66720721 has 2 divisors, whose sum is σ = 66720722. Its totient is φ = 66720720.

The previous prime is 66720707. The next prime is 66720727. The reversal of 66720721 is 12702766.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 37344321 + 29376400 = 6111^2 + 5420^2 .

It is a cyclic number.

It is not a de Polignac number, because 66720721 - 2¹⁵ = 66687953 is a prime.

It is a super-2 number, since 2×66720721² = 8903309221519682, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (66720727) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 33360360 + 33360361.

It is an arithmetic number, because the mean of its divisors is an integer number (33360361).

Almost surely, 2^66720721 is an apocalyptic number.

It is an amenable number.

66720721 is a deficient number, since it is larger than the sum of its proper divisors (1).

66720721 is an equidigital number, since it uses as much as digits as its factorization.

66720721 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 7056, while the sum is 31.

The square root of 66720721 is about 8168.2752769480. The cubic root of 66720721 is about 405.5896932239.

The spelling of 66720721 in words is "sixty-six million, seven hundred twenty thousand, seven hundred twenty-one".