# Graph theory

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Graph theory

In mathematics and computer science, graph theory is the study of "graphs": mathematical structures used to model pairwise relations between objects from a certain collection. A "graph" in this context refers to a collection of vertices or 'nodes' and a collection of "edges" that connect pairs of vertices. A graph may be "undirected", meaning that there is no distinction between the two vertices associated with each edge, or its edges may be "directed" from one vertex to another; see graph (mathematics) for more detailed definitions and for other variations in the types of graphs that are commonly considered. The graphs studied in graph theory should not be confused with "graphs of functions" and other kinds of graphs.

Please refer to Glossary of graph theory for some basic definitions in graph theory.

History

The paper written by Leonhard Euler on the "Seven Bridges of Königsberg" and published in 1736 is regarded as the first paper in the history of graph theory.cite book|author=Biggs, N.; Lloyd, E. and Wilson, R.|title=Graph Theory, 1736-1936|publisher=Oxford University Press|year=1986] This paper, as well as the one written by Vandermonde on the "knight problem," carried on with the "analysis situs" initiated by Leibniz. Euler's formula relating the number of edges, vertices, and faces of a convex polyhedron was studied and generalized by Cauchycite journal|author=Cauchy, A.L.|year=1813|title=Recherche sur les polyèdres - premier mémoire|journal=Journal de l'Ecole Polytechnique|volume= 9 (Cahier 16)|pages=66–86] and L'Huillier,cite journal|author=L'Huillier, S.-A.-J.|title=Mémoire sur la polyèdrométrie|journal=Annales de Mathématiques|volume=3|year=1861|pages=169–189] and is at the origin of topology.

More than one century after Euler's paper on the bridges of Königsberg and while Listing introduced topology, Cayley was led by the study of particular analytical forms arising from differential calculus to study a particular class of graphs, the "trees". This study had many implications in theoretical chemistry. The involved techniques mainly concerned the enumeration of graphs having particular properties. Enumerative graph theory then rose from the results of Cayley and the fundamental results published by Pólya between 1935 and 1937 and the generalization of these by De Bruijn in 1959. Cayley linked his results on trees with the contemporary studies of chemical composition.cite journal|author=Cayley, A.|year=1875|journal=Berichte der deutschen Chemischen Gesellschaft|title=Ueber die Analytischen Figuren, welche in der Mathematik Bäume genannt werden und ihre Anwendung auf die Theorie chemischer Verbindungen|volume=8|pages=1056–1059|doi=10.1002/cber.18750080252] The fusion of the ideas coming from mathematics with those coming from chemistry is at the origin of a part of the standard terminology of graph theory. In particular, the term "graph" was introduced by Sylvester in a paper published in 1878 in "Nature".cite journal|author=Sylvester, J.J.|year=1878|journal=Nature|title=Chemistry and Algebra|volume=17|pages=284|doi=10.1038/017284a0]

One of the most famous and productive problems of graph theory is the four color problem: "Is it true that any map drawn in the plane may have its regions colored with four colors, in such a way that any two regions having a common border have different colors?". This problem remained unsolved for more than a century and the proof was given by Kenneth Appel and Wolfgang Haken in 1976cite journal|author=Appel, K. and Haken, W.|title=Every planar map is four colorable. Part I. Discharging|journal=Illinois J. Math.|volume=21|year=1977|pages=429–490] cite journal|author=Appel, K. and Haken, W.|title=Every planar map is four colorable. Part II. Reducibility|journal=Illinois J. Math.|volume=21|year=1977|pages=491–567] (determination of 1936 types of configurations of which study is sufficient and checking of the properties of these configurations by computer) did not convince all the community. A simpler proof considering far fewer configurations was given twenty years later by Robertson, Seymour, Sanders and Thomas.cite journal|author=Robertson, N.; Sanders, D.; Seymour, P. and Thomas, R.|title=The four color theorem|journal=Journal of Combinatorial Theory Series B|volume=70|year=1997|pages=2–44|doi=10.1006/jctb.1997.1750]

This problem was first posed by Francis Guthrie in 1852 and the first written record of this problem is a letter of De Morgan addressed to Hamilton the same year. Many incorrect proofs have been proposed, including those by Cayley, Kempe, and others.The study and the generalization of this problem by Tait, Heawood, Ramsey and Hadwiger has in particular led to the study of the colorings of the graphs embedded on surfaces with arbitrary genus. Tait's reformulation generated a new class of problems, the "factorization problems", particularly studied by Petersen and Kőnig. The works of Ramsey on colorations and more specially the results obtained by Turán in 1941 is at the origin of another branch of graph theory, the "extremal graph theory".

