"The mathematician Paul Erdős wrote around 1500 mathematical articles in his lifetime, mostly co-written. He had 509 direct collaborators; these are the people with Erdős number 1. The people who have collaborated with them (but not with Erdős himself) have an Erdős number of 2 (6,984 people). " And so on.
My Erdős number is 4 as I have coauthored with Krzysztof Burdzy and Ady Pauzner, each of whom has an Erdős number of 3.
Which just goes to show that the world is really small in some ways.
- David Frankel