The 24th mersenne prime.
The 24th Mersenne prime M(p) = 2(p) - 1, and currently the largest known prime, is 2(19937) - 1. Primality was shown by the Lucas-Lehmer test on an IBM 360/91 computer. The 24th even perfect number is (2(19937) - 1).2(19936).
Tuckerman
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IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598.