starting element and the end element in a set, if the writer believes that the reader can easily guess what ... represents; for example, $\{1,2,3,\dots ,100\}$ presumably denotes the set ofintegers from 1 to 100. Even in this case, however, it is stillpossible to list all the elements, because the number of elements in the set is finite. If we number the elements of the set 1, 2, and so on, up to $n {\displaystyle n}$ , this gives us the usual definition of "sets of size $n {\displaystyle n}$ ".

Bijective mapping from integer to even numbers

Some sets areinfinite; these sets have more than $n {\displaystyle n}$ elements where $n {\displaystyle n}$ is any integer that can be specified. (No matter how large the specified integer $n {\displaystyle n}$ is, such as $n=10^{1000}$ , infinite sets have more than $n {\displaystyle n}$ elements.) For example, the set of natural numbers, denotable by $\{0,1,2,3,4,5,\dots \}$ ,^[a] has infinitely many elements, and we cannot use any natural number to give its size. It might seem natural to divide the sets into different classes: put all the sets containing one element together; all the sets containing two elements together; ...; finally, put together all infinite sets and consider them as having the same size. This view works well for countably infinite sets and was the prevailing assumption before Georg Cantor's work. For example, there are infinitely many odd integers, infinitely many even integers, and also infinitely many integers overall. We can consider all these sets to have the same "size" because we can arrange things such that, for every integer, there is a distinct even integer: $\ldots \,-\!2\!\rightarrow \!-\!4,\,-\!1\!\rightarrow \!-\!2,\,0\!\rightarrow \!0,\,1\!\rightarrow \!2,\,2\!\rightarrow \!4\,\cdots$ or, more generally, $n\rightarrow 2n$ (see picture). What we have done here is arrange the integers and the even integers into aone-to-one correspondence (orbijection), which is afunction that maps between two sets such that each element of each set corresponds to a single element in the other set. This mathematical notion of "size", cardinality, is that two sets are of the same size if and only if there is a bijection between them. We call all sets that are in one-to-one correspondence with the integerscountably infinite and say they have cardinality $\aleph _{0}$ .

Georg Cantor showed that not all infinite sets are countably infinite. For example, the real numbers cannot be put into one-to-one correspondence with the natural numbers (non-negative integers). The set of real numbers has a greater cardinality than the set of natural numbers and is said to be uncountable.

Formal overview

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By definition, a set $S {\displaystyle S}$ iscountable if there exists abijection between $S {\displaystyle S}$ and a subset of thenatural numbers $\mathbb {N} =\{0,1,2,\dots \}$ . For example, define the correspondence $a\leftrightarrow 1,\ b\leftrightarrow 2,\ c\leftrightarrow 3$ Since every element of $S=\{a,b,c\}$ is paired withprecisely one element of $\{1,2,3\}$ ,and vice versa, this defines a bijection, and shows that $S {\displaystyle S}$ is countable. Similarly we can show all finite sets to be countable.

As for the case of infinite sets, a set $S {\displaystyle S}$ is countably infinite if there is abijection between $S {\displaystyle S}$ and all of $\mathbb {N}$ . As examples, consider the sets $A=\{1,2,3,\dots \}$ , the set of positiveintegers, and $B=\{0,2,4,6,\dots \}$ , the set of even integers. We can show these sets are countably infinite by exhibiting a bijection to the natural numbers. This can be achieved using the assignments $n\leftrightarrow n+1$ and $n\leftrightarrow 2n$ , so that ${\begin{matrix}0\leftrightarrow 1,&1\leftrightarrow 2,&2\leftrightarrow 3,&3\leftrightarrow 4,&4\leftrightarrow 5,&\ldots \\[6pt]0\leftrightarrow 0,&1\leftrightarrow 2,&2\leftrightarrow 4,&3\leftrightarrow 6,&4\leftrightarrow 8,&\ldots \end{matrix}}$ Every countably infinite set is countable, and every infinite countable set is countably infinite. Furthermore, any subset of the natural numbers is countable, and more generally:

Theorem—A subset of a countable set is countable.^[20]

The set of allordered pairs of natural numbers (theCartesian product of two sets of natural numbers, $\mathbb {N} \times \mathbb {N}$ ) is countably infinite, as can be seen by following a path like the one in the picture:

TheCantor pairing function assigns one natural number to each pair of natural numbers

The resultingmapping proceeds as follows:

$0\leftrightarrow (0,0),1\leftrightarrow (1,0),2\leftrightarrow (0,1),3\leftrightarrow (2,0),4\leftrightarrow (1,1),5\leftrightarrow (0,2),6\leftrightarrow (3,0),\ldots$ This mapping covers all such ordered pairs.

