Paul Erdös (1913-1996) is one of the most prolific mathmeticians. He wrote about 1500 papers in his lifetime. He collaborated with over 500 people.
As a tribute, his friends created the Erdös number.
Erdös himself has an Erdös number of 0. A co-author with Erdös has an Erdös number of 1, if you have written a paper with one of those co-authors you have an Erdös number of 2.
My Erdös number is 3 (unless anybody knows differently), via these papers.
Nathan Linial has an Erdös number of 1
Amotz Bar-noy has an Erdös number of 2
Graham Kendall has an Erdös number of 3
If you are interested in finding out what your Erdös number is, I found two very good web sites: