Erdős number.

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Interesting thing discussed in comment to this post. About Erdős number.

From Wikipedia: Paul Erdős published more papers during his life (at least 1400) than any other mathematician in history.

To be assigned an Erdős number, an author must co-write a mathematical paper with an author with a finite Erdős number:

• Paul Erdős is the one person having an Erdős number of zero.
• For any author other than Erdős, if the lowest Erdős number of all of his coauthors is k, then the author's Erdős number is k + 1.

More info:

• Erdős number on Wikipedia
• List of people by Erdős number on Wikipedia
• The Erdös Number Project, which studies research collaboration among mathematicians