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| | My name is Elba (21 years old) and my hobbies are Kart racing and Photography.<br><br>My homepage; [http://46.102.241.201/avarcade/profile/228335/andorsett how to get free fifa 15 coins] |
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| !colspan="2"|Example graphs
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| ! Planar
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| ! Nonplanar
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| | align="center" | [[Image:Butterfly graph.svg|100px]] <br> [[Butterfly graph]]
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| | align="center" | [[Image:Complete graph K5.svg|100px]] <br>[[Complete graph]] ''K''<sub>5</sub>
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| | align="center" | [[Image:CGK4PLN.svg|100px]] <br> [[Complete graph]]<br> ''K''<sub>4</sub>
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| | align="center" | [[Image:Biclique K 3 3.svg|100px]] <br>[[Utility graph]] ''K''<sub>3,3</sub>
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| |}
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| In [[graph theory]], a '''planar graph''' is a [[graph (mathematics)|graph]] that can be [[graph embedding|embedded]] in the plane, i.e., it can be drawn on the plane in such a way that its edges intersect only at their endpoints. In other words, it can be drawn in such a way that no edges cross each other.<ref>{{cite book|last=Trudeau|first=Richard J.|title=Introduction to Graph Theory|year=1993|publisher=Dover Pub.|location=New York|isbn=978-0-486-67870-2|pages=64|url=http://store.doverpublications.com/0486678709.html|edition=Corrected, enlarged republication.|accessdate=8 August 2012|quote=Thus a planar graph, when drawn on a flat surface, either has no edge-crossings or can be redrawn without them.}}</ref> Such a drawing is called a '''plane graph''' or '''planar embedding of the graph'''. A plane graph can be defined as a planar graph with a mapping from every node to a point on a plane, and from every edge to a [[plane curve]] on that plane, such that the extreme points of each curve are the points mapped from its end nodes, and all curves are disjoint except on their extreme points.
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| Every graph that can be drawn on a plane can be drawn on the [[sphere]] as well, and vice versa.
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| Plane graphs can be encoded by [[combinatorial map]]s.
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| The [[equivalence class]] of [[topologically equivalent]] drawings on the sphere is called a '''planar map'''. Although a plane graph has an '''external''' or '''unbounded''' face, none of the faces of a planar map have a particular status.
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| A generalization of planar graphs are graphs which can be drawn on a surface of a given [[genus (mathematics)|genus]]. In this terminology, planar graphs have [[graph genus]] 0, since the plane (and the sphere) are surfaces of genus 0. See "[[graph embedding]]" for other related topics.
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| == Kuratowski's and Wagner's theorems ==
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| The [[Poland|Polish]] mathematician [[Kazimierz Kuratowski]] provided a characterization of planar graphs in terms of [[Forbidden graph characterization|forbidden graphs]], now known as [[Kuratowski's theorem]]:
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| :A [[graph (mathematics)#Finite and infinite graphs|finite graph]] is planar [[if and only if]] it does not contain a [[Glossary of graph theory#Subgraphs|subgraph]] that is a [[subdivision (graph theory)|subdivision]] of ''K''<sub>5</sub> (the [[complete graph]] on five [[vertex (graph theory)|vertices]]) or ''K''<sub>3,3</sub> ([[complete bipartite graph]] on six vertices, three of which connect to each of the other three, also known as the [[utility graph]]).
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| A [[subdivision (graph theory)|subdivision]] of a graph results from inserting vertices into edges (for example, changing an edge •——• to •—•—•) zero or more times.