The autonomous development of topology from 1860 and 1930 fertilized graph theory back through the works of Jordan, Kuratowski and Whitney. Another important factor of common development of graph theory and topology came from the use of the techniques of modern algebra. The first example of such a use comes from the work of the physicist Gustav Kirchhoff, who published in 1845 his Kirchhoff's circuit laws for calculating the voltage and current in electric circuits.

The introduction of probabilistic methods in graph theory, especially in the study of Erdős and Rényi of the asymptotic probability of graph connectivity, gave rise to yet another branch, known as "random graph theory", which has been a fruitful source of graph-theoretic results.

Drawing graphs

Graphs are represented graphically by drawing a dot for every vertex, and drawing an arc between two vertices if they are connected by an edge. If the graph is directed, the direction is indicated by drawing an arrow.

A graph drawing should not be confused with the graph itself (the abstract, non-graphical structure) as there are several ways to structure the graph drawing. All that matters is which vertices are connected to which others by how many edges and not the exact layout. In practice it is often difficult to decide if two drawings represent the same graph. Depending on the problem domain some layouts may be better suited and easier to understand than others.

Graph-theoretic data structures

There are different ways to store graphs in a computer system. The data structure used depends on both the graph structure and the algorithm used for manipulating the graph. Theoretically one can distinguish between list and matrix structures but in concrete applications the best structure is often a combination of both. List structures are often preferred for sparse graphs as they have smaller memory requirements. Matrix structures on the other hand provide faster access for some applications but can consume huge amounts of memory .

List structures

; Incidence list : The edges are represented by an array containing pairs (ordered if directed) of vertices (that the edge connects) and possibly weight and other data. Vertices connected by an edge are said to be "adjacent".; Adjacency list : Much like the incidence list, each vertex has a list of which vertices it is adjacent to. This causes redundancy in an undirected graph: for example, if vertices A and B are adjacent, A's adjacency list contains B, while B's list contains A. Adjacency queries are faster, at the cost of extra storage space.

Matrix structures

; Incidence matrix : The graph is represented by a matrix of size |"V"| (number of vertices) by |"E"| (number of edges) where the entry [vertex, edge] contains the edge's endpoint data (simplest case: 1 - connected, 0 - not connected).; Adjacency matrix : This is the "n" by "n" matrix "A", where "n" is the number of vertices in the graph. If there is an edge from some vertex "x" to some vertex "y", then the element $a_\left\{x, y\right\}$ is 1 (or in general the number of "xy" edges), otherwise it is 0. In computing, this matrix makes it easy to find subgraphs, and to reverse a directed graph.; Laplacian matrix or Kirchhoff matrix or Admittance matrix : This is defined as "D" − "A", where "D" is the diagonal degree matrix. It explicitly contains both adjacency information and degree information.; Distance matrix : A symmetric "n" by "n" matrix "D" whose element $d_\left\{x, y\right\}$ is the length of a shortest path between "x" and "y"; if there is no such path $d_\left\{x, y\right\}$ = infinity. It can be derived from powers of "A": $d_\left\{x,y\right\}=min\left\{nmid A^n \left[x,y\right] e 0\right\}.$

Problems in graph theory

Enumeration

There is a large literature on graphical enumeration: the problem of counting graphs meeting specified conditions. Some of this work is found in Harary and Palmer (1973).

ubgraphs, induced subgraphs, and minors

A common problem, called the subgraph isomorphism problem, is finding a fixed graph as a subgraph in a given graph. One reason to be interested in such a question is that many graph properties are "hereditary" for subgraphs, which means that a graph has the property if and only if all subgraphs, or all induced subgraphs, have it too. Unfortunately, finding maximal subgraphs of a certain kind is often an NP-complete problem.

