- Cardinal number
:"This article describes cardinal numbers in mathematics. For cardinals in linguistics, see

Names of numbers in English ."In

mathematics ,**cardinal numbers**, or**cardinals**for short, are generalizednumber s used to measure the(size) of sets. Forcardinality finite set s, the cardinality is given by anatural number , which is simply the number of elements in the set. There are also "transfinite " cardinal numbers that describe the sizes of infinite sets.Cardinality is defined in terms of

bijective function s. Two sets have the same cardinal number if and only if there is a bijection between them. In the case of finite sets, this agrees with the intuitive notion of size. In the case of infinite sets, the behavior is more complex. A fundamental theorem due toGeorg Cantor shows that it is possible for infinite sets to have different cardinalities, and in particular the set ofreal number s and the set ofnatural number s do not have the same cardinal number. It is also possible for a proper subset of an infinite set to have the same cardinality as the original set, something that cannot happen with proper subsets of finite sets.There is a transfinite sequence of cardinal numbers: :$0,\; 1,\; 2,\; 3,\; cdots,\; n,\; cdots\; ;\; aleph\_0,\; aleph\_1,\; aleph\_2,\; cdots,\; aleph\_\{alpha\},\; cdots.$This sequence starts with the

natural number s (finite cardinals), which are followed by thealeph number s (infinite cardinals of well-ordered sets). The aleph numbers are indexed byordinal number s. Under the assumption of theaxiom of choice , this transfinite sequence includes every cardinal number. If the axiom of choice fails, the situation is more complicated, with additional infinite cardinals that are not alephs.Cardinality is studied for its own sake as part of

set theory . It is also a tool used in branches of mathematics includingcombinatorics ,abstract algebra , andmathematical analysis .**History**The notion of cardinality, as now understood, was formulated by

Georg Cantor , the originator ofset theory , in 1874–1884. Cantor first established cardinality as an instrument to compare finite sets; e.g. the sets {1,2,3} and {2,3,4} are not "equal", but have the "same cardinality", three.Cantor identified the fact that one-to-one correspondence is the way to tell that two sets have the same size, called "cardinality", in the case of finite sets. Using this one-to-one correspondence, he applied the concept to infinite sets; e.g. the set of natural numbers

**N**= {0, 1, 2, 3, ...}. He called these cardinal numberstransfinite cardinal numbers , and defined all sets having a one-to-one correspondence with**N**to be denumerable (countably infinite) sets.Naming this cardinal number $aleph\_0$, aleph-null, Cantor proved that any unbounded subset of

**N**has the same cardinality as**N**, even if this might appear at first view, to run contrary to intuition. He also proved that the set of allordered pair s of natural numbers is denumerable (which implies that the set of allrational number s is denumerable), and later proved that the set of allalgebraic number s is also denumerable. Each algebraic number "z" may be encoded as a finite sequence of integers which are the coefficients in the polynomial equation of which it is the solution, i.e. the ordered n-tuple $(a\_0,\; a\_1,\; ...,\; a\_n),;\; a\_i\; in\; mathbb\{Z\},$ together with a pair of rationals $(b\_0,\; b\_1)$ such that "z" is the unique root of the polynomial with coefficients $(a\_0,\; a\_1,\; ...,\; a\_n)$ that lies in the interval $(b\_0,\; b\_1)$.In his 1874 paper, Cantor proved that there exist higher-order cardinal numbers by showing that the set of real numbers has cardinality greater than that of

**N**. His original presentation used a complex argument withnested intervals , but in an 1891 paper he proved the same result using his ingenious but simple diagonal argument. This new cardinal number, called thecardinality of the continuum , was termed "c" by Cantor.Cantor also developed a large portion of the general theory of cardinal numbers; he proved that there is a smallest transfinite cardinal number ($aleph\_0$, aleph-null) and that for every cardinal number, there is a next-larger cardinal $(aleph\_1,\; aleph\_2,\; aleph\_3,\; cdots)$.

