639661 has 4 divisors (see below), whose sum is σ = 697824. Its totient is φ = 581500.

The previous prime is 639647. The next prime is 639671. The reversal of 639661 is 166936.

639661 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, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 639661 - 2⁷ = 639533 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (639601) 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, 29065 + ... + 29086.

It is an arithmetic number, because the mean of its divisors is an integer number (174456).

2⁶³⁹⁶⁶¹ is an apocalyptic number.

It is an amenable number.

639661 is a deficient number, since it is larger than the sum of its proper divisors (58163).

639661 is a wasteful number, since it uses less digits than its factorization.

639661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 58162.

The product of its digits is 5832, while the sum is 31.

The square root of 639661 is about 799.7880969357. The cubic root of 639661 is about 86.1621692190.

The spelling of 639661 in words is "six hundred thirty-nine thousand, six hundred sixty-one".