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| [[Image:Nonplanar no subgraph K 3 3.svg|thumb|An example of a graph which doesn't have ''K''<sub>5</sub> or ''K''<sub>3,3</sub> as its subgraph. However, it has a subgraph that is homeomorphic to ''K''<sub>3,3</sub> and is therefore not planar.]] | |
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| Instead of considering subdivisions, [[Wagner's theorem]] deals with [[minor (graph theory)|minors]]:
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| :A finite graph is planar if and only if it does not have ''K''<sub>5</sub> or ''K''<sub>3,3</sub> as a [[minor (graph theory)|minor]]. | |
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| [[File:Kuratowski.gif|thumb|484px|An animation showing that the [[Petersen graph]] contains a minor isomorphic to the K<sub>3,3</sub> graph]]
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| [[Klaus Wagner (mathematician)|Klaus Wagner]] asked more generally whether any minor-closed class of graphs is determined by a finite set of "[[forbidden minor]]s". This is now the [[Robertson–Seymour theorem]], proved in a long series of papers. In the language of this theorem, ''K''<sub>5</sub> and ''K''<sub>3,3</sub> are the forbidden minors for the class of finite planar graphs.
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| == Other planarity criteria ==
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| In practice, it is difficult to use Kuratowski's criterion to quickly decide whether a given graph is planar. However, there exist fast [[algorithm]]s for this problem: for a graph with ''n'' vertices, it is possible to determine in time [[Big O notation|O]](''n'') (linear time) whether the graph may be planar or not (see [[planarity testing]]).
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| For a simple, connected, planar graph with ''v'' vertices and ''e'' edges, the following simple conditions hold:
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| : Theorem 1. If ''v'' ≥ 3 then ''e'' ≤ 3''v'' − 6;
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| : Theorem 2. If ''v'' ≥ 3 and there are no cycles of length 3, then ''e'' ≤ 2''v'' − 4.
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| In this sense, planar graphs are [[sparse graph]]s, in that they have only O(''v'') edges, asymptotically smaller than the maximum O(''v''<sup>2</sup>). The graph ''K''<sub>3,3</sub>, for example, has 6 vertices, 9 edges, and no cycles of length 3. Therefore, by Theorem 2, it cannot be planar. Note that these theorems provide necessary conditions for planarity that are not sufficient conditions, and therefore can only be used to prove a graph is not planar, not that it is planar. If both theorem 1 and 2 fail, other methods may be used.
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| * [[Whitney's planarity criterion]] gives a characterization based on the existence of an algebraic dual;
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| * [[MacLane's planarity criterion]] gives an algebraic characterization of finite planar graphs, via their [[cycle space]]s;
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| * The [[Fraysseix–Rosenstiehl planarity criterion]] gives a characterization based on the existence of a bipartition of the cotree edges of a depth-first search tree. It is central to the '''left-right planarity testing algorithm''';
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| * [[Schnyder's theorem]] gives a characterization of planarity in terms of [[Order dimension|partial order dimension]];
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| * [[Colin de Verdière graph invariant|Colin de Verdière's planarity criterion]] gives a characterization based on the maximum multiplicity of the second eigenvalue of certain Schrödinger operators defined by the graph.
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| === Euler's formula ===
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| {{main|Euler characteristic#Planar graphs}}
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| '''Euler's formula''' states that if a finite, [[Connectivity (graph theory)|connected]], planar graph is drawn in the plane without any edge intersections, and ''v'' is the number of vertices, ''e'' is the number of edges and ''f'' is the number of [[Glossary of graph theory#Genus|faces]] (regions bounded by edges, including the outer, infinitely large region), then
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| :<big>''v'' − ''e'' + ''f'' = 2</big>.
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| As an illustration, in the butterfly graph given above, ''v'' = 5, ''e'' = 6 and ''f'' = 3. If the second graph is redrawn without edge intersections, it has ''v'' = 4, ''e'' = 6 and ''f'' = 4.
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| In general, if the property holds for all planar graphs of ''f'' faces, any change to the graph that creates an additional face while keeping the graph planar would keep ''v'' − ''e'' + ''f'' an invariant. Since the property holds for all graphs with ''f'' = 2, by [[mathematical induction]] it holds for all cases. Euler's formula can also be proved as follows: if the graph isn't a [[tree (graph theory)|tree]], then remove an edge which completes a [[cycle (graph theory)|cycle]]. This lowers both ''e'' and ''f'' by one, leaving ''v'' − ''e'' + ''f'' constant. Repeat until the remaining graph is a tree; trees have ''v'' = ''e'' + 1 and ''f'' = 1, yielding ''v'' − ''e'' + ''f'' = 2. i.e. the [[Euler characteristic]] is 2.
