666655721 has 4 divisors (see below), whose sum is σ = 666733200. Its totient is φ = 666578244.

The previous prime is 666655709. The next prime is 666655727. The reversal of 666655721 is 127556666.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-666655721 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (666655727) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 23951 + ... + 43668.

It is an arithmetic number, because the mean of its divisors is an integer number (166683300).

Almost surely, 2^666655721 is an apocalyptic number.

It is an amenable number.

666655721 is a deficient number, since it is larger than the sum of its proper divisors (77479).

666655721 is an equidigital number, since it uses as much as digits as its factorization.

666655721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 77478.

The product of its digits is 453600, while the sum is 44.

The square root of 666655721 is about 25819.6770119225. The cubic root of 666655721 is about 873.5756837498.

The spelling of 666655721 in words is "six hundred sixty-six million, six hundred fifty-five thousand, seven hundred twenty-one".