Cardinal number: Difference between revisions

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[[Image:Aleph0.svg|thumb|right|150px|[[Aleph null]], the smallest infinite cardinal]]
In [[mathematics]], '''cardinal numbers''', or '''cardinals''' for short, are a generalization of the [[natural number]]s used to measure the [[cardinality]] (size) of [[Set (mathematics)|sets]]. The cardinality of a [[finite set]] is a natural number – the number of elements in the set. The ''[[transfinite number|transfinite]]'' cardinal numbers describe the sizes of [[infinite set]]s.
 
Cardinality is defined in terms of [[bijective function]]s. Two sets have the same cardinality 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 to [[Georg Cantor]] shows that it is possible for infinite sets to have different cardinalities, and in particular the cardinality of the set of [[real number]]s is greater than the cardinality of the set of [[natural number]]s. 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:
:<math>0, 1, 2, 3, \ldots, n, \ldots ; \aleph_0, \aleph_1, \aleph_2, \ldots, \aleph_{\alpha}, \ldots.\ </math>
This sequence starts with the [[natural number]]s including zero (finite cardinals), which are followed by the [[aleph number]]s (infinite cardinals of [[well-ordering|well-ordered sets]]). The aleph numbers are indexed by [[ordinal number]]s.  Under the assumption of the [[axiom of choice]], this [[transfinite]] sequence includes every cardinal number.  If one [[axiom of choice#Independence|rejects]] that axiom, 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 including [[combinatorics]], [[abstract algebra]], and [[mathematical analysis]]. In [[category theory]], the cardinal numbers form a [[Skeleton (category theory)|skeleton]] of the [[category of sets]].
 
==History==
 
The notion of cardinality, as now understood, was formulated by [[Georg Cantor]], the originator of [[set theory]], in 1874–1884. Cardinality can be used to compare an aspect of finite sets; e.g. the sets {1,2,3} and {4,5,6} are not ''equal'', but have the ''same cardinality'', namely three (this is established by the existence of a [[bijection]], i.e. a one-to-one correspondence, between the two sets; e.g. {1->4, 2->5, 3->6}).
 
Cantor applied his concept of bijection to infinite sets;<ref>{{harvnb|Dauben|1990|loc=pg. 54}}</ref> e.g. the set of natural numbers '''N''' = {0, 1, 2, 3, ...}. Thus, all sets having a bijection with '''N''' he called [[Countable set|denumerable (countably infinite) sets]] and they all have the same cardinal number. This cardinal number is called <math>\aleph_0</math>, [[Aleph number|aleph-null]]. He called the cardinal numbers of these infinite sets, [[transfinite cardinal numbers]].
 
Cantor proved that any unbounded subset of '''N''' has the same cardinality as '''N''', even though this might appear to run contrary to intuition. He also proved that the set of all [[ordered pair]]s of natural numbers is denumerable (which implies that the set of all [[rational number]]s is denumerable), and later proved that the set of all [[algebraic 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''<sub>0</sub>, ''a''<sub>1</sub>, ..., ''a<sub>n</sub>''), ''a<sub>i</sub>'' ∈ '''Z''' together with a pair of rationals (''b''<sub>0</sub>, ''b''<sub>1</sub>) such that ''z'' is the unique root of the polynomial with coefficients (''a''<sub>0</sub>, ''a''<sub>1</sub>, ..., ''a<sub>n</sub>'') that lies in the interval (''b''<sub>0</sub>, ''b''<sub>1</sub>).
 
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 [[Cantor's first uncountability proof|original presentation]] used a complex argument with [[nested intervals]], but in an 1891 paper he proved the same result using his ingenious but simple [[Cantor's diagonal argument|diagonal argument]]. The new cardinal number of the set of real numbers is called the [[cardinality of the continuum]] and Cantor used the symbol <math>\mathfrak{c}</math> for it.
 