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| In a finite, [[Connectivity (graph theory)|connected]], ''[[simple graph|simple]]'', planar graph, any face (except possibly the outer one) is bounded by at least three edges and every edge touches at most two faces; using Euler's formula, one can then show that these graphs are ''sparse'' in the sense that ''e'' ≤ 3''v'' − 6 if ''v'' ≥ 3.
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| [[File:Dodecahedron schlegel diagram.png|thumb|A [[Schlegel diagram]] of a regular [[dodecahedron]], forming a planar graph from a convex polyhedron.]]
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| Euler's formula is also valid for [[convex polyhedron|convex polyhedra]]. This is no coincidence: every convex polyhedron can be turned into a connected, simple, planar graph by using the [[Schlegel diagram]] of the polyhedron, a [[perspective projection]] of the polyhedron onto a plane with the center of perspective chosen near the center of one of the polyhedron's faces. Not every planar graph corresponds to a convex polyhedron in this way: the trees do not, for example. [[Steinitz's theorem]] says that the [[polyhedral graph]]s formed from convex polyhedra are precisely the finite [[Connectivity (graph theory)|3-connected]] simple planar graphs. More generally, Euler's formula applies to any polyhedron whose faces are [[simple polygon]]s that form a surface [[homeomorphism|topologically equivalent]] to a sphere, regardless of its convexity.
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| === Average degree ===
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| From <big>''v'' − ''e'' + ''f'' = 2</big> and <math>3f \le 2e</math> (one face has minimum 3 edges and each edge has maximum two faces) it follows via algebraic transformations that the average degree is strictly less than 6. Otherwise the given graph can't be planar.
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| === Coin graphs ===
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| {{main|Circle packing theorem}}
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| [[File:Circle packing theorem K5 minus edge example.svg|thumb|Example of the circle packing theorem on K<sub>5</sub>, the complete graph on five vertices, minus one edge.]]
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| We say that two circles drawn in a plane ''kiss'' (or ''[[Osculating circle|osculate]]'') whenever they intersect in exactly one point. A "coin graph" is a graph formed by a set of circles, no two of which have overlapping interiors, by making a vertex for each circle and an edge for each pair of circles that kiss. The [[circle packing theorem]], first proved by [[Paul Koebe]] in 1936, states that a graph is planar if and only if it is a coin graph.
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| This result provides an easy proof of [[Fáry's theorem]], that every planar graph can be embedded in the plane in such a way that its edges are straight [[line segment]]s that do not cross each other. If one places each vertex of the graph at the center of the corresponding circle in a coin graph representation, then the line segments between centers of kissing circles do not cross any of the other edges.
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| ==Related families of graphs==
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| ===Maximal planar graphs===
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| [[File:Goldner-Harary graph.svg|thumb|240px|The [[Goldner–Harary graph]] is maximal planar. All its faces are bounded by three edges.]]
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| A simple graph is called '''maximal planar''' if it is planar but adding any edge (on the given vertex set) would destroy that property. All faces (including the outer one) are then bounded by three edges, explaining the alternative term '''plane triangulation'''. The alternative names "triangular graph"<ref>{{citation|first=W.|last=Schnyder|title=Planar graphs and poset dimension|journal=[[Order (journal)|Order]]|volume=5|year=1989|pages=323–343|doi=10.1007/BF00353652|mr=1010382}}.</ref> or "triangulated graph"<ref>{{citation|journal=Algorithmica|volume=3|issue=1–4|year=1988|pages=247–278|doi= 10.1007/BF01762117|title=A linear algorithm to find a rectangular dual of a planar triangulated graph|first1=Jayaram|last1=Bhasker|first2=Sartaj|last2=Sahni}}.</ref> have also been used, but are ambiguous, as they more commonly refer to the [[line graph]] of a [[complete graph]] and to the [[chordal graph]]s respectively.
