644640 has 96 divisors (see below), whose sum is σ = 2177280. Its totient is φ = 159744.

The previous prime is 644629. The next prime is 644647. The reversal of 644640 is 46446.

It is a tau number, because it is divible by the number of its divisors (96).

It is a Harshad number since it is a multiple of its sum of digits (24).

It is a self number, because there is not a number n which added to its sum of digits gives 644640.

It is not an unprimeable number, because it can be changed into a prime (644647) by changing a digit.

644640 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 8121 + ... + 8199.

It is an arithmetic number, because the mean of its divisors is an integer number (22680).

2⁶⁴⁴⁶⁴⁰ is an apocalyptic number.

644640 is a gapful number since it is divisible by the number (60) 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 644640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (1088640).

644640 is an abundant number, since it is smaller than the sum of its proper divisors (1532640).

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

644640 is a wasteful number, since it uses less digits than its factorization.

644640 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 114 (or 106 counting only the distinct ones).

The product of its (nonzero) digits is 2304, while the sum is 24.

The square root of 644640 is about 802.8947627180. The cubic root of 644640 is about 86.3851483389.

The spelling of 644640 in words is "six hundred forty-four thousand, six hundred forty".