665721 has 12 divisors (see below), whose sum is σ = 1099072. Its totient is φ = 380376.

The previous prime is 665719. The next prime is 665723. The reversal of 665721 is 127566.

665721 is a `hidden beast` number, since 6 + 657 + 2 + 1 = 666.

It is an interprime number because it is at equal distance from previous prime (665719) and next prime (665723).

It is not a de Polignac number, because 665721 - 2¹ = 665719 is a prime.

It is a Duffinian number.

It is a Curzon number.

665721 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (665723) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 5221 + ... + 5346.

2⁶⁶⁵⁷²¹ is an apocalyptic number.

It is an amenable number.

665721 is a deficient number, since it is larger than the sum of its proper divisors (433351).

665721 is a wasteful number, since it uses less digits than its factorization.

665721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10580 (or 10577 counting only the distinct ones).

The product of its digits is 2520, while the sum is 27.

The square root of 665721 is about 815.9172752185. The cubic root of 665721 is about 87.3167211312.

The spelling of 665721 in words is "six hundred sixty-five thousand, seven hundred twenty-one".