644688 has 90 divisors (see below), whose sum is σ = 2036762. Its totient is φ = 190080.

The previous prime is 644687. The next prime is 644701. The reversal of 644688 is 886446.

644688 is a `hidden beast` number, since 644 + 6 + 8 + 8 = 666.

It can be written as a sum of positive squares in only one way, i.e., 627264 + 17424 = 792^2 + 132^2 .

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a nude number because it is divisible by every one of its digits.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (644687) by changing a digit.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 17406 + ... + 17442.

2⁶⁴⁴⁶⁸⁸ 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 644688, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (1018381).

644688 is an abundant number, since it is smaller than the sum of its proper divisors (1392074).

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

644688 is a wasteful number, since it uses less digits than its factorization.

644688 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 73 (or 53 counting only the distinct ones).

The product of its digits is 36864, while the sum is 36.

The square root of 644688 is about 802.9246539994. The cubic root of 644688 is about 86.3872923697.

The spelling of 644688 in words is "six hundred forty-four thousand, six hundred eighty-eight".