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| If a maximal planar graph has ''v'' vertices with ''v'' > 2, then it has precisely 3''v'' − 6 edges and 2''v'' − 4 faces.
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| [[Apollonian network]]s are the maximal planar graphs formed by repeatedly splitting triangular faces into triples of smaller triangles. Equivalently, they are the planar [[k-tree|3-trees]].
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| [[Strangulated graph]]s are the graphs in which every [[peripheral cycle]] is a triangle. In a maximal planar graph (or more generally a polyhedral graph) the peripheral cycles are the faces, so maximal planar graphs are strangulated. The strangulated graphs include also the [[chordal graph]]s, and are exactly the graphs that can be formed by [[clique-sum]]s (without deleting edges) of [[complete graph]]s and maximal planar graphs.<ref>{{citation
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| | last1 = Seymour | first1 = P. D.
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| | last2 = Weaver | first2 = R. W.
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| | doi = 10.1002/jgt.3190080206
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| | issue = 2
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| | journal = Journal of Graph Theory
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| | mr = 742878
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| | pages = 241–251
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| | title = A generalization of chordal graphs
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| | volume = 8
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| | year = 1984}}.</ref>
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| ===Outerplanar graphs===
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| [[Outerplanar graph]]s are graphs with an embedding in the plane such that all vertices belong to the unbounded face of the embedding. Every outerplanar graph is planar, but the converse is not true: ''K''<sub>4</sub> is planar but not outerplanar. A theorem similar to Kuratowski's states that a finite graph is outerplanar if and only if it does not contain a subdivision of ''K''<sub>4</sub> or of ''K''<sub>2,3</sub>.
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| A 1-outerplanar embedding of a graph is the same as an outerplanar embedding. For ''k'' > 1 a planar embedding is ''k''-outerplanar if removing the vertices on the outer face results in a (''k'' − 1)-outerplanar embedding. A graph is ''k''-outerplanar if it has a ''k''-outerplanar embedding.
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| ===Halin graphs===
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| A [[Halin graph]] is a graph formed from an undirected plane tree (with no degree-two nodes) by connecting its leaves into a cycle, in the order given by the plane embedding of the tree. Equivalently, it is a [[polyhedral graph]] in which one face is adjacent to all the others. Every Halin graph is planar. Like outerplanar graphs, Halin graphs have low [[treewidth]], making many algorithmic problems on them more easily solved than in unrestricted planar graphs.<ref>{{citation
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| | last1 = Sysło | first1 = Maciej M.
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| | last2 = Proskurowski | first2 = Andrzej
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| | contribution = On Halin graphs
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| | doi = 10.1007/BFb0071635
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| | pages = 248–256
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| | publisher = Springer-Verlag
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| | series = Lecture Notes in Mathematics
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| | title = Graph Theory: Proceedings of a Conference held in Lagów, Poland, February 10–13, 1981
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| | volume = 1018
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| | year = 1983}}.</ref>
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| ===Other related families===
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| An [[apex graph]] is a graph that may be made planar by the removal of one vertex, and a ''k''-apex graph is a graph that may be made planar by the removal of at most ''k'' vertices.
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| A [[1-planar graph]] is a graph that may be drawn in the plane with at most one simple crossing per edge, and a ''k''-planar graph is a graph that may be drawn with at most ''k'' simple crossings per edge.
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| A [[toroidal graph]] is a graph that can be embedded without crossings on the [[torus]]. More generally, the [[genus (mathematics)|genus]] of a graph is the minimum genus of a two-dimensional graph onto which the graph may be embedded; planar graphs have genus zero and nonplanar toroidal graphs have genus one.
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| Any graph may be embedded into three-dimensional space without crossings. However, a three-dimensional analogue of the planar graphs is provided by the [[linkless embedding|linklessly embeddable graphs]], graphs that can be embedded into three-dimensional space in such a way that no two cycles are [[linking number|topologically linked]] with each other. In analogy to Kuratowski's and Wagner's characterizations of the planar graphs as being the graphs that do not contain ''K''<sub>5</sub> or ''K''<sub>3,3</sub> as a minor, the linklessly embeddable graphs may be characterized as the graphs that do not contain as a minor any of the seven graphs in the [[Petersen family]]. In analogy to the characterizations of the outerplanar and planar graphs as being the graphs with [[Colin de Verdière graph invariant]] at most two or three, the linklessly embeddable graphs are the graphs that have Colin de Verdière invariant at most four.
