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{{short description|A closed curve divides the plane into two regions}}
[[Image:Jordan curve theorem.svg|thumb|200px|Illustration of the Jordan curve theorem. The Jordan curve (drawn in black) divides the plane into an "inside" region (light blue) and an "outside" region (pink).]]

In [[topology]], a '''Jordan curve''', sometimes called a ''plane simple closed curve'', is a non-self-intersecting [[loop (topology)|continuous loop]] in the plane.<ref>{{Cite book|url=https://books.google.com/books?id=0Q9mbXCQRyoC&pg=PA7|title=Depth, Crossings and Conflicts in Discrete Geometry|last=Sulovský|first=Marek|date=2012|publisher=Logos Verlag Berlin GmbH|isbn=9783832531195|page=7|language=en}}</ref> The '''Jordan curve theorem''' asserts that every Jordan curve divides the plane into an "interior" region bounded by the curve and an "exterior" region containing all of the nearby and far away exterior points, so that every [[path (topology)|continuous path]] connecting a point of one region to a point of the other intersects with that loop somewhere. While the statement of this [[theorem]] seems to be intuitively obvious, it takes some ingenuity to prove it by elementary means. ''"Although the JCT is one of the best known topological theorems, there are many, even among professional mathematicians, who have never read a proof of it."'' ({{harvtxt|Tverberg|1980|loc=Introduction}}). More transparent proofs rely on the mathematical machinery of [[algebraic topology]], and these lead to generalizations to higher-dimensional spaces.

The Jordan curve theorem is named after the [[mathematician]] [[Camille Jordan]] (1838–1922), who found its first proof. For decades, mathematicians generally thought that this proof was flawed and that the first rigorous proof was carried out by [[Oswald Veblen]]. However, this notion has been overturned by [[Thomas Callister Hales|Thomas C. Hales]] and others.

== Definitions and the statement of the Jordan theorem ==

A ''Jordan curve'' or a ''simple closed curve'' in the plane '''R'''<sup>2</sup> is the [[image (mathematics)|image]] ''C'' of an [[injective]] [[continuous map]] of a [[circle]] into the plane, ''φ'': ''S''<sup>1</sup> → '''R'''<sup>2</sup>. A '''Jordan arc''' in the plane is the image of an injective continuous map of a closed and bounded interval {{math|[''a'', ''b'']}} into the plane. It is a [[plane curve]] that is not necessarily [[Curve#Differential curve|smooth]] nor [[algebraic curve|algebraic]].

Alternatively, a Jordan curve is the image of a continuous map ''φ'': [0,1] → '''R'''<sup>2</sup> such that ''φ''(0) = ''φ''(1) and the restriction of ''φ'' to [0,1) is injective. The first two conditions say that ''C'' is a continuous loop, whereas the last condition stipulates that ''C'' has no self-intersection points.

With these definitions, the Jordan curve theorem can be stated as follows:

<blockquote>
Let ''C'' be a Jordan curve in the plane '''R'''<sup>2</sup>. Then its [[complement (set theory)|complement]], '''R'''<sup>2</sup>&nbsp;\&nbsp;''C'', consists of exactly two [[connected component (topology)|connected component]]s. One of these components is [[bounded set|bounded]] (the '''interior''') and the other is unbounded (the '''exterior'''), and the curve ''C'' is the [[boundary (topology)|boundary]] of each component.
</blockquote>

In contrast, the complement of a Jordan ''arc'' in the plane is connected.

== Proof and generalizations ==

The Jordan curve theorem was independently generalized to higher dimensions by [[H. Lebesgue]] and [[L.E.J. Brouwer]] in 1911, resulting in the '''Jordan–Brouwer separation theorem'''.

