644864 has 36 divisors (see below), whose sum is σ = 1410360. Its totient is φ = 291840.

The previous prime is 644863. The next prime is 644867. The reversal of 644864 is 468446.

It is a Harshad number since it is a multiple of its sum of digits (32).

Its product of digits (18432) is a multiple of the sum of its prime factors (256).

It is a self number, because there is not a number n which added to its sum of digits gives 644864.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (644863) 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, 2702 + ... + 2930.

2⁶⁴⁴⁸⁶⁴ is an apocalyptic number.

644864 is a gapful number since it is divisible by the number (64) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 644864, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (705180).

644864 is an abundant number, since it is smaller than the sum of its proper divisors (765496).

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

644864 is a wasteful number, since it uses less digits than its factorization.

644864 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 256 (or 242 counting only the distinct ones).

The product of its digits is 18432, while the sum is 32.

The square root of 644864 is about 803.0342458451. The cubic root of 644864 is about 86.3951529059.

The spelling of 644864 in words is "six hundred forty-four thousand, eight hundred sixty-four".