639979 has 4 divisors (see below), whose sum is σ = 648160. Its totient is φ = 631800.

The previous prime is 639959. The next prime is 639983. The reversal of 639979 is 979936.

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

It is a cyclic number.

It is not a de Polignac number, because 639979 - 2⁷ = 639851 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 15.

It is not an unprimeable number, because it can be changed into a prime (639949) 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, 3972 + ... + 4129.

It is an arithmetic number, because the mean of its divisors is an integer number (162040).

2⁶³⁹⁹⁷⁹ is an apocalyptic number.

639979 is a deficient number, since it is larger than the sum of its proper divisors (8181).

639979 is an equidigital number, since it uses as much as digits as its factorization.

639979 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 8180.

The product of its digits is 91854, while the sum is 43.

The square root of 639979 is about 799.9868748923. The cubic root of 639979 is about 86.1764450258.

Subtracting from 639979 its sum of digits (43), we obtain a palindrome (639936).

It can be divided in two parts, 6399 and 79, that multiplied together give a square (505521 = 711²).

The spelling of 639979 in words is "six hundred thirty-nine thousand, nine hundred seventy-nine".