<blockquote>
Let ''X'' be a ''[[topological sphere]]'' in the (''n''+1)-dimensional [[Euclidean space]] '''R'''<sup>''n''+1</sup> (''n'' > 0), i.e. the image of an injective continuous mapping of the [[n-sphere|''n''-sphere]] ''S<sup>n</sup>'' into '''R'''<sup>''n''+1</sup>.  Then the complement ''Y'' of ''X'' in '''R'''<sup>''n''+1</sup> consists of exactly two connected components.  One of these components is bounded (the interior) and the other is unbounded (the exterior).  The set ''X'' is their common boundary.
</blockquote>

The proof uses [[homology theory]]. It is first established that, more generally, if ''X'' is homeomorphic to the ''k''-sphere, then the [[reduced homology|reduced integral homology]] groups of ''Y'' = '''R'''<sup>''n''+1</sup> \ ''X'' are as follows:

: <math>\tilde{H}_{q}(Y)= \begin{cases}\mathbb{Z},\quad q=n-k\text{ or }n \\ 0,\quad \text{otherwise}.\end{cases}</math>

This is proved by induction in ''k'' using the [[Mayer–Vietoris sequence]]. When ''n'' = ''k'', the zeroth reduced homology of ''Y'' has rank 1, which means that ''Y'' has 2 connected components (which are, moreover, [[path connected]]), and with a bit of extra work, one shows that their common boundary is ''X''. A further generalization was found by [[James Waddell Alexander II|J. W. Alexander]], who established the [[Alexander duality]] between the reduced homology of a [[compact space|compact]] subset ''X'' of '''R'''<sup>''n''+1</sup> and the reduced cohomology of its complement. If ''X'' is an ''n''-dimensional compact connected submanifold of '''R'''<sup>''n''+1</sup> (or '''S'''<sup>''n''+1</sup>) without boundary, its complement has 2 connected components.

There is a strengthening of the Jordan curve theorem, called the [[Jordan–Schönflies theorem]], which states that the interior and the exterior planar regions determined by a Jordan curve in '''R'''<sup>2</sup> are [[homeomorphic]] to the interior and exterior of the [[unit disk]]. In particular, for any point ''P'' in the interior region and a point ''A'' on the Jordan curve, there exists a Jordan arc connecting ''P'' with ''A'' and, with the exception of the endpoint ''A'', completely lying in the interior region. An alternative and equivalent formulation of the Jordan–Schönflies theorem asserts that any Jordan curve ''&phi;'': ''S''<sup>1</sup> → '''R'''<sup>2</sup>, where ''S''<sup>1</sup> is viewed as the [[unit circle]] in the plane, can be extended to a homeomorphism ''&psi;'': '''R'''<sup>2</sup> → '''R'''<sup>2</sup> of the plane. Unlike Lebesgues' and Brouwer's generalization of the Jordan curve theorem, this statement becomes ''false'' in higher dimensions: while the exterior of the unit ball in '''R'''<sup>3</sup> is [[simply connected]], because it [[deformation retract|retracts]] onto the unit sphere, the [[Alexander horned sphere]] is a subset of '''R'''<sup>3</sup> homeomorphic to a [[sphere]], but so twisted in space that the unbounded component of its complement in '''R'''<sup>3</sup> is not simply connected, and hence not homeomorphic to the exterior of the unit ball.

== History and further proofs ==

The statement of the Jordan curve theorem may seem obvious at first, but it is a rather difficult theorem to prove.
{{citation needed span|text=[[Bernard Bolzano]] was the first to formulate a precise conjecture, observing that it was not a self-evident statement, but that it required a proof.|date=March 2019}}
It is easy to establish this result for [[polygon]]s, but the problem came in generalizing it to all kinds of badly behaved curves, which include [[nowhere differentiable]] curves, such as the [[Koch snowflake]] and other [[fractal curve]]s, or even [[Osgood curve|a Jordan curve of positive area]] constructed by {{harvtxt|Osgood|1903}}.

