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There is a prime number that is one less than a power of two

The prime number 2^127 - 1 is a Mersenne prime, a type of prime number that is one less than a power of two. Discovered in 1947 by Raphael M. Robinson, this prime has 39 digits and is the largest known Mersenne prime. Its enormity is due to the fact that 2^127 is the largest power of two that has not been tested for primality, and its subtraction by one yields a prime number.