644840 has 64 divisors (see below), whose sum is σ = 1728000. Its totient is φ = 216384.

The previous prime is 644837. The next prime is 644843. The reversal of 644840 is 48446.

It is an interprime number because it is at equal distance from previous prime (644837) and next prime (644843).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (644843) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 13697 + ... + 13743.

It is an arithmetic number, because the mean of its divisors is an integer number (27000).

2⁶⁴⁴⁸⁴⁰ is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 644840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (864000).

644840 is an abundant number, since it is smaller than the sum of its proper divisors (1083160).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

644840 is a wasteful number, since it uses less digits than its factorization.

644840 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 79 (or 61 counting only the distinct ones).

The product of its (nonzero) digits is 3072, while the sum is 26.

The square root of 644840 is about 803.0193023832. The cubic root of 644840 is about 86.3940810989.

The spelling of 644840 in words is "six hundred forty-four thousand, eight hundred forty".