The first proof of this theorem was given by [[Camille Jordan]] in his lectures on [[real analysis]], and was published in his book ''Cours d'analyse de l'École Polytechnique''.<ref>{{harvs|txt|authorlink=Camille Jordan|first=Camille|last= Jordan|year=1887}}</ref> There is some controversy about whether Jordan's proof was complete: the majority of commenters on it have claimed that the first complete proof was given later by [[Oswald Veblen]], who said the following about Jordan's proof:

:His proof, however, is unsatisfactory to many mathematicians. It assumes the theorem without proof in the important special case of a simple polygon, and of the argument from that point on, one must admit at least that all details are not given.<ref>{{harvs|txt|authorlink=Oswald Veblen|first=Oswald |last=Veblen|year=1905}}</ref>

However, [[Thomas Callister Hales|Thomas C. Hales]] wrote:

:Nearly every modern citation that I have found agrees that the first correct proof is due to Veblen... In view of the heavy criticism of Jordan’s proof, I was surprised when I sat down to read his proof to find nothing objectionable about it. Since then, I have contacted a number of the authors who have criticized Jordan, and each case the author has admitted to having no direct knowledge of an error in Jordan’s proof.<ref>{{harvtxt|Hales|2007b}}</ref>

Hales also pointed out that the special case of simple polygons is not only an easy exercise, but was not really used by Jordan anyway, and quoted Michael Reeken as saying:

:Jordan’s proof is essentially correct... Jordan’s proof does not present the details in a satisfactory way. But the idea is right, and with some polishing the proof would be impeccable.<ref>{{harvtxt|Hales|2007b}}</ref>

Earlier, Jordan's proof and another early proof by [[de la Vallée Poussin]] had already been critically analyzed and completed by Schoenflies (1924).<ref>{{cite journal |author=A. Schoenflies |authorlink=Arthur Moritz Schoenflies |title=Bemerkungen zu den Beweisen von C. Jordan und Ch. J. de la Vallée Poussin |journal=Jahresber. Deutsch. Math.-Verein |volume=33 |year=1924 |pages=157–160}}</ref>

Due to the importance of the Jordan curve theorem in [[low-dimensional topology]] and [[complex analysis]], it received much attention from prominent mathematicians of the first half of the 20th century. Various proofs of the theorem and its generalizations were constructed by [[James Waddell Alexander II|J. W. Alexander]], [[Louis Antoine]], [[Ludwig Bieberbach]], [[Luitzen Brouwer]], [[Arnaud Denjoy]], [[Friedrich Hartogs]], [[Béla Kerékjártó]], [[Alfred Pringsheim]], and [[Arthur Moritz Schoenflies]].

New elementary proofs of the Jordan curve theorem, as well as simplifications of the earlier proofs, continue to be carried out.

* Elementary proofs were presented by {{harvtxt|Filippov|1950}} and {{harvtxt|Tverberg|1980}}.
* A proof using [[non-standard analysis]] by {{harvtxt|Narens|1971}}.
* A proof using constructive mathematics by  {{harvs | txt|last1=Berg | first1=Gordon O. | last2=Julian | first2=W. | last3=Mines | first3=R. | last4=Richman | first4=Fred | title=The constructive Jordan curve theorem | mr=0410701 | year=1975 | journal=[[Rocky Mountain Journal of Mathematics]] | issn=0035-7596 | volume=5 | pages=225–236}}.
* A proof using the [[Brouwer fixed point theorem]] by {{harvtxt|Maehara|1984}}.
* A proof using [[planar graph|non-planarity]] of the [[complete bipartite graph]] K<sub>3,3</sub> was given by {{harvtxt| Thomassen| 1992}}.