His

continuum hypothesis is the proposition that "c" is the same as $aleph\_1$, but this has been found to be independent of the standard axioms of mathematical set theory; it can neither be proved nor disproved under the standard assumptions.**Motivation**In informal use, a

**cardinal number**is what is normally referred to as a "counting number ", provided that 0 is included: 0, 1, 2, .... They may be identified with thenatural numbers beginning with 0.The counting numbers are exactly what can be defined formally as the finite cardinal numbers. Infinite cardinals only occur in higher-level mathematics and logic.More formally, a non-zero number can be used for two purposes: to describe the size of a set, or to describe the position of an element in a sequence. For finite sets and sequences it is easy to see that these two notions coincide, since for every number describing a position in a sequence we can construct a set which has exactly the right size, e.g. 3 describes the position of 'c' in the sequence <'a','b','c','d',...>, and we can construct the set {a,b,c} which has 3 elements. However when dealing with

infinite set s it is essential to distinguish between the two — the two notions are in fact different for infinite sets. Considering the position aspect leads to ordinal numbers, while the size aspect is generalized by the**cardinal numbers**described here.The intuition behind the formal definition of cardinal is the construction of a notion of the relative size or "bigness" of a set without reference to the kind of members which it has. For finite sets this is easy; one simply counts the number of elements a set has. In order to compare the sizes of larger sets, it is necessary to appeal to more subtle notions.

A set "Y" is at least as big as, or greater than or equal to a set "X" if there is an injective (one-to-one) mapping from the elements of "X" to the elements of "Y". A one-to-one mapping identifies each element of the set "X" with a unique element of the set "Y". This is most easily understood by an example; suppose we have the sets "X" = {1,2,3} and "Y" = {a,b,c,d}, then using this notion of size we would observe that there is a mapping:: 1 → a: 2 → b: 3 → cwhich is one-to-one, and hence conclude that "Y" has cardinality greater than or equal to "X". Note the element d has no element mapping to it, but this is permitted as we only require a one-to-one mapping, and not necessarily a one-to-one and

onto mapping. The advantage of this notion is that it can be extended to infinite sets.We can then extend this to an equality-style relation.Two sets "X" and "Y" are said to have the same

**cardinality**if there exists abijection between "X" and "Y". By theSchroeder-Bernstein theorem , this is equivalent to there being "both" a one-to-one mapping from "X" to "Y" "and" a one-to-one mapping from "Y" to "X". We then write | "X" | = | "Y" |. The cardinal number of "X" itself is often defined as the least ordinal "a" with | "a" | = | "X" |. This is called thevon Neumann cardinal assignment ; for this definition to make sense, it must be proved that every set has the same cardinality as "some" ordinal; this statement is thewell-ordering principle . It is however possible to discuss the relative cardinality of sets without explicitly assigning names to objects.The classic example used is that of the infinite hotel paradox, also called

Hilbert's paradox of the Grand Hotel . Suppose you are an innkeeper at a hotel with an infinite number of rooms. The hotel is full, and then a new guest arrives. It's possible to fit the extra guest in by asking the guest who was in room 1 to move to room 2, the guest in room 2 to move to room 3, and so on, leaving room 1 vacant. We can explicitly write a segment of this mapping:: 1 ↔ 2: 2 ↔ 3: 3 ↔ 4: ...: n ↔ n+1: ...In this way we can see that the set {1,2,3,...} has the same cardinality as the set {2,3,4,...} since a bijection between the first and the second has been shown. This motivates the definition of an infinite set being any set which has a proper subset of the same cardinality; in this case {2,3,4,...} is a proper subset of {1,2,3,...}.When considering these large objects, we might also want to see if the notion of counting order coincides with that of cardinal defined above for these infinite sets. It happens that it doesn't; by considering the above example we can see that if some object "one greater than infinity" exists, then it must have the same cardinality as the infinite set we started out with. It is possible to use a different formal notion for number, called

ordinal s, based on the ideas of counting and considering each number in turn, and we discover that the notions of cardinality and ordinality are divergent once we move out of the finite numbers.It can be proved that the cardinality of the

real number s is greater than that of the natural numbers just described. This can be visualized usingCantor's diagonal argument ; classic questions of cardinality (for instance thecontinuum hypothesis ) are concerned with discovering whether there is some cardinal between some pair of other infinite cardinals. In more recent times mathematicians have been describing the properties of larger and larger cardinals.Since cardinality is such a common concept in mathematics, a variety of names are in use. Sameness of cardinality is sometimes referred to as

