http://en.m.wikipedia.org/wiki/Paul_Erd%C5%91s
What is Erdős Number ?
Amazon Book: “My Brain Is Open”
Paul Erdős (er-dish) was one of the greatest mathematicians in 21st Century, a Jewish Hungarian, single and no home. He traveled around the world in one small suitcase containing his mathematical papers. He would knock at the door of his former students or math collaborator impromptu, started working and published the papers, then moved on to next destination the moment his overstay became unwelcome by the host’s wife. In this way he published 1,500 articles in his lifetime.
Example:
Einstein had the Erdős Number 2 , through his assistant who co-wrote a paper with Paul Erdős.
https://www.youtube.com/watch?v=5iflQseSSfA&feature=youtube_gdata_player
Ref:
Mathtuition88 