The root of the difficulty is explained in {{harvtxt|Tverberg|1980}} as follows. It is relatively simple to prove that the Jordan curve theorem holds for every Jordan polygon (Lemma 1), and every Jordan curve can be approximated arbitrarily well by a Jordan polygon (Lemma 2). A Jordan polygon is a [[polygonal chain]], the boundary of a bounded connected [[open set]], call it the open polygon, and its [[Closure (topology)|closure]], the closed polygon. Consider the diameter <math>\delta</math> of the largest disk contained in the closed polygon. Evidently, <math>\delta</math> is positive. Using a sequence of Jordan polygons (that converge to the given Jordan curve) we have a sequence <math>\delta_1, \delta_2, \dots</math> ''presumably'' converging to a positive number, the diameter <math>\delta</math> of the largest disk contained in the [[closed region]] bounded by the Jordan curve. However, we have to ''prove'' that the sequence <math>\delta_1, \delta_2, \dots</math> does not converge to zero, using only the given Jordan curve, not the region ''presumably'' bounded by the curve. This is the point of Tverberg's Lemma 3. Roughly, the closed polygons should not thin to zero everywhere. Moreover, they should not thin to zero somewhere, which is the point of Tverberg's Lemma 4.

The first [[formal proof]] of the Jordan curve theorem was created by {{harvtxt|Hales|2007a}} in the [[HOL Light]] system, in January 2005, and contained about 60,000 lines. Another rigorous 6,500-line formal proof was produced in 2005 by an international team of mathematicians using the [[Mizar system]]. Both the Mizar and the HOL Light proof rely on libraries of previously proved theorems, so these two sizes are not comparable. {{harvs|txt | last1=Sakamoto | first1=Nobuyuki | last2=Yokoyama | first2=Keita | title=The Jordan curve theorem and the Schönflies theorem in weak second-order arithmetic | doi=10.1007/s00153-007-0050-6 | mr=2321588 | year=2007 | journal=Archive for Mathematical Logic | issn=0933-5846 | volume=46 | issue=5 | pages=465–480}} showed that the Jordan curve theorem is equivalent in proof-theoretic strength to the [[weak König's lemma]].

== See also ==
* [[Denjoy–Riesz theorem]], a description of certain sets of points in the plane that can be subsets of Jordan curves
* [[Lakes of Wada]]
* [[Quasi-Fuchsian group]], a mathematical group that preserves a Jordan curve
* [[Complex analysis]]

