21644720 has 40 divisors (see below), whose sum is σ = 51559200. Its totient is φ = 8445440.

The previous prime is 21644717. The next prime is 21644731. The reversal of 21644720 is 2744612.

It is a tau number, because it is divible by the number of its divisors (40).

It is a self number, because there is not a number n which added to its sum of digits gives 21644720.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 20 + ... + 6579.

It is an arithmetic number, because the mean of its divisors is an integer number (1288980).

Almost surely, 2^21644720 is an apocalyptic number.

21644720 is a gapful number since it is divisible by the number (20) 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 21644720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25779600).

21644720 is an abundant number, since it is smaller than the sum of its proper divisors (29914480).

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

21644720 is a wasteful number, since it uses less digits than its factorization.

21644720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6653 (or 6647 counting only the distinct ones).

The product of its (nonzero) digits is 2688, while the sum is 26.

The square root of 21644720 is about 4652.3886338095. The cubic root of 21644720 is about 278.6873936899.

The spelling of 21644720 in words is "twenty-one million, six hundred forty-four thousand, seven hundred twenty".