639687691 has 4 divisors (see below), whose sum is σ = 639763672. Its totient is φ = 639611712.

The previous prime is 639687677. The next prime is 639687703. The reversal of 639687691 is 196786936.

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 639687691 - 2⁹ = 639687179 is a prime.

It is a super-2 number, since 2×639687691² = 818400684033822962, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (639687791) 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, 23526 + ... + 42811.

It is an arithmetic number, because the mean of its divisors is an integer number (159940918).

Almost surely, 2^639687691 is an apocalyptic number.

639687691 is a deficient number, since it is larger than the sum of its proper divisors (75981).

639687691 is an equidigital number, since it uses as much as digits as its factorization.

639687691 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 75980.

The product of its digits is 2939328, while the sum is 55.

The square root of 639687691 is about 25292.0479795528. The cubic root of 639687691 is about 861.6336762587.

The spelling of 639687691 in words is "six hundred thirty-nine million, six hundred eighty-seven thousand, six hundred ninety-one".