== Notes ==
<references/>

== References ==

*{{Citation | doi=10.1216/RMJ-1975-5-2-225 | last1=Berg | first1=Gordon O. | last2=Julian | first2=W. | last3=Mines | first3=R. | last4=Richman | first4=Fred | title=The constructive Jordan curve theorem | mr=0410701 | year=1975 | journal=[[Rocky Mountain Journal of Mathematics]] | issn=0035-7596 | volume=5 | issue=2 | pages=225–236}}
*{{Citation | last=Filippov | first= A. F. | authorlink=Aleksei Fedorovich Filippov | year=1950 | title=An elementary proof of Jordan's theorem | url=http://www.mathnet.ru/links/9bd143c40db8f868858992d40913c648/rm8482.pdf | journal=Uspekhi Mat. Nauk | language=Russian | volume=5 | number=5 | pages=173–176}}
*{{Citation | last1=Hales | first1=Thomas C. | author1-link=Thomas Callister Hales | title=The Jordan curve theorem, formally and informally | mr=2363054 | year=2007a | journal=[[American Mathematical Monthly|The American Mathematical Monthly]] | issn=0002-9890 | volume=114 | issue=10 | pages=882–894}}
*{{Citation | last1=Hales | first1=Thomas | author1-link=Thomas Callister Hales | title=Jordan's proof of the Jordan Curve theorem | url=http://mizar.org/trybulec65/4.pdf | year=2007b | journal=Studies in Logic, Grammar and Rhetoric | volume=10 | issue=23}}
*{{Citation | last1=Jordan | first1=Camille | author1-link=Camille Jordan |title= Cours d'analyse  | url=http://www.maths.ed.ac.uk/~aar/jordan/jordan.pdf | year=1887|pages=587–594}}
*{{Citation | doi=10.2307/2323369 | last1=Maehara | first1=Ryuji | title=The Jordan Curve Theorem Via the Brouwer Fixed Point Theorem | mr=0769530 | year=1984 | journal=[[American Mathematical Monthly|The American Mathematical Monthly]] | issn=0002-9890 | volume=91 | issue=10 | pages=641–643 | jstor=2323369}}
*{{Citation | last1=Narens | first1=Louis | title=A nonstandard proof of the Jordan curve theorem | url=http://projecteuclid.org/euclid.pjm/1102971282 | mr=0276940 | year=1971 | journal=[[Pacific Journal of Mathematics]] | issn=0030-8730 | volume=36 | pages=219–229 | doi=10.2140/pjm.1971.36.219}}
*{{Citation | last1=Osgood | first1=William F. | title=A Jordan Curve of Positive Area | jstor=1986455 | jfm=34.0533.02 | year=1903 | journal=[[Transactions of the American Mathematical Society]] | issn=0002-9947 | volume=4 | issue=1 | pages=107–112 | doi=10.2307/1986455}}
*{{Citation | last1=Ross | first1=Fiona | last2=Ross | first2=William T. | year=2011 | title=The Jordan curve theorem is non-trivial | journal=[[Journal of Mathematics and the Arts]] | volume=5 | issue=4 | pages=213–219 | doi=10.1080/17513472.2011.634320}}. [https://facultystaff.richmond.edu/~wross/PDF/Jordan-revised.pdf author's site]
*{{Citation | last1=Sakamoto | first1=Nobuyuki | last2=Yokoyama | first2=Keita | title=The Jordan curve theorem and the Schönflies theorem in weak second-order arithmetic | doi=10.1007/s00153-007-0050-6 | mr=2321588 | year=2007 | journal=Archive for Mathematical Logic | issn=0933-5846 | volume=46 | issue=5 | pages=465–480}}
*{{Citation | doi=10.2307/2324180 | last=Thomassen | first=Carsten | author-link=Carsten Thomassen | title=The Jordan–Schönflies theorem and the classification of surfaces |year=1992 | journal=American Mathematical Monthly | volume=99 | issue=2 | pages=116–130 | jstor=2324180}}
*{{Citation | last= Tverberg |first= Helge |authorlink= Helge Tverberg |year= 1980 |title= A proof of the Jordan curve theorem |url= http://www.maths.ed.ac.uk/~aar/jordan/tverberg.pdf |journal= [[Bulletin of the London Mathematical Society]] |volume= 12 |number= 1 |pages= 34–38 |doi= 10.1112/blms/12.1.34 |citeseerx= 10.1.1.374.2903 }}
* {{Citation | last1=Veblen | first1=Oswald | author1-link=Oswald Veblen | title=Theory on Plane Curves in Non-Metrical Analysis Situs | jstor=1986378 | year=1905 | journal=[[Transactions of the American Mathematical Society]] | volume=6 | issue=1 | pages=83–98 | mr=1500697 | doi=10.2307/1986378}}

== External links ==

* {{eom|id=j/j054370|author=M.I. Voitsekhovskii|title=Jordan theorem}}
* [http://mizar.uwb.edu.pl/version/7.11.07_4.156.1112/html/jordan.html The full 6,500 line formal proof of Jordan's curve theorem] in [[Mizar system|Mizar]].
* [http://www.maths.ed.ac.uk/~aar/jordan Collection of proofs of the Jordan curve theorem] at Andrew Ranicki's homepage
* [http://www.math.auckland.ac.nz/class750/section5.pdf A simple proof of Jordan curve theorem] (PDF) by David B. Gauld
* [https://arxiv.org/abs/1404.0556arXiv:1404.0556 Brown, R.  and Antolino-Camarena, O. "Corrigendum to "Groupoids, the Phragmen-Brouwer Property, and the Jordan Curve Theorem", J. Homotopy and Related Structures 1 (2006) 175-183." J. Homotopy and Related Structures, (2015) 10:669-672, DOI: 10.100715.40062-014-0089-0 ]{{dead link|date=December 2017 |bot=InternetArchiveBot |fix-attempted=yes }}
[[Category:Theorems in topology]]
[[Category:Curves]]