* Finding the largest complete graph is called the clique problem (NP-complete).

A similar problem is finding induced subgraphs in a given graph. Again, some important graph properties are hereditary with respect to induced subgraphs, which means that a graph has a property if and only if all induced subgraphs also have it. Finding maximal induced subgraphs of a certain kind is also often NP-complete. For example,

* Finding the largest edgeless induced subgraph, or independent set, called the independent set problem (NP-complete).

Still another such problem, the "minor containment problem", is to find a fixed graph as a minor of a given graph. A minor or subcontraction of a graph is any graph obtained by taking a subgraph and contracting some (or no) edges. Many graph properties are hereditary for minors, which means that a graph has a property if and only if all minors have it too. A famous example:

* A graph is planar if it contains as a minor neither the complete bipartite graph $K_\left\{3,3\right\}$ (See the Three-cottage problem) nor the complete graph $K_\left\{5\right\}$.

Another class of problems has to do with the extent to which various species and generalizations of graphs are determined by their "point-deleted subgraphs", for example:

* The reconstruction conjecture

Graph coloring

Many problems have to do with various ways of coloring graphs, for example:

* The four-color theorem
* The strong perfect graph theorem
* The Erdős–Faber–Lovász conjecture (unsolved)
* The total coloring conjecture (unsolved)
* The list coloring conjecture (unsolved)

Route problems

* Hamiltonian path and cycle problems
* Minimum spanning tree
* Route inspection problem (also called the "Chinese Postman Problem")
* Seven Bridges of Königsberg
* Shortest path problem
* Steiner tree
* Three-cottage problem
* Traveling salesman problem (NP-complete)

Network flow

There are numerous problems arising especially from applications that have to do with various notions of flows in networks, for example:

* Max flow min cut theorem

Visibility graph problems

* Museum guard problem

Covering problems

Covering problems are specific instances of subgraph-finding problems, and they tend to be closely related to the clique problem or the independent set problem.

* Set cover problem
* Vertex cover problem

Applications

Applications of graph theory are primarily, but not exclusively, concerned with labeled graphs and various specializations of these.

Structures that can be represented as graphs are ubiquitous, and many problems of practical interest can be represented by graphs. The link structure of a website could be represented by a directed graph: the vertices are the web pages available at the website and a directed edge from page "A" to page "B" exists if and only if "A" contains a link to "B". A similar approach can be taken to problems in travel, biology, computer chip design, and many other fields. The development of algorithms to handle graphs is therefore of major interest in computer science. There, the transformation of graphs is often formalized and represented by graph rewrite systems. They are either directly used or properties of the rewrite systems(e.g. confluence) are studied.

A graph structure can be extended by assigning a weight to each edge of the graph. Graphs with weights, or weighted graphs, are used to represent structures in which pairwise connections have some numerical values. For example if a graph represents a road network, the weights could represent the length of each road. A digraph with weighted edges in the context of graph theory is called a network.

Networks have many uses in the practical side of graph theory, network analysis (for example, to model and analyze traffic networks). Within network analysis, the definition of the term "network" varies, and may often refer to a simple graph.

Many applications of graph theory exist in the form of network analysis. These split broadly into three categories. Firstly, analysis to determine structural properties of a network, such as the distribution of vertex degrees and the diameter of the graph. A vast number of graph measures exist, and the production of useful ones for various domains remains an active area of research. Secondly, analysis to find a measurable quantity within the network, for example, for a transportation network, the level of vehicular flow within any portion of it. Thirdly, analysis of dynamical properties of networks.

Graph theory is also used to study molecules in chemistry and physics. In condensed matter physics, the three dimensional structure of complicated simulated atomic structures can be studied quantitatively by gathering statistics on graph-theoretic properties related to the topology of the atoms. For example, Franzblau's shortest-path (SP) rings. In chemistry a graph makes a natural model for a molecule, where vertices represent atoms and edges bonds. This approach is especially used in computer processing of molecular structures, ranging from chemical editors to database searching.

Graph theory is also widely used in sociology as a way, for example, to measure actors' prestige or to explore diffusion mechanisms, notably through the use of social network analysis software.

