For example, "31 = 2 − 1", and 5 is a prime number, so 31 is a Mersenne number; and 31 is also a Mersenne prime because it is a prime number. But the Mersenne number "2047 = 2 − 1" is not a prime because it is divisible by 89 and 23. And "2 − 1 = 15" can be shown to be composite because 4 is not prime.