http://en.wikipedia.org/wiki/Erd%C5%91s%E2%80%93Bacon_number

A person's Erdős–Bacon number is the sum of one's Erdős number—which measures the "collaborative distance" in authoring mathematical papers between that person and Hungarian mathematician Paul Erdős—and one's Bacon number—which represents the number of links, through roles in films, by which the individual is separated from American actor Kevin Bacon. The lower the number, the closer a person is to Erdős and Bacon, and this reflects a small world phenomenon in academia and entertainment.

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Notable scientists with defined Erdős–Bacon numbers include popular astronomer Carl Sagan. One of the best-known actors with a number is actress Natalie Portman, whose authorship of a psychology paper during her Harvard degree in psychology earned her an Erdős–Bacon number of 6 (see table below).