Website for the Erdős Number Project, which studies research collaboration among mathematicians. Most practicing mathematicians are familiar with the definition of one’s Erdös number. Paul Erdös (1913–1996), the widely-traveled and incredibly prolific Hungarian mathematician of the highest caliber, wrote hundreds of mathematical research papers in many different areas, many in collaboration with others. His Erdös number is 0. Erdös’s coauthors have Erdös number 1. People other than Erdös who have written a joint paper with someone with Erdös number 1 but not with Erdös have Erdös number 2, and so on. If there is no chain of coauthorships connecting someone with Erdös, then that person’s Erdös number is said to be infinite.