The Erdos Number
Paul Erdos was an eccentric mathematician, and known for his many collaborations in the field. He placed little value on possessions, and so he lived his life traveling between homes of colleagues throughout the world. For only a few days at a time, he would stay to collaborate on a paper before moving on to the next doorstep. Erdos resultingly became one of the most prolific contributors to mathematics, producing around 1,500 mathematical articles and having 511 different collaborators in his lifetime. In tribute to his output, Erdos’ friends created the Erdos number, the degree of separation of collaborations with Erdos.
To illustrate, a person who is a co-author of a research paper with Erdos would have an Erdos number of 1. Someone who colloborates with this person (and not with Erdos directly) would then have an Erdos number of 2, and so on. In terms of networks, each person can be represented as a node, and each edge connects them with the co-authors of all the research papers that they wrote. Their Erdos number would therefore be the number of edges that leads to Erdos in a path. In forming this network, Erdos’ influence across many fields become apparent. 56 Field Medal winners have an Erdos number of 5 or less. About 130 Nobel Laureates in medicine, chemistry, physics and economics have an Erdos number of 8 or less. Therefore, much of today’s advancements in math and science can be linked back to the contributions of Paul Erdos.
http://content.time.com/time/magazine/article/0,9171,990598,00.html
http://www.oakland.edu/enp/erdpaths/