639721 has 4 divisors (see below), whose sum is σ = 645700. Its totient is φ = 633744.

The previous prime is 639719. The next prime is 639731. The reversal of 639721 is 127936.

639721 is nontrivially palindromic in base 3 and base 7.

639721 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

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 235225 + 404496 = 485^2 + 636^2 .

It is a cyclic number.

It is not a de Polignac number, because 639721 - 2¹ = 639719 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 15.

It is a self number, because there is not a number n which added to its sum of digits gives 639721.

It is not an unprimeable number, because it can be changed into a prime (639701) 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, 2826 + ... + 3043.

It is an arithmetic number, because the mean of its divisors is an integer number (161425).

2⁶³⁹⁷²¹ is an apocalyptic number.

It is an amenable number.

639721 is a deficient number, since it is larger than the sum of its proper divisors (5979).

639721 is a wasteful number, since it uses less digits than its factorization.

639721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5978.

The product of its digits is 2268, while the sum is 28.

The square root of 639721 is about 799.8256059917. The cubic root of 639721 is about 86.1648631295.

The spelling of 639721 in words is "six hundred thirty-nine thousand, seven hundred twenty-one".