Mersenne primes (MPs)

We know only 51 Mersenne primes (MPs), primes of the form $2ᵖ − 1$, including the largest known prime $2⁸²⁵⁸⁹⁹³³ − 1$ ($24,862,048$ digits long). 

A double MP is a prime of the form $2ᵖ − 1$ where p is also a MP. 

$2¹²⁷ − 1$ is the largest double — and only quadruple — MP known.