This form of triangular mappingrecursively generalizes to $n {\displaystyle n}$ -tuples of natural numbers, i.e., $(a_{1},a_{2},a_{3},\dots ,a_{n})$ where $a_{i}$ and $n {\displaystyle n}$ are natural numbers, by repeatedly mapping the first two elements of an $n {\displaystyle n}$ -tuple to a natural number. For example, $(0,2,3)$ can be written as $((0,2),3)$ . Then $(0,2)$ maps to 5 so $((0,2),3)$ maps to $(5,3)$ , then $(5,3)$ maps to 39. Since a different 2-tuple, that is a pair such as $(a,b)$ , maps to a different natural number, a difference between two n-tuples by a single element is enough to ensure the n-tuples being mapped to different natural numbers. So, an injection from the set of $n {\displaystyle n}$ -tuples to the set of natural numbers $\mathbb {N}$ is proved. For the set of $n {\displaystyle n}$ -tuples made by the Cartesian product of finitely many different sets, each element in each tuple has the correspondence to a natural number, so every tuple can be written in natural numbers then the same logic is applied to prove the theorem.

Theorem—TheCartesian product of finitely many countable sets is countable.^[21]^[b]

The set of allintegers $\mathbb {Z}$ and the set of allrational numbers $\mathbb {Q}$ may intuitively seem much bigger than $\mathbb {N}$ . But looks can be deceiving. If a pair is treated as thenumerator anddenominator of avulgar fraction (a fraction in the form of $a/b$ where $a {\displaystyle a}$ and $b\neq 0$ are integers), then for every positive fraction, we can come up with a distinct natural number corresponding to it. This representation also includes the natural numbers, since every natural number $n {\displaystyle n}$ is also a fraction $n/1$ . So we can conclude that there are exactly as many positive rational numbers as there are positive integers. This is also true for all rational numbers, as can be seen below.

Theorem— $\mathbb {Z}$ (the set of all integers) and $\mathbb {Q}$ (the set of all rational numbers) are countable.^[c]

In a similar manner, the set ofalgebraic numbers is countable.^[23]^[d]

Sometimes more than one mapping is useful: if a set $A {\displaystyle A}$ to be shown as countable is one-to-one mapped (injection) to another set $B {\displaystyle B}$ , then $A {\displaystyle A}$ is proved as countable if $B {\displaystyle B}$ is one-to-one mapped to the set of natural numbers. For example, the set of positiverational numbers can easily be one-to-one mapped to the set of natural number pairs (2-tuples) because $p/q$ maps to $(p,q)$ . Since the set of natural number pairs is one-to-one mapped (actually one-to-one correspondence or bijection) to the set of natural numbers as shown above, the positive rational number set is proved as countable.

Theorem—Any finiteunion of countable sets is countable.^[24]^[25]^[e]

With the foresight of knowing that there are uncountable sets, we can wonder whether or not this last result can be pushed any further. The answer is "yes" and "no", we can extend it, but we need to assume a new axiom to do so.

Theorem—(Assuming theaxiom of countable choice) The union of countably many countable sets is countable.^[f]

Enumeration for countable number of countable sets

For example, given countable sets ${\textbf {a}},{\textbf {b}},{\textbf {c}},\dots$ , we first assign each element of each set a tuple, then we assign each tuple an index using a variant of the triangular enumeration we saw above: ${\begin{array}{c|c|c }{\text{Index}}&{\text{Tuple}}&{\text{Element}}\\\hline 0&(0,0)&{\textbf {a}}_{0}\\1&(0,1)&{\textbf {a}}_{1}\\2&(1,0)&{\textbf {b}}_{0}\\3&(0,2)&{\textbf {a}}_{2}\\4&(1,1)&{\textbf {b}}_{1}\\5&(2,0)&{\textbf {c}}_{0}\\6&(0,3)&{\textbf {a}}_{3}\\7&(1,2)&{\textbf {b}}_{2}\\8&(2,1)&{\textbf {c}}_{1}\\9&(3,0)&{\textbf {d}}_{0}\\10&(0,4)&{\textbf {a}}_{4}\\\vdots &&\end{array}}$

We need theaxiom of countable choice to indexall the sets ${\textbf {a}},{\textbf {b}},{\textbf {c}},\dots$ simultaneously.

