The Big Number Duel was a large number-naming contest between associate professors Agustín Rayo of MIT and Adam N. Elga of Princeton University.
Elga went first, writing the number 1 on the board. Rayo wrote a string of ones on the board, and Elga turned a half of the ones into factorials. After that, they began to create their own notations.
Rayo won the contest; his winning entry is known as Rayo's number.