Cantor also developed a large portion of the general theory of cardinal numbers; he proved that there is a smallest transfinite cardinal number (<math>\aleph_0</math>, aleph-null) and that for every cardinal number, there is a next-larger cardinal
 
:<math>(\aleph_1, \aleph_2, \aleph_3, \cdots).\ </math>
 
His [[continuum hypothesis]] is the proposition that <math>\mathfrak{c}</math> is the same as <math>\aleph_1</math>. This hypothesis has been found to be independent of the standard axioms of mathematical set theory; it can neither be proved nor disproved from 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 the [[natural numbers]] beginning with 0. The counting numbers are exactly what can be defined formally as the [[finite set|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 &mdash; 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 a set ''X'' if there is an [[injective function|injective]] [[map (mathematics)|mapping]] from the elements of ''X'' to the elements of ''Y''. An injective 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 → c
which is injective, 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 an injective mapping, and not necessarily an injective 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 [[Set (mathematics)|sets]] ''X'' and ''Y'' are said to have the same '''cardinality''' if there exists a [[bijection]] between ''X'' and ''Y''.  By the [[Cantor–Bernstein–Schroeder theorem|Schroeder–Bernstein theorem]], this is equivalent to there being ''both'' an injective mapping from ''X'' to ''Y'' ''and'' an injective 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 the [[von 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 the [[well-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 number|ordinals]], 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 using [[Cantor's diagonal argument]];
classic questions of cardinality (for instance the [[continuum 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 the [[von 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 and [[Principia Mathematica]]) is as the class ''[X]'' of all sets that are equinumerous with ''X''. This does not work in [[ZFC]] or other related systems of [[axiomatic set theory]] because if ''X'' is non-empty, this collection is too large to be a set. In fact, for ''X &ne; &empty;'' there is an injection from the universe into ''[X]'' by mapping a set ''m'' to ''{m} &times; X'' and so by the [[Axiom of limitation of size|axiom of limitation of size]], ''[X]'' is a proper class. The definition does work however in [[type theory]] and in [[New Foundations]] and related systems.  However, if we restrict from this class to those equinumerous with ''X'' that have the least [[rank (set theory)|rank]], then it will work (this is a trick due to [[Dana Scott]]:<ref>{{cite journal|last1=Deiser|first1=Oliver|title=On the Development of the Notion of a Cardinal Number|journal=History and Philosophy of Logic|doi=10.1080/01445340903545904 |volume=31|issue=2|pages=123–143|date=May 2010}}</ref>  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''. The [[Cantor–Bernstein–Schroeder theorem]] states that if |''X''| ≤ |''Y''| and |''Y''| ≤ |''X''| then |''X''| = |''Y''|. The [[axiom of choice]] is equivalent to the statement that given two sets ''X'' and ''Y'', either |''X''| ≤ |''Y''| or |''Y''| ≤ |''X''|.<ref name="Enderton">Enderton, Herbert. "Elements of Set Theory", Academic Press Inc., 1977. ISBN 0-12-238440-7</ref><ref>{{cite | author=Friedrich M. Hartogs | editor=Felix Klein, Walther von Dyck, David Hilbert, Otto Blumenthal | title=Über das Problem der Wohlordnung | journal=Math.&nbsp;Ann | volume=Bd.&nbsp;76 | number=4 | publisher=B.&nbsp;G. Teubner | location=Leipzig | year=1915 | pages=438–443 | ISSN=0025-5831 |url=http://gdz.sub.uni-goettingen.de/index.php?id=11&PPN=PPN235181684_0076&DMDID=DMDLOG_0037&L=1}}</ref>
 
