A Mersenne number is a number of the form \(2^n - 1\). A Mersenne prime is a Mersenne number that is prime. As of January 2016, there are 49 known Mersenne primes, with \(2^{74,207,281} - 1\) being the largest. A double Mersenne number is a Mersenne number whose exponent is a Mersenne prime. As of 2016, it is known that the first four double Mersenne numbers, \(M_{M_2}\), \(M_{M_3}\), \(M_{M_5}\), \(M_{M_7}\), are prime and the next four, \(M_{M_{13}}\), \(M_{M_{17}}\), \(M_{M_{19}}\), \(M_{M_{31}}\) are composite. The primality status of other known double Mersenne numbers remain unknown.