Theorem—The set of all finite-lengthsequences of natural numbers is countable.

This set is the union of the length-1 sequences, the length-2 sequences, the length-3 sequences, and so on, each of which is a countable set (finite Cartesian product). Thus the set is a countable union of countable sets, which is countable by the previous theorem.

Theorem—The set of all finitesubsets of the natural numbers is countable.

The elements of any finite subset can be ordered into a finite sequence. There are only countably many finite sequences, so also there are only countably many finite subsets.

Theorem—Let $S {\displaystyle S}$ and $T {\displaystyle T}$ be sets.

If the function $f:S\to T$ is injective and $T {\displaystyle T}$ is countable then $S {\displaystyle S}$ is countable.
If the function $g:S\to T$ is surjective and $S {\displaystyle S}$ is countable then $T {\displaystyle T}$ is countable.

These follow from the definitions of countable set as injective / surjective functions.^[g]

Cantor's theorem asserts that if $A {\displaystyle A}$ is a set and ${\mathcal {P}}(A)$ is itspower set, i.e. the set of all subsets of $A {\displaystyle A}$ , then there is no surjective function from $A {\displaystyle A}$ to ${\mathcal {P}}(A)$ . A proof is given in the articleCantor's theorem. As an immediate consequence of this and the Basic Theorem above we have:

Proposition—The set ${\mathcal {P}}(\mathbb {N} )$ is not countable; i.e. it isuncountable.

For an elaboration of this result seeCantor's diagonal argument.

The set ofreal numbers is uncountable,^[h] and so is the set of all infinitesequences of natural numbers.

Minimal model of set theory is countable

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If there is a set that is a standard model (seeinner model) of ZFC set theory, then there is a minimal standard model (seeConstructible universe). TheLöwenheim–Skolem theorem can be used to show that this minimal model is countable. The fact that the notion of "uncountability" makes sense even in this model, and in particular that this modelM contains elements that are:

subsets ofM, hence countable,
but uncountable from the point of view ofM,

was seen as paradoxical in the early days of set theory; seeSkolem's paradox for more.

The minimal standard model includes all thealgebraic numbers and all effectively computabletranscendental numbers, as well as many other kinds of numbers.

Total orders

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Countable sets can betotally ordered in various ways, for example:

Well-orders (see alsoordinal number):
- The usual order of natural numbers (0, 1, 2, 3, 4, 5, ...)
- The integers in the order (0, 1, 2, 3, ...; −1, −2, −3, ...)
Other (not well orders):
- The usual order of integers (..., −3, −2, −1, 0, 1, 2, 3, ...)
- The usual order of rational numbers (Cannot be explicitly written as an ordered list!)

In both examples of well orders here, any subset has aleast element; and in both examples of non-well orders,some subsets do not have aleast element.This is the key definition that determines whether a total order is also a well order.