A set ''X'' is [[Dedekind-infinite]] if there exists a [[proper subset]] ''Y'' of ''X'' with |''X''| = |''Y''|, and [[Dedekind-finite]] if such a subset doesn't exist.  The [[finite set|finite]] cardinals are just the [[natural numbers]], i.e., a set ''X'' is finite if and only if |''X''| = |''n''| = ''n'' for some natural number ''n''.  Any other set is [[infinite set|infinite]].  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 <math>\aleph_0</math> ([[aleph null]] or aleph-0, where aleph is the first letter in the [[Hebrew alphabet]], represented <math>\aleph</math>) of the set of natural numbers is the smallest infinite cardinal, i.e. that any infinite set has a subset of cardinality <math>\aleph_0.</math>  The next larger cardinal is denoted by <math>\aleph_1</math> and so on. For every [[Ordinal number|ordinal]] α there is a cardinal number <math>\aleph_{\alpha},</math> 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 ===
{{Details|Successor cardinal}}
 
If the axiom of choice holds, every cardinal κ has a successor κ<sup>+</sup> > 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 the [[successor ordinal]].
 
=== Cardinal addition ===
If ''X'' and ''Y'' are [[Disjoint sets|disjoint]], addition is given by the [[union (set theory)|union]] of ''X'' and ''Y''.  If the two sets are not already disjoint, then they can be replaced by disjoint sets of the same cardinality, e.g., replace ''X'' by ''X''&times;{0} and ''Y'' by ''Y''&times;{1}.
:<math>|X| + |Y| = | X \cup Y|.</math>
 
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:
:<math>(\kappa \le \mu) \rightarrow ((\kappa + \nu \le \mu + \nu) \mbox{ and } (\nu + \kappa \le \nu + \mu)).</math>
 
Assuming the axiom of choice, addition of infinite cardinal numbers is easy.  If either κ or μ is infinite, then
:<math>\kappa + \mu = \max\{\kappa, \mu\}\,.</math>
 
==== Subtraction ====
Assuming the axiom of choice and, given an infinite cardinal σ and a cardinal μ, there exists 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]].
:<math>|X|\cdot|Y| = |X \times Y|</math>
 
''κ''·0 = 0·''κ'' = 0.
 
''κ''·''μ'' = 0 → (''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'' → (''κ''·''ν'' ≤ ''μ''·''ν'' and ''ν''·''κ'' ≤ ''ν''·''μ'').
 
Multiplication [[distributivity|distributes]] over addition:
''κ''·(''μ'' + ''ν'') = ''κ''·''μ'' + ''κ''·''ν'' and
(''μ'' + ''ν'')·''κ'' = ''μ''·''κ'' + ''ν''·''κ''.
 
Assuming the axiom of choice, multiplication of infinite cardinal numbers is also easy.  If either ''κ''  or ''μ'' is infinite and both are non-zero, then
:<math>\kappa\cdot\mu = \max\{\kappa, \mu\}.</math>
 
==== Division ====
Assuming the axiom of choice and, given an infinite cardinal ''π'' and a non-zero cardinal μ, there exists 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
:<math>|X|^{|Y|} = \left|X^Y\right|</math>
where ''X<sup>Y</sup>'' is the set of all [[function (mathematics)|functions]] from ''Y'' to ''X''.
 
:κ<sup>0</sup> = 1  (in particular 0<sup>0</sup> = 1), see [[empty function]].
:If 1 ≤ μ, then 0<sup>μ</sup> = 0.
:1<sup>μ</sup> = 1.
:κ<sup>1</sup> = κ.
:κ<sup>μ + ν</sup> = ''κ''<sup>''μ''</sup>·''κ''<sup>''ν''</sup>.
:κ<sup>μ · ν</sup> = (''κ''<sup>''μ''</sup>)<sup>''ν''</sup>.
:(''κ''·''μ'')<sup>''ν''</sup> = ''κ''<sup>''ν''</sup>·''μ''<sup>''ν''</sup>.
Exponentiation is non-decreasing in both arguments:
:(1 ≤ ''ν'' and ''κ'' ≤ ''μ'') → (''ν''<sup>''κ''</sup> ≤ ''ν''<sup>''μ''</sup>) and
:(''κ'' ≤ ''μ'') → (''κ''<sup>''ν''</sup> ≤ ''μ''<sup>''ν''</sup>).
 
