644639 has 4 divisors (see below), whose sum is σ = 656856. Its totient is φ = 632424.

The previous prime is 644629. The next prime is 644647. The reversal of 644639 is 936446.

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 644639 - 2⁸ = 644383 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 (644629) 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, 6029 + ... + 6134.

It is an arithmetic number, because the mean of its divisors is an integer number (164214).

2⁶⁴⁴⁶³⁹ is an apocalyptic number.

644639 is a deficient number, since it is larger than the sum of its proper divisors (12217).

644639 is a wasteful number, since it uses less digits than its factorization.

644639 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 12216.

The product of its digits is 15552, while the sum is 32.

The square root of 644639 is about 802.8941399711. The cubic root of 644639 is about 86.3851036704.

The spelling of 644639 in words is "six hundred forty-four thousand, six hundred thirty-nine".