Notes

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^^a ^bSince there is an obviousbijection between $\mathbb {N}$ and $\mathbb {N} ^{*}=\{1,2,3,\dots \}$ , it makes no difference whether one considers 0 a natural number or not. In any case, this article followsISO 31-11 and the standard convention inmathematical logic, which takes 0 as a natural number.
^Proof: Observe that $\mathbb {N} \times \mathbb {N}$ is countable as a consequence of the definition because the function $f:\mathbb {N} \times \mathbb {N} \to \mathbb {N}$ given by $f(m,n)=2^{m}\cdot 3^{n}$ is injective.^[22] It then follows that the Cartesian product of any two countable sets is countable, because if $A {\displaystyle A}$ and $B {\displaystyle B}$ are two countable sets there are surjections $f:\mathbb {N} \to A$ and $g:\mathbb {N} \to B$ . So $f\times g:\mathbb {N} \times \mathbb {N} \to A\times B$ is a surjection from the countable set $\mathbb {N} \times \mathbb {N}$ to the set $A\times B$ and the Corollary implies $A\times B$ is countable. This result generalizes to the Cartesian product of any finite collection of countable sets and the proof follows byinduction on the number of sets in the collection.
^Proof: The integers $\mathbb {Z}$ are countable because the function $f:\mathbb {Z} \to \mathbb {N}$ given by $f(n)=2^{n}$ if $n {\displaystyle n}$ is non-negative and $f(n)=3^{-n}$ if $n {\displaystyle n}$ is negative, is an injective function. The rational numbers $\mathbb {Q}$ are countable because the function $g:\mathbb {Z} \times \mathbb {N} \to \mathbb {Q}$ given by $g(m,n)=m/(n+1)$ is a surjection from the countable set $\mathbb {Z} \times \mathbb {N}$ to the rationals $\mathbb {Q}$ .
^Proof: Per definition, every algebraic number (including complex numbers) is a root of a polynomial with integer coefficients. Given an algebraic number $\alpha$ , let $a_{0}x^{0}+a_{1}x^{1}+a_{2}x^{2}+\cdots +a_{n}x^{n}$ be a polynomial with integer coefficients such that $\alpha$ is the $k {\displaystyle k}$ -th root of the polynomial, where the roots are sorted by absolute value from small to big, then sorted by argument from small to big. We can define an injection (i. e. one-to-one) function $f:\mathbb {A} \to \mathbb {Q}$ given by $f(\alpha )=2^{k-1}\cdot 3^{a_{0}}\cdot 5^{a_{1}}\cdot 7^{a_{2}}\cdots {p_{n+2}}^{a_{n}}$ , where $p_{n}$ is the $n {\displaystyle n}$ -thprime.
^Proof: If $A_{i}$ is a countable set for each $i {\displaystyle i}$ in $I=\{1,\dots ,n\}$ , then for each $i {\displaystyle i}$ there is a surjective function $g_{i}:\mathbb {N} \to A_{i}$ and hence the function $G:I\times \mathbf {N} \to \bigcup _{i\in I}A_{i},$ given by $G(i,m)=g_{i}(m)$ is a surjection. Since $I\times \mathbb {N}$ is countable, the union ${\textstyle \bigcup _{i\in I}A_{i}}$ is countable.
^Proof: As in the finite case, but $I=\mathbb {N}$ and we use theaxiom of countable choice to pick for each $i {\displaystyle i}$ in $\mathbb {N}$ a surjection $g_{i}$ from the non-empty collection of surjections from $\mathbb {N}$ to $A_{i}$ .^[26] Note that since we are considering the surjection $G:\mathbf {N} \times \mathbf {N} \to \bigcup _{i\in I}A_{i}$ , rather than an injection, there is no requirement that the sets be disjoint.
^Proof: For (1) observe that if $T {\displaystyle T}$ is countable there is an injective function $h:T\to \mathbb {N}$ . Then if $f:S\to T$ is injective the composition $h\circ f:S\to \mathbb {N}$ is injective, so $S {\displaystyle S}$ is countable.For (2) observe that if $S {\displaystyle S}$ is countable, either $S {\displaystyle S}$ is empty or there is a surjective function $h:\mathbb {N} \to S$ . Then if $g:S\to T$ is surjective, either $S {\displaystyle S}$ and $T {\displaystyle T}$ are both empty, or the composition $g\circ h:\mathbb {N} \to T$ is surjective. In either case $T {\displaystyle T}$ is countable.
^SeeCantor's first uncountability proof, and alsoFinite intersection property#Applications for a topological proof.