Note that 2<sup>|''X''|</sup> is the cardinality of the [[power set]] of the set ''X'' and [[Cantor's diagonal argument]] shows that 2<sup>|''X''|</sup> > |''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<sup>κ</sup>). In fact, the [[class (set theory)|class]] of cardinals is a [[proper class]].  (This proof fails in some set theories, notably [[New Foundations]].)
 
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''<sup>''ν''</sup> = ''μ''<sup>''ν''</sup>.
:If ''κ'' is infinite and ''μ'' is finite and non-zero, then ''κ''<sup>''μ''</sup> = ''κ''.
 
If 2 ≤ κ and 1 ≤ μ and at least one of them is infinite, then:
:Max (κ, 2<sup>μ</sup>) ≤ κ<sup>μ</sup> ≤ Max (2<sup>κ</sup>, 2<sup>μ</sup>).
 
Using [[König's theorem (set theory)|König's theorem]], one can prove κ < κ<sup>cf(κ)</sup> and κ < cf(2<sup>κ</sup>) for any infinite cardinal κ, where cf(κ) is the [[cofinality]] of κ.
 
====Roots====
Assuming the axiom of choice and, given an infinite cardinal κ and a finite cardinal μ greater than 0, the cardinal ν satisfying <math>{\nu}^{\mu}={\kappa}</math> will be κ.
 
====Logarithms====
Assuming the axiom of choice and, given an infinite cardinal κ and a finite cardinal μ greater than 1, there may or may not be a cardinal λ satisfying <math>{\mu}^{\lambda}={\kappa}</math>. But, if such a cardinal exists, it is infinite and less than κ and any finite cardinality ν greater than 1 will also satisfy <math>{\nu}^{\lambda}={\kappa}</math>.
 
The logarithm of an infinite cardinal number κ is defined as the least cardinal number μ such that κ ≤ 2<sup>μ</sup>. 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.<ref>Robert A. McCoy and Ibula Ntantu, Topological Properties of Spaces of Continuous Functions, Lecture Notes in Mathematics 1315, [[Springer-Verlag]].</ref><ref>[[Eduard Čech]], Topological Spaces, revised by Zdenek Frolík and Miroslav Katetov, John Wiley & Sons, 1966.</ref><ref>D.A. Vladimirov, Boolean Algebras in Analysis, Mathematics and Its Applications, Kluwer Academic Publishers.</ref>
 
==The continuum hypothesis==
The [[continuum hypothesis]] (CH) states that there are no cardinals strictly between <math>\aleph_0</math> and <math>2^{\aleph_0}.</math> The latter cardinal number is also often denoted by <math>\mathfrak{c}</math>; it is the [[cardinality of the continuum]] (the set of [[real number]]s). In this case <math>2^{\aleph_0} = \aleph_1.</math> The [[generalized continuum hypothesis]] (GCH) states that for every infinite set ''X'', there are no cardinals strictly between |&nbsp;''X''&nbsp;| and 2<sup>|&nbsp;''X''&nbsp;|</sup>. The continuum hypothesis is independent of the usual axioms of set theory, the Zermelo-Fraenkel axioms together with the axiom of choice ([[Zermelo-Fraenkel set theory|ZFC]]).
 
==See also==
{{Col-begin}}
{{Col-1-of-3}}
* [[Counting]]
* [[Names of numbers in English]]
* [[Large cardinal]]
* [[Inclusion-exclusion principle]]
{{Col-2-of-3}}
* [[Nominal number]]
* [[Ordinal number]]
* [[Regular cardinal]]
{{Col-3-of-3}}
{{Portal|Mathematics}}
* [[Cantor's paradox|The paradox of the greatest cardinal]]
* [[Aleph number]]
* [[Beth number]]
{{Col-end}}
 
==References==
'''Notes'''
{{Reflist}}
 
'''Bibliography'''
*{{citation|last=Dauben|first=Joseph Warren|title=Georg Cantor: His Mathematics and Philosophy of the Infinite|publisher=Princeton University Press|place=Princeton|year=1990|isbn=0691-02447-2}}
*{{aut|[[Hans Hahn (mathematician)|Hahn, Hans]]}}, ''Infinity'', Part IX, Chapter 2, Volume 3 of ''The World of Mathematics''. New York: Simon and Schuster, 1956.
*{{aut|[[Paul Halmos|Halmos, Paul]]}}, ''[[Naive Set Theory (book)|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).
 
==External links==
* {{springer|title=Cardinal number|id=p/c020370}}
* {{MathWorld | urlname=CardinalNumber| title=Cardinal Number}}
*[http://www.apronus.com/provenmath/cardinality.htm Cardinality at ProvenMath] proofs of the basic theorems on cardinality.
 
 
{{Number Systems}}
{{Set theory}}
 
{{DEFAULTSORT:Cardinal Number}}
[[Category:Cardinal numbers| ]]
 
[[ru:Кардинальное число]]

Revision as of 16:37, 2 February 2014

29 yr old Orthopaedic Surgeon Grippo from Saint-Paul, spends time with interests including model railways, top property developers in singapore developers in singapore and dolls. Finished a cruise ship experience that included passing by Runic Stones and Church.

Aleph null, the smallest infinite cardinal

In mathematics, cardinal numbers, or cardinals for short, are a generalization of the natural numbers used to measure the cardinality (size) of sets. The cardinality of a finite set is a natural number – the number of elements in the set. The transfinite cardinal numbers describe the sizes of infinite sets.

Cardinality is defined in terms of bijective functions. Two sets have the same cardinality 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 to Georg Cantor shows that it is possible for infinite sets to have different cardinalities, and in particular the cardinality of the set of real numbers is greater than the cardinality of the set of natural numbers. 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:

This sequence starts with the natural numbers including zero (finite cardinals), which are followed by the aleph numbers (infinite cardinals of well-ordered sets). The aleph numbers are indexed by ordinal numbers. Under the assumption of the axiom of choice, this transfinite sequence includes every cardinal number. If one rejects that axiom, 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 including combinatorics, abstract algebra, and mathematical analysis. In category theory, the cardinal numbers form a skeleton of the category of sets.

History

The notion of cardinality, as now understood, was formulated by Georg Cantor, the originator of set theory, in 1874–1884. Cardinality can be used to compare an aspect of finite sets; e.g. the sets {1,2,3} and {4,5,6} are not equal, but have the same cardinality, namely three (this is established by the existence of a bijection, i.e. a one-to-one correspondence, between the two sets; e.g. {1->4, 2->5, 3->6}).

Cantor applied his concept of bijection to infinite sets;[1] e.g. the set of natural numbers N = {0, 1, 2, 3, ...}. Thus, all sets having a bijection with N he called denumerable (countably infinite) sets and they all have the same cardinal number. This cardinal number is called , aleph-null. He called the cardinal numbers of these infinite sets, transfinite cardinal numbers.

Cantor proved that any unbounded subset of N has the same cardinality as N, even though this might appear to run contrary to intuition. He also proved that the set of all ordered pairs of natural numbers is denumerable (which implies that the set of all rational numbers is denumerable), and later proved that the set of all algebraic numbers 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 (a0, a1, ..., an), aiZ together with a pair of rationals (b0, b1) such that z is the unique root of the polynomial with coefficients (a0, a1, ..., an) that lies in the interval (b0, b1).

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 with nested intervals, but in an 1891 paper he proved the same result using his ingenious but simple diagonal argument. The new cardinal number of the set of real numbers is called the cardinality of the continuum and Cantor used the symbol for it.