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| An [[Upward planar drawing|upward planar graph]] is a [[directed acyclic graph]] that can be drawn in the plane with its edges as non-crossing curves that are consistently oriented in an upward direction. Not every planar directed acyclic graph is upward planar, and it is NP-complete to test whether a given graph is upward planar.
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| == Other facts and definitions ==
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| Every planar graph without loops is 4-partite, or 4-[[graph coloring|colorable]]; this is the graph-theoretical formulation of the [[four color theorem]].
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| [[Fáry's theorem]] states that every simple planar graph admits an embedding in the plane such that all edges are [[straight line]] segments which don't intersect. A [[universal point set]] is a set of points such that every planar graph with ''n'' vertices has such an embedding with all vertices in the point set; there exist universal point sets of quadratic size, formed by taking a rectangular subset of the [[integer lattice]]. Every simple outerplanar graph admits an embedding in the plane such that all vertices lie on a fixed circle and all edges are straight line segments that lie inside the disk and don't intersect, so ''n''-vertex [[regular polygon]]s are universal for outerplanar graphs.
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| [[Image:dual graphs.svg|thumb|100px|A planar graph and its [[Dual graph|dual]]]]
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| Given an embedding ''G'' of a (not necessarily simple) connected graph in the plane without edge intersections, we construct the '''[[dual graph]]''' ''G''* as follows: we choose one vertex in each face of ''G'' (including the outer face) and for each edge ''e'' in ''G'' we introduce a new edge in ''G''* connecting the two vertices in ''G''* corresponding to the two faces in ''G'' that meet at ''e''. Furthermore, this edge is drawn so that it crosses ''e'' exactly once and that no other edge of ''G'' or ''G''* is intersected. Then ''G''* is again the embedding of a (not necessarily simple) planar graph; it has as many edges as ''G'', as many vertices as ''G'' has faces and as many faces as ''G'' has vertices. The term "dual" is justified by the fact that ''G''** = ''G''; here the equality is the equivalence of embeddings on the [[sphere]]. If ''G'' is the planar graph corresponding to a convex polyhedron, then ''G''* is the planar graph corresponding to the dual polyhedron.
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| Duals are useful because many properties of the dual graph are related in simple ways to properties of the original graph, enabling results to be proven about graphs by examining their dual graphs.
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| While the dual constructed for a particular embedding is unique (up to [[isomorphism]]), graphs may have different (i.e. non-isomorphic) duals, obtained from different (i.e. non-[[homeomorphic]]) embeddings.
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| A ''Euclidean graph'' is a graph in which the vertices represent points in the plane, and the edges are assigned lengths equal to the Euclidean distance between those points; see [[Geometric graph theory]].
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| A plane graph is said to be ''convex'' if all of its faces (including the outer face) are [[convex polygon]]s. A planar graph may be drawn convexly if and only if it is a [[Subdivision (graph theory)|subdivision]] of a [[k-vertex-connected graph|3-vertex-connected]] planar graph.
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| [[Scheinerman's conjecture]] (now a theorem) states that every planar graph can be represented as an [[intersection graph]] of [[line segment]]s in the plane.
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| The [[planar separator theorem]] states that every ''n''-vertex planar graph can be partitioned into two [[Glossary of graph theory#Subgraphs|subgraphs]] of size at most 2''n''/3 by the removal of O(√''n'') vertices. As a consequence, planar graphs also have [[treewidth]] and [[branch-width]] O(√''n'').