Citations

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^Manetti, Marco (19 June 2015).Topology. Springer. p. 26.ISBN 978-3-319-16958-3.
^Rudin 1976, Chapter 2
^Tao 2016, p. 181
^Kamke 1950, p. 2
^^a ^bLang 1993, §2 of Chapter I
^Apostol 1969, p. 23, Chapter 1.14
^Thierry, Vialar (4 April 2017).Handbook of Mathematics. BoD - Books on Demand. p. 24.ISBN 978-2-9551990-1-5.
^Mukherjee, Subir Kumar (2009).First Course in Real Analysis. Academic Publishers. p. 22.ISBN 978-81-89781-90-3.
^^a ^b ^cYaqub, Aladdin M. (24 October 2014).An Introduction to Metalogic. Broadview Press.ISBN 978-1-4604-0244-3.
^Singh, Tej Bahadur (17 May 2019).Introduction to Topology. Springer. p. 422.ISBN 978-981-13-6954-4.
^^a ^bKatzourakis, Nikolaos; Varvaruca, Eugen (2 January 2018).An Illustrative Introduction to Modern Analysis. CRC Press.ISBN 978-1-351-76532-9.
^Halmos 1960, p. 91
^Kamke 1950, p. 2
^Dlab, Vlastimil; Williams, Kenneth S. (9 June 2020).Invitation To Algebra: A Resource Compendium For Teachers, Advanced Undergraduate Students And Graduate Students In Mathematics. World Scientific. p. 8.ISBN 978-981-12-1999-3.
^Tao 2016, p. 182
^Stillwell, John C. (2010),Roads to Infinity: The Mathematics of Truth and Proof, CRC Press, p. 10,ISBN 9781439865507,Cantor's discovery of uncountable sets in 1874 was one of the most unexpected events in the history of mathematics. Before 1874, infinity was not even considered a legitimate mathematical subject by most people, so the need to distinguish between countable and uncountable infinities could not have been imagined.
^Cantor 1878, p. 242.
^Ferreirós 2007, pp. 268, 272–273.
^"What Are Sets and Roster Form?".expii. 2021-05-09.Archived from the original on 2020-09-18.
^Halmos 1960, p. 91
^Halmos 1960, p. 92
^Avelsgaard 1990, p. 182
^Kamke 1950, pp. 3–4
^Avelsgaard 1990, p. 180
^Fletcher & Patty 1988, p. 187
^Hrbacek, Karel; Jech, Thomas (22 June 1999).Introduction to Set Theory, Third Edition, Revised and Expanded. CRC Press. p. 141.ISBN 978-0-8247-7915-3.

References

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Apostol, Tom M. (June 1969),Multi-Variable Calculus and Linear Algebra with Applications, vol. 2 (2nd ed.), New York: John Wiley + Sons,ISBN 978-0-471-00007-5
Avelsgaard, Carol (1990),Foundations for Advanced Mathematics, Scott, Foresman and Company,ISBN 0-673-38152-8
Cantor, Georg (1878),"Ein Beitrag zur Mannigfaltigkeitslehre",Journal für die Reine und Angewandte Mathematik,1878 (84):242–248,doi:10.1515/crelle-1878-18788413,S2CID 123695365
Ferreirós, José (2007),Labyrinth of Thought: A History of Set Theory and Its Role in Mathematical Thought (2nd revised ed.), Birkhäuser,ISBN 978-3-7643-8349-7
Fletcher, Peter; Patty, C. Wayne (1988),Foundations of Higher Mathematics, Boston: PWS-KENT Publishing Company,ISBN 0-87150-164-3
Halmos, Paul R. (1960),Naive Set Theory, D. Van Nostrand Company, Inc Reprinted by Springer-Verlag, New York, 1974.ISBN 0-387-90092-6 (Springer-Verlag edition). Reprinted by Martino Fine Books, 2011.ISBN 978-1-61427-131-4 (Paperback edition).
Kamke, Erich (1950),Theory of Sets, Dover series in mathematics and physics, New York: Dover,ISBN 978-0486601410{{citation}}:ISBN / Date incompatibility (help)
Lang, Serge (1993),Real and Functional Analysis, Berlin, New York: Springer-Verlag,ISBN 0-387-94001-4
Rudin, Walter (1976),Principles of Mathematical Analysis, New York: McGraw-Hill,ISBN 0-07-054235-X
Tao, Terence (2016)."Infinite sets".Analysis I. Texts and Readings in Mathematics. Vol. 37 (Third ed.). Singapore: Springer. pp. 181–210.doi:10.1007/978-981-10-1789-6_8.ISBN 978-981-10-1789-6.

Look upcountable in Wiktionary, the free dictionary.

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