639540821 has 8 divisors (see below), whose sum is σ = 645283584. Its totient is φ = 633811360.

The previous prime is 639540793. The next prime is 639540851. The reversal of 639540821 is 128045936.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-639540821 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 (639540851) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 110225 + ... + 115881.

It is an arithmetic number, because the mean of its divisors is an integer number (80660448).

Almost surely, 2^639540821 is an apocalyptic number.

It is an amenable number.

639540821 is a deficient number, since it is larger than the sum of its proper divisors (5742763).

639540821 is a wasteful number, since it uses less digits than its factorization.

639540821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6651.

The product of its (nonzero) digits is 51840, while the sum is 38.

The square root of 639540821 is about 25289.1443311157. The cubic root of 639540821 is about 861.5677285438.

The spelling of 639540821 in words is "six hundred thirty-nine million, five hundred forty thousand, eight hundred twenty-one".