644 can be written using four 4's:
See also 113
644 has 12
divisors (see below), whose sum is σ = 1344
Its totient is φ = 264
The previous prime is 643. The next prime is 647. The reversal of 644 is 446.
It is a happy number.
644 is nontrivially palindromic in base 3 and base 6.
644 is an admirable number.
It is a hoax number, since the sum of its digits (14) coincides with the sum of the digits of its distinct prime factors.
It is a Harshad number since it is a multiple of its sum of digits (14).
Its product of digits (96) is a multiple of the sum of its prime divisors (32).
It is the 23-rd Perrin number.
It is a plaindrome in base 12 and base 15.
It is a nialpdrome in base 10.
It is a congruent number.
It is not an unprimeable number, because it can be changed into a prime (641) by changing a digit.
It is a pernicious number, because its binary representation contains a prime number (3) of ones.
It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 17 + ... + 39.
It is an arithmetic number, because the mean of its divisors is an integer number (112).
It is an amenable number.
It is a practical number, because each smaller number is the sum of distinct divisors of 644, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (672).
644 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
644 is a wasteful number, since it uses less digits than its factorization.
644 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 34 (or 32 counting only the distinct ones).
The product of its digits is 96, while the sum is 14.
The square root of 644 is about 25.3771550809.
The cubic root of 644 is about 8.6356551085.
Subtracting from 644 its sum of digits (14), we obtain a triangular number (630 = T35).
It can be divided in two parts, 64 and 4, that multiplied together give a 8-th power (256 = 28).
The spelling of 644 in words is "six hundred forty-four", and thus it is an aban number.