Mersenne prime
31 is the third Mersenne prime (25 − 1) and the eighth Mersenne prime exponent, as well as the fourth primorial prime, and together with twenty-nine, a primorial prime, it comprises a twin prime. As a Mersenne prime, 31 is related to the perfect number 496, since 496 = 2(5 − 1)(25 − 1). 31 is also the 4th lucky prime and the 11th supersingular prime.