666556521 has 4 divisors (see below), whose sum is σ = 888742032. Its totient is φ = 444371012.

The previous prime is 666556507. The next prime is 666556523. The reversal of 666556521 is 125655666.

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 not a de Polignac number, because 666556521 - 2⁵ = 666556489 is a prime.

It is not an unprimeable number, because it can be changed into a prime (666556523) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 111092751 + ... + 111092756.

It is an arithmetic number, because the mean of its divisors is an integer number (222185508).

Almost surely, 2^666556521 is an apocalyptic number.

It is an amenable number.

666556521 is a deficient number, since it is larger than the sum of its proper divisors (222185511).

666556521 is a wasteful number, since it uses less digits than its factorization.

666556521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 222185510.

The product of its digits is 324000, while the sum is 42.

The square root of 666556521 is about 25817.7559249444. The cubic root of 666556521 is about 873.5323515351.

The spelling of 666556521 in words is "six hundred sixty-six million, five hundred fifty-six thousand, five hundred twenty-one".