756839

This number is a prime.

2^756839-1 is the largest Mersenne prime with distinct digits in the exponent. [Dobb]

The second smallest and largest known Mersenne prime exponent which exceeds the sum of all smaller ones. The only other known number with this property is 521. [Terr]

The Mersenne prime with exponent 756839 is the smaller of four consecutive Mersenne primes that exist between consecutive perfect numbers. This is the one and only (not to mention the largest) known case of 4. Observation by Metin Sariyar of Turkey.