# Complex network

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In the context of network theory, a **complex network** is a network (graph) with non-trivial topological features—features that do not occur in simple networks such as lattices or random graphs. The study of complex networks is a young and active area of scientific research inspired largely by the empirical study of real-world networks such as computer networks and social networks.

## Contents |

## [edit] Definition

Most social, biological, and technological networks display substantial non-trivial topological features, with patterns of connection between their elements that are neither purely regular nor purely random. Such features include a heavy tail in the degree distribution, a high clustering coefficient, assortativity or disassortativity among vertices, community structure, and hierarchical structure. In the case of directed networks these features also include reciprocity, triad significance profile and other features. In contrast, many of the mathematical models of networks that have been studied in the past, such as lattices and random graphs, do not show these features.

Two well-known and much studied classes of complex networks are scale-free networks and small-world networks, whose discovery and definition are canonical case-studies in the field. Both are characterized by specific structural features—power-law degree distributions for the former and short path lengths and high clustering for the latter. However, as the study of complex networks has continued to grow in importance and popularity, many other aspects of network structure have attracted attention as well.

The field continues to develop at a brisk pace, and has brought together researchers from many areas including mathematics, physics, biology, computer science, sociology, epidemiology, and others. Ideas from network science have been applied to the analysis of metabolic and genetic regulatory networks, the design of robust and scalable communication networks both wired and wireless, the development of vaccination strategies for the control of disease, and a broad range of other practical issues. Research on networks has seen regular publication in some of the most visible scientific journals and vigorous funding in many countries, has been the topic of conferences in a variety of different fields, and has been the subject of numerous books both for the lay person and for the expert.

## [edit] Scale-free networks

A network is named scale-free if its degree distribution, i.e., the probability that a node selected uniformly at random has a certain number of links (degree), follows a particular mathematical function called a power law. The power law implies that the degree distribution of these networks has no characteristic scale. In contrast, network with a single well-defined scale are somewhat similar to a lattice in that every node has (roughly) the same degree. Examples of networks with a single scale include the Erdős–Rényi random graph and hypercubes. In a network with a scale-free degree distribution, some vertices have a degree that is orders of magnitude larger than the average - these vertices are often called "hubs", although this is a bit misleading as there is no inherent threshold above which a node can be viewed as a hub. If there were, then it wouldn't be a scale-free distribution!

Interest in scale-free networks began in the late 1990s with the apparent discovery of a power-law degree distribution in many real world networks such as the World Wide Web, the network of Autonomous systems (ASs), some network of Internet routers, protein interaction networks, email networks, etc. Although many of these distributions are not unambiguously power laws, their breadth, both in degree and in domain, shows that networks exhibiting such a distribution are clearly very different from what you would expect if edges existed independently and at random (a Poisson distribution). Indeed, there are many different ways to build a network with a power-law degree distribution. The Yule process is a canonical generative process for power laws, and has been known since 1925. However, it is known by many other names due to its frequent reinvention, e.g., The Gibrat principle by Herbert Simon, the Matthew effect, cumulative advantage and, most recently, preferential attachment by Barabási and Albert for power-law degree distributions.

Networks with a power-law degree distribution can be highly resistant to the random deletion of vertices, i.e., the vast majority of vertices remain connected together in a giant component. Such networks can also be quite sensitive to targeted attacks aimed at fracturing the network quickly. When the graph is uniformly random except for the degree distribution, these critical vertices are the ones with the highest degree, and have thus been implicated in the spread of disease (natural and artificial) in social and communication networks, and in the spread of fads (both of which are modeled by a percolation or branching process).

## [edit] Small-world networks

A network is called a small-world network by analogy with the small-world phenomenon (popularly known as six degrees of separation). The small world hypothesis, which was first described by the Hungarian writer Frigyes Karinthy in 1929, and tested experimentally by Stanley Milgram (1967), is the idea that two arbitrary people are connected by only six degrees of separation, i.e. the diameter of the corresponding graph of social connections is not much larger than six. In 1998, Duncan J. Watts and Steven Strogatz published the first small-world network model, which through a single parameter smoothly interpolates between a random graph to a lattice. Their model demonstrated that with the addition of only a small number of long-range links, a regular graph, in which the diameter is proportional to the size of the network, can be transformed into a "small world" in which the average number of edges between any two vertices is very small (mathematically, it should grow as the logarithm of the size of the network), while the clustering coefficient stays large. It is known that a wide variety of abstract graphs exhibit the small-world property, e.g., random graphs and scale-free networks. Further, real world networks such as the World Wide Web and the metabolic network also exhibit this property.