Cantor also developed a large portion of the general theory of cardinal numbers; he proved that there is a smallest transfinite cardinal number (, aleph-null) and that for every cardinal number, there is a next-larger cardinal

His continuum hypothesis is the proposition that is the same as . This hypothesis has been found to be independent of the standard axioms of mathematical set theory; it can neither be proved nor disproved from 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 the natural 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 sets 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 a set X if there is an injective mapping from the elements of X to the elements of Y. An injective 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 → c

which is injective, 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 an injective mapping, and not necessarily an injective 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 a bijection between X and Y. By the Schroeder–Bernstein theorem, this is equivalent to there being both an injective mapping from X to Y and an injective 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 the von 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 the well-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
...
nn + 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 ordinals, 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 numbers is greater than that of the natural numbers just described. This can be visualized using Cantor's diagonal argument; classic questions of cardinality (for instance the continuum 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 the von 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 and Principia Mathematica) is as the class [X] of all sets that are equinumerous with X. This does not work in ZFC or other related systems of axiomatic set theory because if X is non-empty, this collection is too large to be a set. In fact, for X ≠ ∅ there is an injection from the universe into [X] by mapping a set m to {m} × X and so by the axiom of limitation of size, [X] is a proper class. The definition does work however in type theory and in New 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 to Dana Scott:[2] 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. The Cantor–Bernstein–Schroeder theorem states that if |X| ≤ |Y| and |Y| ≤ |X| then |X| = |Y|. The axiom of choice is equivalent to the statement that given two sets X and Y, either |X| ≤ |Y| or |Y| ≤ |X|.[3][4]

A set X is Dedekind-infinite if there exists a proper subset Y of X with |X| = |Y|, and Dedekind-finite if such a subset doesn't exist. The finite cardinals are just the natural numbers, i.e., a set X is finite if and only if |X| = |n| = n for some natural number n. Any other set is infinite. 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 null or aleph-0, where aleph is the first letter in the Hebrew alphabet, represented ) of the set of natural numbers is the smallest infinite cardinal, i.e. that any infinite set has a subset of cardinality The next larger cardinal is denoted by and so on. For every ordinal α there is a cardinal number 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

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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 the successor 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 of the same cardinality, e.g., replace X by X×{0} and Y by Y×{1}.

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:

Assuming the axiom of choice, addition of infinite cardinal numbers is easy. If either κ or μ is infinite, then

Subtraction

Assuming the axiom of choice and, given an infinite cardinal σ and a cardinal μ, there exists 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.

κ·0 = 0·κ = 0.

κ·μ = 0 → (κ = 0 or μ = 0).

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

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

Multiplication is commutative κ·μ = μ·κ.

Multiplication is non-decreasing in both arguments: κμ → (κ·νμ·ν and ν·κν·μ).

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

Assuming the axiom of choice, multiplication of infinite cardinal numbers is also easy. If either κ or μ is infinite and both are non-zero, then

Division

Assuming the axiom of choice and, given an infinite cardinal π and a non-zero cardinal μ, there exists 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

where XY is the set of all functions from Y to X.

κ0 = 1 (in particular 00 = 1), see empty function.
If 1 ≤ μ, then 0μ = 0.
1μ = 1.
κ1 = κ.
κμ + ν = κμ·κν.
κμ · ν = (κμ)ν.
(κ·μ)ν = κν·μν.

Exponentiation is non-decreasing in both arguments:

(1 ≤ ν and κμ) → (νκνμ) and
(κμ) → (κνμν).

Note that 2|X| is the cardinality of the power set of the set X and Cantor'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 2κ). In fact, the class of cardinals is a proper class. (This proof fails in some set theories, notably New Foundations.)

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 κν = μν.
If κ is infinite and μ is finite and non-zero, then κμ = κ.

If 2 ≤ κ and 1 ≤ μ and at least one of them is infinite, then:

Max (κ, 2μ) ≤ κμ ≤ Max (2κ, 2μ).

Using König's theorem, one can prove κ < κcf(κ) and κ < cf(2κ) for any infinite cardinal κ, where cf(κ) is the cofinality of κ.

Roots

Assuming the axiom of choice and, given an infinite cardinal κ and a finite cardinal μ greater than 0, the cardinal ν satisfying will be κ.

Logarithms

Assuming the axiom of choice and, given an infinite cardinal κ and a finite cardinal μ greater than 1, there may or may not be a cardinal λ satisfying . But, if such a cardinal exists, it is infinite and less than κ and any finite cardinality ν greater than 1 will also satisfy .

The logarithm of an infinite cardinal number κ is defined as the least cardinal number μ such that κ ≤ 2μ. 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.[5][6][7]

The continuum hypothesis

The continuum hypothesis (CH) states that there are no cardinals strictly between and The latter cardinal number is also often denoted by ; it is the cardinality of the continuum (the set of real numbers). In this case The generalized continuum hypothesis (GCH) states that for every infinite set X, there are no cardinals strictly between | X | and 2X |. The continuum hypothesis is independent of the usual axioms of set theory, the Zermelo-Fraenkel axioms together with the axiom of choice (ZFC).

See also

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Template:Col-3-of-3 Sportspersons Hyslop from Nicolet, usually spends time with pastimes for example martial arts, property developers condominium in singapore singapore and hot rods. Maintains a trip site and has lots to write about after touring Gulf of Porto: Calanche of Piana.

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References

Notes 43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

Bibliography

  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

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    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
  • Template:Aut, Infinity, Part IX, Chapter 2, Volume 3 of The World of Mathematics. New York: Simon and Schuster, 1956.
  • Template:Aut, 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).

External links

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  • Cardinality at ProvenMath proofs of the basic theorems on cardinality.


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ru:Кардинальное число

  1. Template:Harvnb
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  3. Enderton, Herbert. "Elements of Set Theory", Academic Press Inc., 1977. ISBN 0-12-238440-7
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    Singaporeans have historically been averse to wealth redistribution, partly because of the idea that focussing on equalising life opportunities is sufficient. When property taxes were abolished in 2008, Singapore grew to become one of the few nations that doesn't have capital features (together with property) or property taxes. Take into account the idea that Singapore wants many rich billionaires to be able to seed and develop businesses. This misunderstands the character of entrepreneurship. Henry Ford and Invoice Gates came from humble origins to construct giant corporations. Their descendants, billionaires, have not created something comparable. Much better for Singapore to foster a pleasant investment local weather and an open, innovative atmosphere. New Launch 2014 in March / April Lush Acres EC

    The Peak @ Cairnhill - is the following upcoming Residential condominium in Singapore District 9. It is a Uncommon Freehold Development that can outshine the Read More b) Singapore Everlasting Residents (SPR) who already own # 1 or more residential properties must pay ABSD of three% on the acquisition or acquisition of one other residential property. SGDeveloper.com showcase unique developer new properties launches in Singapore or abroad. Our group of skilled real property consultants from varied businesses will make it easier to in securing your new dream dwelling or in your property investment wants. N ew Condominium Urban Vista Tanah Merah Launch 2013 Wishing you the very best in your search for that very best new launch property in Singapore! Click right here to learn property market information

    For Mark Shen, 36, the brand new MAS regulations have dented his upgrading plans. He lives together with his wife Annie and two daughters in a resale HDB flat. "We'd planned to buy a three-bedroom rental by taking the maximum bank mortgage and stretching it over the longest period we can. However with these new measures, the loan quantum we now qualify for can probably only get us a two-bedroom unit," says Mark. quick time period good points. The impact of the SSD is especially important as it is payable regardless whether or not the property is ultimately offered at a achieve or loss. HOUSE consumers will now must stump up much more cash upfront after the Government moved to cool the property market in an surroundings of "terribly low" rates of interest. Queenstown Condominium by Hong Leong & CDL
  5. Robert A. McCoy and Ibula Ntantu, Topological Properties of Spaces of Continuous Functions, Lecture Notes in Mathematics 1315, Springer-Verlag.
  6. Eduard Čech, Topological Spaces, revised by Zdenek Frolík and Miroslav Katetov, John Wiley & Sons, 1966.
  7. D.A. Vladimirov, Boolean Algebras in Analysis, Mathematics and Its Applications, Kluwer Academic Publishers.