Networks in The Real World

Complex network 研究之所以被發掘而且迅速地廣為被研究，最重要的時間點在 1998 年，Watts D. J. and Strogatz S. H. 在 Nature 雜誌上發表一篇名為 "Collective dynamics of 'small-world' networks" (Nature 393, 440-442)，該篇論文著重於分析複雜網路的特性，並嘗試將它以數學模型來解釋。

2003 年，Mark Newman 進一步將真實世界的 Complex network 歸納為四大類：
(M. E. J. Newman, The structure and function of complex networks, SIAM Review 2003)

1. Social Network
2. Information Network
3. Technological Network
4. Biological Network

[Social Network]
以人，或者由人所組成團體，為基本單位，藉由特定基本單位間各種互動關係建立彼此的連結，所形成的網路，稱之。典型的幾個例子：film actors, co-authorship, sexual contacts, email messages, blogosphere。Social Network 最為著名的體現是 Stanley Milgram 的 small-world experiments，得出世界上任何兩個人之間，至多存在五個人，便可建立關連，即"六度分離" (Six degree of separation)。在 Stanley Milgram 的實驗後，科學家發現傳統 social network analysis 的 sample size 都很小，加上資料來源可能受到人為主觀意識的影響，鑒於此，又時逢 WWW 迅速發展，因此資料來源轉為 affiliation network，也就是透過眾人的力量所組織成的 knowledge base。舉兩個social network的例子來說明：

Science coauthorship (Node: author; Link: write paper together)



Blogosphere (Node: blog (color means host); Link: (a-)reciprocal link (thickness)



[Information Network]
Also called "Knowledge Network"，典型的幾個例子為：citation network, WWW, word co-occurrence, preference network (bipartite graph)

World Wide Web (Node: webpage; Link: hyperlink)



[Technological Network]
Man-made networks designed for distribution of some commodities or resources. 典型的幾個例子為：Internet, power grid, train routes, P2P network, electronic circuits

Power grid (Node: generator, substations; Link: high-voltage power transmission lines [Note: Line thickness and color indicate the voltage level])



Internet (Node: computer, router; Link: physical lines)



[Biological Network]
Biological System，典型的幾個例子為：metabolic network, protein interactions, food web, neural network

Food web (Node: trophic species; Link: trophic interactions)



metabolic network, protein interactions