Paul Erdős
was one of the most prolific mathematicians of all time.
He authored or coauthored around 1500 articles and books.
The
Erdős Number
measures the distance of a
given mathematician from Erdős on a graph whose edges
denote the relationship of coauthorship.
My Erdős number is 2,
because I've coauthored a paper with
John Gimbel,
who in turn has coauthored a paper with Paul Erdős
(and so has Erdős number 1):

Erdős, Paul;
Gimbel, John.
Some problems and results in cochromatic theory.
In:
Quo vadis, graph theory?,
Ann. Discrete Math. 55 (1993) 261264.

de Figueiredo, Celina M. H.;
Gimbel, John;
Mello, Célia P.;
Szwarcfiter, Jayme L.
Sources and sinks in comparability graphs.
Order 14 #1 (1997) 7583.
A different proof is given by a more recent paper coauthored with
Yoshiharu Kohayakawa,
who also has Erdős number 1:

Erdős, Paul;
Gyárfás, András;
Kohayakawa, Yoshiharu.
The size of the largest bipartite subgraphs.
Discrete Math. 177 #13 (1997) 267271.

de Figueiredo, Celina M. H.;
Klein, Sulamita;
Kohayakawa, Yoshiharu;
Reed, Bruce.
Finding skew partitions efficiently.
Journal of Algorithms 37 (2000) 505521.
See the
MathSciNet data
on my Erdős number.