**equipotence**,**equipollence**, or**equinumerosity**. It is thus said that two sets with the same cardinality are, respectively,**equipotent**,**equipollent**, or**equinumerous**.**Formal definition**Formally, assuming the

axiom of choice , the cardinality of a set "X" is the least ordinal α such that there is a bijection between "X" and α. This definition is known as thevon Neumann cardinal assignment . If the axiom of choice is not assumed we need to do something different. The oldest definition of the cardinality of a set "X" (implicit in Cantor and explicit in Frege andPrincipia Mathematica ) is as the set of all sets which are equinumerous with "X": this does not work inZFC or other related systems ofaxiomatic set theory because this collection is too large to be a set, but it does work intype theory and inNew Foundations and related systems. However, if we restrict from this class to those equinumerous with "X" that have the least rank, then it will work (this is a trick due toDana Scott : it works because the collection of objects with any given rank is a set).Formally, the order among cardinal numbers is defined as follows: | "X" | ≤ | "Y" | means that there exists an

injective function from "X" to "Y". TheCantor–Bernstein–Schroeder theorem states that if | "X" | ≤ | "Y" | and | "Y" | ≤ | "X" | then | "X" | = | "Y" |. Theaxiom of choice is equivalent to the statement that given two sets "X" and "Y", either | "X" | ≤ | "Y" | or | "Y" | ≤ | "X" |.A set "X" is

Dedekind-infinite if there exists aproper subset "Y" of "X" with | "X" | = | "Y" |, andDedekind-finite if such a subset doesn't exist. The finite cardinals are just thenatural numbers , i.e., a set "X" is finite if and only if | "X" | = | "n" | = "n" for some natural number "n". Any other set isinfinite . Assuming the axiom of choice, it can be proved that the Dedekind notions correspond to the standard ones. It can also be proved that the cardinal $aleph\_0$ (aleph-0, where aleph is the first letter in theHebrew alphabet , represented $aleph$) of the set of natural numbers is the smallest infinite cardinal, i.e. that any infinite set has a subset of cardinality $aleph\_0.$ The next larger cardinal is denoted by $aleph\_1$ and so on. For everyordinal α there is a cardinal number $aleph\_\{alpha\},$ and this list exhausts all infinite cardinal numbers.**Cardinal arithmetic**We can define

arithmetic operations on cardinal numbers that generalize the ordinary operations for natural numbers. It can be shown that for finite cardinals these operations coincide with the usual operations for natural numbers. Furthermore, these operations share many properties with ordinary arithmetic.**Successor cardinal**If the axiom of choice holds, every cardinal κ has a successor κ

^{+}> κ, and there are no cardinals between κ and its successor. For finite cardinals, the successor is simply κ+1. For infinite cardinals, the successor cardinal differs from thesuccessor ordinal .**Cardinal addition**If "X" and "Y" are disjoint, addition is given by the union of "X" and "Y". If the two sets are not already disjoint, then they can be replaced by disjoint sets, i.e. replace "X" by "X"×{0} and "Y" by "Y"×{1}.:$|X|\; +\; |Y|\; =\; |\; X\; cup\; Y|.$

Zero is an additive identity "κ" + 0 = 0 + "κ" = "κ".

Addition is

associative ("κ" + "μ") + "ν" = "κ" + ("μ" + "ν").Addition is

commutative "κ" + "μ" = "μ" + "κ".Addition is non-decreasing in both arguments::$(kappa\; le\; mu)\; ightarrow\; ((kappa\; +\; u\; le\; mu\; +\; u)\; mbox\{\; and\; \}\; (\; u\; +\; kappa\; le\; u\; +\; mu)).$

If the axiom of choice holds, addition of infinite cardinal numbers is easy. If either $kappa$ or $mu$ is infinite, then:$kappa\; +\; mu\; =\; max\{kappa,\; mu\},.$

**Subtraction**If the axiom of choice holds and given an infinite cardinal "σ" and a cardinal "μ", there will be a cardinal "κ" such that "μ" + "κ" = "σ" if and only if "μ" ≤ "σ". It will be unique (and equal to "σ") if and only if "μ" < "σ".

**Cardinal multiplication**The product of cardinals comes from the

cartesian product .:$|X|cdot|Y|\; =\; |X\; imes\; Y|$"κ"·0 = 0·"κ" = 0.

"κ"·"μ" = 0 $ightarrow$ ("κ" = 0 or "μ" = 0).

One is a multiplicative identity "κ"·1 = 1·"κ" = "κ".

Multiplication is associative ("κ"·"μ")·"ν" = "κ"·("μ"·"ν").

Multiplication is

commutative "κ"·"μ" = "μ"·"κ".Multiplication is non-decreasing in both arguments:"κ" ≤ "μ" $ightarrow$ ("κ"·"ν" ≤ "μ"·"ν" and "ν"·"κ" ≤ "ν"·"μ").

Multiplication distributes over addition:"κ"·("μ" + "ν") = "κ"·"μ" + "κ"·"ν" and ("μ" + "ν")·"κ" = "μ"·"κ" + "ν"·"κ".

