666391991 has 4 divisors (see below), whose sum is σ = 680570592. Its totient is φ = 652213392.

The previous prime is 666391969. The next prime is 666392009. The reversal of 666391991 is 199193666.

It is a happy number.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 666391991 - 2⁶ = 666391927 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 (666391091) 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, 7089230 + ... + 7089323.

It is an arithmetic number, because the mean of its divisors is an integer number (170142648).

Almost surely, 2^666391991 is an apocalyptic number.

666391991 is a deficient number, since it is larger than the sum of its proper divisors (14178601).

666391991 is a wasteful number, since it uses less digits than its factorization.

666391991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 14178600.

The product of its digits is 472392, while the sum is 50.

The square root of 666391991 is about 25814.5693553079. The cubic root of 666391991 is about 873.4604726071.

The spelling of 666391991 in words is "six hundred sixty-six million, three hundred ninety-one thousand, nine hundred ninety-one".