665991 has 6 divisors (see below), whose sum is σ = 962000. Its totient is φ = 443988.

The previous prime is 665983. The next prime is 665993. The reversal of 665991 is 199566.

665991 is a `hidden beast` number, since 66 + 599 + 1 = 666.

It is not a de Polignac number, because 665991 - 2³ = 665983 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 665991.

It is not an unprimeable number, because it can be changed into a prime (665993) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 36991 + ... + 37008.

2⁶⁶⁵⁹⁹¹ is an apocalyptic number.

665991 is a deficient number, since it is larger than the sum of its proper divisors (296009).

665991 is a wasteful number, since it uses less digits than its factorization.

665991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 74005 (or 74002 counting only the distinct ones).

The product of its digits is 14580, while the sum is 36.

The square root of 665991 is about 816.0827163958. The cubic root of 665991 is about 87.3285240377.

The spelling of 665991 in words is "six hundred sixty-five thousand, nine hundred ninety-one".