666721 has 4 divisors (see below), whose sum is σ = 727344. Its totient is φ = 606100.

The previous prime is 666707. The next prime is 666727. The reversal of 666721 is 127666.

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 not a de Polignac number, because 666721 - 2¹⁵ = 633953 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 666721.

It is not an unprimeable number, because it can be changed into a prime (666727) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 30295 + ... + 30316.

It is an arithmetic number, because the mean of its divisors is an integer number (181836).

2⁶⁶⁶⁷²¹ is an apocalyptic number.

It is an amenable number.

666721 is a deficient number, since it is larger than the sum of its proper divisors (60623).

666721 is a wasteful number, since it uses less digits than its factorization.

666721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 60622.

The product of its digits is 3024, while the sum is 28.

The square root of 666721 is about 816.5298524855. The cubic root of 666721 is about 87.3604196361.

The spelling of 666721 in words is "six hundred sixty-six thousand, seven hundred twenty-one".