639821 has 16 divisors (see below), whose sum is σ = 806400. Its totient is φ = 494208.

The previous prime is 639757. The next prime is 639833. The reversal of 639821 is 128936.

639821 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is not a de Polignac number, because 639821 - 2⁶ = 639757 is a prime.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (639851) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 7145 + ... + 7233.

It is an arithmetic number, because the mean of its divisors is an integer number (50400).

2⁶³⁹⁸²¹ is an apocalyptic number.

It is an amenable number.

639821 is a deficient number, since it is larger than the sum of its proper divisors (166579).

639821 is a wasteful number, since it uses less digits than its factorization.

639821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 188.

The product of its digits is 2592, while the sum is 29.

The square root of 639821 is about 799.8881171764. Note that the first 3 decimals coincide. The cubic root of 639821 is about 86.1693526061.

The spelling of 639821 in words is "six hundred thirty-nine thousand, eight hundred twenty-one".