007

Seven, the fourth prime number, is not only a Mersenne prime (since 2 3 − 1 = 7 {\displaystyle 2^{3}-1=7}) 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 Harshad number, a lucky prime, a happy number (happy prime), a safe prime (the only Mersenne safe prime), a Leyland number of the second kind and Leyland prime of the second kind ( 2 5 − 5 2 {\displaystyle 2^{5}-5^{2}}), and the fourth Heegner number.Seven is the lowest natural number that cannot be represented as the sum of the squares of three integers.