639537721 has 4 divisors (see below), whose sum is σ = 655136244. Its totient is φ = 623939200.

The previous prime is 639537707. The next prime is 639537727. The reversal of 639537721 is 127735936.

It is a happy number.

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

It can be written as a sum of positive squares in 2 ways, for example, as 83521321 + 556016400 = 9139^2 + 23580^2 .

It is a cyclic number.

It is not a de Polignac number, because 639537721 - 2⁷ = 639537593 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (639537727) 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, 7799200 + ... + 7799281.

It is an arithmetic number, because the mean of its divisors is an integer number (163784061).

Almost surely, 2^639537721 is an apocalyptic number.

It is an amenable number.

639537721 is a deficient number, since it is larger than the sum of its proper divisors (15598523).

639537721 is a wasteful number, since it uses less digits than its factorization.

639537721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 15598522.

The product of its digits is 238140, while the sum is 43.

The square root of 639537721 is about 25289.0830399206. The cubic root of 639537721 is about 861.5663364699.

The spelling of 639537721 in words is "six hundred thirty-nine million, five hundred thirty-seven thousand, seven hundred twenty-one".