664721 has 4 divisors (see below), whose sum is σ = 678912. Its totient is φ = 650532.

The previous prime is 664711. The next prime is 664739. The reversal of 664721 is 127466.

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-664721 is a prime.

It is a Duffinian number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 664721.

It is not an unprimeable number, because it can be changed into a prime (664711) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 7025 + ... + 7118.

It is an arithmetic number, because the mean of its divisors is an integer number (169728).

2⁶⁶⁴⁷²¹ is an apocalyptic number.

It is an amenable number.

664721 is a deficient number, since it is larger than the sum of its proper divisors (14191).

664721 is a wasteful number, since it uses less digits than its factorization.

664721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 14190.

The product of its digits is 2016, while the sum is 26.

The square root of 664721 is about 815.3042376929. The cubic root of 664721 is about 87.2729788437.

The spelling of 664721 in words is "six hundred sixty-four thousand, seven hundred twenty-one".