644426640 has 360 divisors, whose sum is σ = 2566320120. Its totient is φ = 143953920.

The previous prime is 644426617. The next prime is 644426641. The reversal of 644426640 is 46624446.

It is a happy number.

644426640 is a `hidden beast` number, since 6 + 4 + 4 + 4 + 2 + 6 + 640 = 666.

It can be written as a sum of positive squares in 4 ways, for example, as 2108304 + 642318336 = 1452^2 + 25344^2 .

It is a tau number, because it is divible by the number of its divisors (360).

It is a hoax number, since the sum of its digits (36) 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 (36).

It is a junction number, because it is equal to n+sod(n) for n = 644426595 and 644426604.

It is not an unprimeable number, because it can be changed into a prime (644426641) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 71 ways as a sum of consecutive naturals, for example, 1132276 + ... + 1132844.

It is an arithmetic number, because the mean of its divisors is an integer number (7128667).

Almost surely, 2^644426640 is an apocalyptic number.

644426640 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 644426640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (1283160060).

644426640 is an abundant number, since it is smaller than the sum of its proper divisors (1921893480).

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

644426640 is a wasteful number, since it uses less digits than its factorization.

644426640 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 623 (or 603 counting only the distinct ones).

The product of its (nonzero) digits is 110592, while the sum is 36.

The square root of 644426640 is about 25385.5596747442. The cubic root of 644426640 is about 863.7561683340.

The spelling of 644426640 in words is "six hundred forty-four million, four hundred twenty-six thousand, six hundred forty".