If the axiom of choice holds, multiplication of infinite cardinal numbers is also easy. If either "κ" or "μ" is infinite and both are non-zero, then:$kappacdotmu\; =\; max\{kappa,\; mu\}.$

**Division**If the axiom of choice holds and given an infinite cardinal "π" and a non-zero cardinal "μ", there will be a cardinal "κ" such that "μ" · "κ" = "π" if and only if "μ" ≤ "π". It will be unique (and equal to "π") if and only if "μ" < "π".

**Cardinal exponentiation**Exponentiation is given by:$|X|^\; =\; left|X^Y\; ight|$where "X"

^{"Y"}is the set of all functions from "Y" to "X".:"κ"

^{0}= 1 (in particular 0^{0}= 1), seeempty function .:If 1 ≤ "μ", then 0^{"μ"}= 0.:1^{"μ"}= 1.:"κ"^{1}= "κ".:"κ"^{"μ" + "ν"}= "κ"^{"μ"}·"κ"^{"ν"}.:"κ"^{"μ"·"ν"}= ("κ"^{"μ"})^{"ν"}.:("κ"·"μ")^{"ν"}= "κ"^{"ν"}·"μ"^{"ν"}.Exponentiation is non-decreasing in both arguments::(1 ≤ "ν" and "κ" ≤ "μ") $ightarrow$ ("ν"^{"κ"}≤ "ν"^{"μ"}) and :("κ" ≤ "μ") $ightarrow$ ("κ"^{"ν"}≤ "μ"^{"ν"}).Note that 2

^{| "X" |}is the cardinality of thepower set of the set "X" andCantor's diagonal argument shows that 2^{| "X" |}> | "X" | for any set "X". This proves that no largest cardinal exists (because for any cardinal "κ", we can always find a larger cardinal 2^{"κ"}). In fact, the class of cardinals is aproper class .Neither roots nor logarithms can be defined uniquely for infinite cardinals.

All the remaining propositions in this section assume the axiom of choice:

:If "κ" and "μ" are both finite and greater than 1, and "ν" is infinite, then "κ"

^{"ν"}= "μ"^{"ν"}.:If "κ" is infinite and "μ" is finite and non-zero, then "κ"^{"μ"}= "κ".If 2 ≤ κ and 1 ≤ μ and at least one of them is infinite, then::Max (κ, 2

^{μ}) ≤ κ^{μ}≤ Max (2^{κ}, 2^{μ}).Using König's theorem, one can prove κ < κ

^{cf(κ)}and κ < cf(2^{κ}) for any infinite cardinal κ, where cf(κ) is thecofinality of κ.The logarithm of an infinite cardinal number κ is defined as the least cardinal number μ such that κ ≤ 2

^{μ}. Logarithms of infinite cardinals are useful in some fields of mathematics, for example in the study of cardinal invariants of topological spaces, though they lack some of the properties that logarithms of positive real numbers possess. [*Robert A. McCoy and Ibula Ntantu, Topological Properties of Spaces of Continuous Functions, Lecture Notes in Mathematics 1315, Springer-Verlag.*] [*Eduard Cech, Topological Spaces, revised by Zdenek Frolík and Miroslav Katetov, John Wiley & Sons, 1966.*] [*D.A. Vladimirov, Boolean Algebras in Analysis, Mathematics and Its Applications, Kluwer Academic Publishers.*]**The continuum hypothesis**The

continuum hypothesis (CH) states that there are no cardinals strictly between $aleph\_0$ and $2^\{aleph\_0\}.$The latter cardinal number is also often denoted by "c"; it is thecardinality of the continuum (the set ofreal number s). In this case $2^\{aleph\_0\}\; =\; aleph\_1.$ The generalized continuum hypothesis (GCH)states that for every infinite set "X", there are no cardinals strictly between | "X" | and 2^{| "X" |}.The continuum hypothesis is independent of the usual axioms of set theory,the Zermelo-Fraenkel axioms together with the axiom of choice (ZFC).**ee also***

Counting

*Names of numbers in English

*Large cardinal

*Nominal number

*Ordinal number

*Serial number

* The paradox of the greatest cardinal

*Aleph number

*Beth number **References***Hahn, Hans, "Infinity", Part IX, Chapter 2, Volume 3 of "The World of Mathematics". New York: Simon and Schuster, 1956.

*Halmos, Paul, "Naive set theory". Princeton, NJ: D. Van Nostrand Company, 1960. Reprinted by Springer-Verlag, New York, 1974. ISBN 0-387-90092-6 (Springer-Verlag edition).**Notes****External links***

* [*http://www.apronus.com/provenmath/cardinality.htm Cardinality at ProvenMath*] formal proofs of the basic theorems on cardinality.

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