193707721

This number is a prime.

+ Lucas demonstrated that 2^67 - 1 (which Mersenne had claimed to be prime) was composite in 1876; but he did so without finding its factors. In a 1903 meeting of the American Mathematical Society, F. N. Cole (1861-1926) silently walked to the board and calculated 2^67 - 1. On another board he multiplied the primes 193707721 and 761838257287, and still in silence, sat down to a standing ovation.

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