Three of my collaborators have Erdõs number one: A. Kroó, J. Szabados and my advisor V. Totik.
The Erdõs number, honoring the late Hungarian mathematician Paul Erdõs, one of the most prolific writers of mathematical papers, is a way of describing the "collaborative distance", in regard to mathematical papers, between an author and Erdõs. Erdõs has written around 1500 mathematical articles. He had 509 direct collaborators; these are the people with Erdõs number 1. The people who have collaborated with them (but not with Erdõs himself) have an Erdõs number of 2, and so forth.
It is also customary to define the Erdõs number as the real number E-1+1/m if someone has original Erdõs number E with multiplicity m. Having 4 joint papers with Erdõs would give an Erdõs number 0.25! :)
For more information visit the Erdõs Number Project website.