The Erdős number is the number of "hops" needed to connect the author of a paper with the prolific late mathematician Paul Erdős. An author's Erdős number is 1 if he has co-authored a paper with Erdős, 2 if he has co-authored a paper with someone who has co-authored a paper with Erdős, etc.. Erdős wrote papers with a total of 509 coauthors, meaning 509 people have Erdős number 1.

Erdős-Borwein constant | small world network