665999991 has 12 divisors (see below), whose sum is σ = 962225368. Its totient is φ = 443895984.

The previous prime is 665999987. The next prime is 666000019. The reversal of 665999991 is 199999566.

It is not a de Polignac number, because 665999991 - 2² = 665999987 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (665999911) 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, 63561 + ... + 73293.

Almost surely, 2^665999991 is an apocalyptic number.

665999991 is a deficient number, since it is larger than the sum of its proper divisors (296225377).

665999991 is a wasteful number, since it uses less digits than its factorization.

665999991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 17342 (or 17339 counting only the distinct ones).

The product of its digits is 10628820, while the sum is 63.

The square root of 665999991 is about 25806.9756267564. The cubic root of 665999991 is about 873.2891701959.

The spelling of 665999991 in words is "six hundred sixty-five million, nine hundred ninety-nine thousand, nine hundred ninety-one".