Double Mersenne Primes

 A  Double Mersenne Prime  is a Mersenne Prime of the form 2 p  - 1 for which p is also a  Mersenne Prime. There are currently only 4 known double Mersenne Primes:  7 ,  127, 2147483647, and 170141183460469231731687303715884105727. It is conjectured that these are the only double Mersenne Primes in existence.