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| For two planar graphs with ''v'' vertices, it is possible to determine in time O(''v'') whether they are [[graph theory|isomorphic]] or not (see also [[graph isomorphism problem]]).<ref>I. S. Filotti, Jack N. Mayer. A polynomial-time algorithm for determining the isomorphism of graphs of fixed genus. Proceedings of the 12th Annual ACM Symposium on Theory of Computing, p.236–243. 1980.</ref>
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| ==See also==
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| * [[Planarity]], a puzzle computer game in which the objective is to embed a planar graph onto a plane
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| * [[Sprouts (game)]], a pencil-and-paper game where a planar graph subject to certain constrains is constructed as part of the game play
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| ==Notes==
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| <references />
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| ==References==
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| *{{citation|first=Kazimierz|last=Kuratowski|authorlink=Kazimierz Kuratowski|year=1930|url=http://matwbn.icm.edu.pl/ksiazki/fm/fm15/fm15126.pdf|title=Sur le problème des courbes gauches en topologie|journal=Fund. Math.|volume=15|pages=271–283|language=French}}.
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| *{{citation|first=K.|last=Wagner|year=1937|title=Über eine Eigenschaft der ebenen Komplexe|journal=Math. Ann.|volume=114|pages=570–590|doi=10.1007/BF01594196}}.
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| *{{citation|first1=John M.|last1=Boyer|first2=Wendy J.|last2=Myrvold|year=2005|url=http://jgaa.info/accepted/2004/BoyerMyrvold2004.8.3.pdf|title=On the cutting edge: Simplified O(n) planarity by edge addition|journal=[[Journal of Graph Algorithms and Applications]]|volume=8|issue=3|pages=241–273}}.
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| *{{citation|first1=Brendan|last1=McKay|authorlink1=Brendan McKay|first2=Gunnar|last2=Brinkmann|url=http://cs.anu.edu.au/~bdm/plantri/|title=A useful planar graph generator}}.
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| *{{citation|first1=H.|last1=de Fraysseix|first2=P.|last2=Ossona de Mendez|first3=P.|last3=Rosenstiehl|year=2006|title=Trémaux trees and planarity|journal=International Journal of Foundations of Computer Science|volume=17|issue=5|pages=1017–1029|doi=10.1142/S0129054106004248}}. Special Issue on Graph Drawing.
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| * [[David A. Bader|D.A. Bader]] and S. Sreshta, [http://www.cc.gatech.edu/~bader/papers/planarity2003.html A New Parallel Algorithm for Planarity Testing], UNM-ECE Technical Report 03-002, October 1, 2003.
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| *{{citation|first=Steve|last=Fisk|year=1978|title=A short proof of Chvátal's watchman theorem|journal=J. Comb. Theory, Ser. B|volume=24|pages=374|doi=10.1016/0095-8956(78)90059-X|issue=3}}.
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| ==External links==
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| {{commons category|Planar graphs}}
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| *[http://jgaa.info/accepted/2004/BoyerMyrvold2004.8.3/planarity.zip Edge Addition Planarity Algorithm Source Code, version 1.0] — Free C source code for reference implementation of Boyer–Myrvold planarity algorithm, which provides both a combinatorial planar embedder and Kuratowski subgraph isolator. An open source project with free licensing provides the [http://code.google.com/p/planarity/ Edge Addition Planarity Algorithms, current version].
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| *[http://pigale.sourceforge.net Public Implementation of a Graph Algorithm Library and Editor] — GPL graph algorithm library including planarity testing, planarity embedder and Kuratowski subgraph exhibition in linear time.
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| *[http://www.boost.org/doc/libs/1_40_0/libs/graph/doc/planar_graphs.html Boost Graph Library tools for planar graphs], including linear time planarity testing, embedding, Kuratowski subgraph isolation, and straight-line drawing.
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| *[http://www.cut-the-knot.org/do_you_know/3Utilities.shtml 3 Utilities Puzzle and Planar Graphs]
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| *[http://ccl.northwestern.edu/netlogo/models/Planarity NetLogo Planarity model] — NetLogo version of John Tantalo's game
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| [[Category:Planar graphs| ]]
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| [[Category:Graph families]]
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| [[Category:Intersection classes of graphs]]
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