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An author's Erdös number is 1 if he has co-authored a paper with Erdös, 2 if he has co-authored a paper with someone who has co-authored a paper with Erdös, etc.
References
Grossman, J. and Ion, P.  ``The Erdös Number Project.''  
  http://www.acs.oakland.edu/~grossman/erdoshp.html.