Infinite set: Difference between revisions

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{{Short description|Set that is not a finite set}}

{{RefimproveMore citations needed|date=September 2011}}

{{Refimprove|date=September 2011}}

[[File:Real numbers.svg|alt=Set Theory Image|thumb|Set Theory Image]]

In [[set theory]], an '''infinite set''' is a [[Set (mathematics)|set]] that is not a [[finite set]]. [[Infinity|Infinite]] sets may be [[countable set|countable]] or [[uncountable set|uncountable]].<ref>{{Cite web|url=http://mathworld.wolfram.com/InfiniteSet.html|title=Infinite Set|last=Weisstein|first=Eric W.|website=mathworld.wolfram.com|language=en|access-date=2019-11-29}}</ref><ref name=":0">{{Cite web|url=https:Bagaria//ncatlab.org/nlab/show/infinite+set|title=infinite set in nLab|website=ncatlab.org|access-date=2019-11-29}}</ref>

==Properties==

The set of [[natural numbers]] (whose existence is postulated by the [[axiom of infinity]]) is infinite.<ref name=":0" /><refBagaria>{{Citation|last=Bagaria|first=Joan|title=Set Theory|date=2019|url=https://plato.stanford.edu/archives/fall2019/entries/set-theory/|encyclopedia=The Stanford Encyclopedia of Philosophy|editor-last=Zalta|editor-first=Edward N.|edition=Fall 2019|publisher=Metaphysics Research Lab, Stanford University|access-date=2019-11-30}}</ref> It is the only set that is directly required by the [[axiom]]s to be infinite. The existence of any other infinite set can be proved in [[Zermelo–Fraenkel set theory]] (ZFC), but only by showing that it follows from the existence of the natural numbers.

A set is infinite if and only if for every natural number, the set has a [[subset]] whose [[cardinality]] is that natural number.<ref>{{cncite book |datetitle=AugustLogic, 2020Logic, and Logic |last = Boolos | first = George |edition=illustrated |publisher=Harvard University Press |year=1998 |isbn=978-0-674-53766-8 |page=262 |url=https://books.google.com/books?id=4OPWAAAAMAAJ}}</ref>

If the [[axiom of choice]] holds, then a set is infinite if and only if it includes a countable infinite subset.

If a [[set of sets]] is infinite or contains an infinite element, then its union is infinite. The [[power set]] of an infinite set is infinite.<ref name=":1" /> Any [[subset|superset]] of an infinite set is infinite. If an infinite set is partitioned into finitely many subsets, then at least one of them must be infinite. Any set which can be mapped ''[[onto]]'' an infinite set is infinite. The [[Cartesian product]] of an infinite set and a nonempty set is infinite. The Cartesian product of an infinite number of sets, each containing at least two elements, is either empty or infinite; if the axiom of choice holds, then it is infinite.

If an infinite set is a [[well-ordered set]], then it must have a nonempty, nontrivial subset that has no greatest element.

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If an infinite set is a [[well-orderable set]], then it has many well-orderings which are non-isomorphic.

==History==

InfiniteImportant ideas discussed by David Burton in his book ''The History of Mathematics: An Introduction'' include how to define "elements" or parts of a set, theoryhow involvesto proofsdefine unique elements in the set, and definitionshow to prove infinity.<ref name=":2">{{Cite book |last=Burton |first=David |title=The History of Mathematics: An Introduction |publisher=McGraw Hill |year=2007 |isbn=9780073051895 |edition=6th |___location=Boston |pages=666-689666–689 |language=en}}</ref> Important ideas discussed by Burton include how to define "elements" or parts of a set, how to define unique elements in the set, and how to prove infinity.<ref name=":2" /> Burton also discusses proofs for different types of infinity, including countable and uncountable sets.<ref name=":2" /> Topics used when comparing infinite and finite sets include [[ordered setsset]]s, cardinality, equivalency, [[coordinate planesplane]]s, [[universal setsset]]s, mapping, subsets, continuity, and [[Transcendental number theory|transcendence]].<ref name=":2" /> Candor[[Georg Cantor|Cantor's]] set ideas were influenced by trigonometry and irrational numbers. Other key ideas in infinite set theory mentioned by Burton, Paula, Narli and Rodger include real numbers such as [[pi|{{pi}}]], integers, and [[Euler's number]].<ref name=":2" /><ref>{{Cite journal |lastlast1=Pala |firstfirst1=Ozan |last2=Narli |first2=Serkan |date=2020-12-15 |title=Role of the Formal Knowledge in the Formation of the Proof Image: A Case Study in the Context of Infinite Sets |url=https://dergipark.org.tr/tr/pub/turkbilmat/issue/58294/702540 |journal=Turkish Journal of Computer and Mathematics Education (TURCOMAT) |language=en |volume=11 |issue=3 |pages=584–618 |doi=10.16949/turkbilmat.702540|s2cid=225253469 |doi-access=free }}</ref><ref name=":3">{{Cite book |last=Rodgers |first=Nancy |title=Learning to reason: an introduction to logic, sets and relations |date=2000 |publisher=Wiley |isbn=978-1-118-16570-6 |___location=New York |oclc=757394919}}</ref>

Both Burton and Rogers use finite sets to start to explain infinite sets using proof concepts such as mapping, proof by induction, or proof by contradiction.<ref name=":2" /><ref name=":3" /> [[Tree (set theory)|Mathematical trees]] can also be used to understand infinite sets.<ref>{{Cite journal |lastlast1=Gollin |firstfirst1=J. Pascal |last2=Kneip |first2=Jakob |date=2021-04-01 |title=Representations of Infinite Tree Sets |journal=Order |language=en |volume=38 |issue=1 |pages=79–96 |doi=10.1007/s11083-020-09529-0 |s2cid=201646182 |issn=1572-9273|doi-access=free |arxiv=1908.10327 }}</ref> Burton also discusses proofs of infinite sets including ideas such as unions and subsets.<ref name=":2" />

In Chapter 12 of ''The History of Mathematics: An Introduction'', Burton emphasizes how mathematicians such as [[Ernst Zermelo|Zermelo]], [[Dedekind]], [[Galileo]], [[Leopold Kronecker|Kronecker]], Cantor, and [[Bernard Bolzano|Bolzano]] investigated and influenced infinite set theory.<ref name=":2"Many />of these mathematicians either debated infinity or otherwise added to the ideas of infinite sets. Potential historical influences, such as how Prussia's history in the 1800's1800s, resulted in an increase in scholarly mathematical knowledge, including CandorCantor's theory of infinite sets.<ref name=":2" />

One potential application of infinite set theory is in genetics and biology.<ref>{{Cite journal |lastlast1=Shelah |firstfirst1=Saharon |last2=Strüngmann |first2=Lutz |date=2021-06-01 |title=Infinite combinatorics in mathematical biology |url=https://www.sciencedirect.com/science/article/pii/S0303264721000496 |journal=Biosystems |language=en |volume=204 |pages=104392 |doi=10.1016/j.biosystems.2021.104392 |pmid=33731280 |s2cid=232298447 |issn=0303-2647|doi-access=free |bibcode=2021BiSys.20404392S }}</ref>

Mathematicians including Zermelo, Dedekind, Galileo, Kronecker, Cantor, and Bolzano investigated or influenced infinite set theory. Many of these mathematicians either debated infinity or otherwise added to the ideas of infinite sets.<ref name=":2" />

One potential application of infinite set theory is in genetics and biology.<ref>{{Cite journal |last=Shelah |first=Saharon |last2=Strüngmann |first2=Lutz |date=2021-06-01 |title=Infinite combinatorics in mathematical biology |url=https://www.sciencedirect.com/science/article/pii/S0303264721000496 |journal=Biosystems |language=en |volume=204 |pages=104392 |doi=10.1016/j.biosystems.2021.104392 |issn=0303-2647}}</ref>

==Examples==

===Countably infinite sets===

The set of all [[integer]]s, {..., -&minus;1, 0, 1, 2, ...} is a countably infinite set. The set of all even integers is also a countably infinite set, even if it is a proper subset of the integers.<ref name=":1">{{Cite web|url=https://primes.utm.edu/glossary/page.php?sort=Infinite|title=The Prime Glossary — Infinite|last=Caldwell|first=Chris|website=primes.utm.edu|access-date=2019-11-29}}</ref>

The set of all [[Rationalrational number|rational numbers]]s is a countably infinite set as there is a bijection to the set of integers.<ref name=":1" />

===Uncountably infinite sets===

The set of all [[real number]]s is an uncountably infinite set. The set of all [[Irrationalirrational number|irrational numbers]]s is also an uncountably infinite set.<ref name=":1" />

The set of all subsets of the integers is uncountably infinite.

==See also==

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==References==

{{Reflist}}

== External links ==