639991 has 8 divisors (see below), whose sum is σ = 708624. Its totient is φ = 573120.

The previous prime is 639983. The next prime is 639997. The reversal of 639991 is 199936.

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

It is a cyclic number.

It is not a de Polignac number, because 639991 - 2³ = 639983 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 639991.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (639997) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 405 + ... + 1201.

It is an arithmetic number, because the mean of its divisors is an integer number (88578).

2⁶³⁹⁹⁹¹ is an apocalyptic number.

639991 is a deficient number, since it is larger than the sum of its proper divisors (68633).

639991 is a wasteful number, since it uses less digits than its factorization.

639991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 881.

The product of its digits is 13122, while the sum is 37.

The square root of 639991 is about 799.9943749802. The cubic root of 639991 is about 86.1769836429.

It can be divided in two parts, 63999 and 1, that added together give a cube (64000 = 40³).

The spelling of 639991 in words is "six hundred thirty-nine thousand, nine hundred ninety-one".