Mersenne prime
Seven, the fourth prime number, is not only a Mersenne prime (since ) but also a double Mersenne prime since the exponent, 3, is itself a Mersenne prime. It is also a Newman–Shanks–Williams prime, a Woodall prime, a factorial prime, a lucky prime, a happy number (happy prime), a safe prime (the only Mersenne safe prime), and the fourth Heegner number.