References

* Berge, Claude, "Théorie des graphes et ses applications". Collection Universitaire de Mathématiques, II Dunod, Paris 1958, viii+277 pp. (English edition, Wiley 1961; Methuen & Co, New York 1962; Russian, Moscow 1961; Spanish, Mexico 1962; Roumanian, Bucharest 1969; Chinese, Shanghai 1963; Second printing of the 1962 first English edition. Dover, New York 2001)
* Citation
last=Pelle
first=St&eacute;phane
year=1996
title=La Th&eacute;orie des Graphes
place=Saint-Mand&eacute;
publisher=&Eacute;cole Nationale des Sciences G&eacute;ographiques
url=http://www.ensg.ign.fr/~spelle/TheorieGraphes.pdf

* Chartrand, Gary, "Introductory Graph Theory", Dover. ISBN 0-486-24775-9.
* Biggs, N.; Lloyd, E. & Wilson, R. Graph Theory, 1736-1936 Oxford University Press, 1986
* Harary, Frank, and Palmer, Edgar M., "Graphical Enumeration" (1973), Academic Press, New York, NY.

ee also

* Gallery of named graphs
* Glossary of graph theory
* List of graph theory topics
* Publications in graph theory

Related topics

* Algebraic graph theory
* Conceptual graph
* Data structure
* Disjoint-set data structure
* Entitative graph
* Existential graph
* Graph data structure
* Graph coloring
* Graph drawing
* Graph rewriting
* Logical graph
* Loop
* Null graph
* Quantum graph
* Spectral graph theory
* Strongly regular graphs
* Tree data structure

Algorithms

* Bellman-Ford algorithm
* Dijkstra's algorithm
* Ford-Fulkerson algorithm
* Kruskal's algorithm
* Nearest neighbour algorithm
* Prim's algorithm
* Depth-first search

ubareas

* Algebraic graph theory
* Geometric graph theory
* Extremal graph theory
* Probabilistic graph theory
* Topological graph theory

Related areas of mathematics

* Combinatorics
* Group theory
* Knot theory
* Ramsey theory

Generalizations

* Hypergraph
* Abstract simplicial complex

Prominent graph theorists

Notes

Online textbooks

* [http://www.ecp6.jussieu.fr/pageperso/bondy/books/gtwa/gtwa.html Graph Theory with Applications] (1976) by Bondy and Murty
* [http://www.britannica.com/eb/article-9037754/graph-theory Encyclopaedia Britannica, Graph Theory]
* [http://arxiv.org/pdf/cond-mat/0602129 Phase Transitions in Combinatorial Optimization Problems, Section 3: Introduction to Graphs] (2006) by Hartmann and Weigt
* [http://www.cs.auckland.ac.nz/~ute/220ft/graphalg/graphalg.html An Introduction to Graph Algorithms] 1999 by Waltraut Ute Lorch based on Dr Michael Dinneen's lecture notes
* [http://www.cs.rhul.ac.uk/books/dbook/ Chapters 1,2,4,5,7,10 and 12 of Digraphs: Theory Algorithms and Applications] 2001 by Jorgen Bang-Jensen and Gregory Gutin
* [http://www.math.uni-hamburg.de/home/diestel/books/graph.theory/GraphTheoryIII.pdf Graph Theory, by Reinhard Diestel]

Other resources

* More people and publications at: " [http://www.math.gatech.edu/~sanders/graphtheory/ Graph Theory Resources] "
* [http://www.utm.edu/departments/math/graph/ Graph theory tutorial]
* [http://web.archive.org/web/20060206155001/http://www.nd.edu/~networks/gallery.htm Image gallery: graphs]
* GraphViz open source software to produce graph images from a description of the graph
* [http://graphexploration.cond.org/] GUESS Graph Exploration System( Open Source GPL )
* [http://www.jgrapht.org JGraphT] an open source Java graph theory library
* [http://www.boost.org/libs/graph/doc/index.html Boost] an open source C++ graph theory library
*
* [http://orgnet.com/SocialLifeOfRouters.pdf] Graph theory applied to computer/social networks
* [http://www.babelgraph.org/links.html Concise, annotated list of graph theory resources for researchers]
* [http://gtad.sourceforge.net/ GTAD (Graph Toolkit for Algorithms and Drawings)] C++ graph library

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