• 639 can be written using four 4's:

639 has 6 divisors (see below), whose sum is σ = 936. Its totient is φ = 420.

The previous prime is 631. The next prime is 641. The reversal of 639 is 936.

It is not a de Polignac number, because 639 - 2^{3} = 631 is a prime.

639 is a lucky number.

It is a plaindrome in base 8, base 14 and base 16.

It is a nialpdrome in base 11.

It is a zygodrome in base 8.

It is a congruent number.

It is an inconsummate number, since it does not exist a number *n* which divided by its sum of digits gives 639.

It is not an unprimeable number, because it can be changed into a prime (631) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 27 + ... + 44.

It is an arithmetic number, because the mean of its divisors is an integer number (156).

639 is a deficient number, since it is larger than the sum of its proper divisors (297).

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

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

The sum of its prime factors is 77 (or 74 counting only the distinct ones).

The product of its digits is 162, while the sum is 18.

The square root of 639 is about 25.2784493195. The cubic root of 639 is about 8.6132480148.

It can be divided in two parts, 6 and 39, that added together give a triangular number (45 = T_{9}).

The spelling of 639 in words is "six hundred thirty-nine", and thus it is an aban number and an oban number.

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