In the scientific literature on networks, there is some ambiguity associated with the term "small world." In addition to referring to the size of the diameter of the network, it can also refer to the co-occurrence of a small diameter and a high clustering coefficient. The clustering coefficient is a metric that represents the density of triangles in the network. For instance, sparse random graphs have a vanishingly small clustering coefficient while real world networks often have a coefficient significantly larger. Scientists point to this difference as suggesting that edges are correlated in real world networks.

## [edit] Researchers and scientists

(with papers on **complex networks** cited at least 100 times)

- Luis Amaral
- Alex Arenas
- Albert-László Barabási
- Alain Barrat
- Marc Barthelemy
- Stefano Boccaletti
- Guido Caldarelli
- Jon Kleinberg
- José Mendes
- Adilson E. Motter
- Mark Newman
- Sidney Redner
- Steven Strogatz
- Alessandro Vespignani
- Duncan J. Watts

## [edit] Books

- Albert-László Barabási,
*Linked: How Everything is Connected to Everything Else*, 2004, ISBN 0-452-28439-2 - Alain Barrat, Marc Barthelemy, Alessandro Vespignani,
*Dynamical processes in complex networks*, Cambridge University Press, 2008, ISBN 978-0-521-87950-7 - Stefan Bornholdt (Editor) and Heinz Georg Schuster (Editor),
*Handbook of Graphs and Networks: From the Genome to the Internet*, 2003, ISBN 3-527-40336-1 - Guido Caldarelli,
*Scale-Free Networks*Oxford University Press, 2007, ISBN 0-19-921151-7 - S.N. Dorogovtsev and J.F.F. Mendes,
*Evolution of Networks: From biological networks to the Internet and WWW*, Oxford University Press, 2003, ISBN 0-19-851590-1 - M. E. J. Newman, A.-L. Barabási, and D. J. Watts,
*The Structure and Dynamics of Networks*, Princeton University Press, Princeton, 2006, ISBN 978-0-691-11357-9 - R. Pastor-Satorras and A. Vespignani, "Evolution and Structure of the Internet: A statistical physics approach", Cambridge University Press, 2004, ISBN 0-521-82698-5
- Duncan J. Watts,
*Six Degrees: The Science of a Connected Age*, W. W. Norton & Company, 2003, ISBN 0-393-04142-5 - Duncan J. Watts,
*Small Worlds: The Dynamics of Networks between Order and Randomness*, Princeton University Press, 2003, ISBN 0-691-11704-7

## [edit] References

- R. Albert and A.-L. Barabási, "Statistical mechanics of complex networks" Reviews of Modern Physics 74, (2002) 47
- S. N. Dorogovtsev, A. V. Goltsev, and J. F. F. Mendes,
*Critical phenomena in complex networks*, Rev. Mod. Phys. 80, 1275, (2008) - M. E. J. Newman The structure and function of complex networks (Review article)
- A. Barabasi and E. Bonabeau,
*Scale-Free Networks*, Scientific American, (May 2003), 50-59 - S. H. Strogatz,
*Exploring Complex Networks*, Nature Vol 410 (2001) 268-276 - D. J. Watts and S. H. Strogatz.,
*Collective dynamics of 'small-world' networks*, Nature Vol 393 (1998) 440-442 - S. N. Dorogovtsev and J.F.F. Mendes,
*Evolution of Networks*, Adv. Phys. 51, 1079 (2002) - S. Boccaletti et al.,
*Complex Networks: Structure and Dynamics*, Phys. Rep., 424 (2006), 175-308.

## [edit] External links

This article's external links may not follow Wikipedia's content policies or guidelines. Please improve this article by removing excessive or inappropriate external links. |

- complexnetworks.fr — French research group on Complex Networks
- Network Science — United States Military Academy - Network Science Center
- Resources in Complex Networks — University of São Paulo - Institute of Physics at São Carlos
- Cx-Nets — Complex Networks Collaboratory
- Networks Wiki
- GNET — Group of Complex Systems & Random Networks
- UCLA Human Complex Systems Program
- New England Complex Systems Institute
- Santa Fe Institute Networks Group
- Barabasi Networks Group
- Scale-Free and Small-World Networks
- Cosin Project Codes, Papers and Data on Complex Networks
- Complex network on arxiv.org
- [1] — Anna Nagurney's Virtual Center for Supernetworks
- BIOREL resource for quantitative estimation of the network bias in relation to external information
- Complexity Virtual Laboratory (VLAB)