 In addition to the aforementioned tests that apply to any natural number, some numbers of a special form can be tested for primality more quickly.For example, the Lucas–Lehmer primality test can determine whether a Mersenne number (one less than a power of two) is prime, deterministically,
in the same time as a single iteration of the Miller–Rabin test.This is why since 1992 () the largest "known" prime has always been a Mersenne prime.