21664728 has 72 divisors (see below), whose sum is σ = 60477300. Its totient is φ = 7006080.

The previous prime is 21664717. The next prime is 21664729. The reversal of 21664728 is 82746612.

21664728 is a `hidden beast` number, since 2 + 1 + 6 + 647 + 2 + 8 = 666.

It is a tau number, because it is divible by the number of its divisors (72).

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 = 21664692 and 21664701.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21664729) 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, 120943 + ... + 121121.

Almost surely, 2^21664728 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 21664728, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30238650).

21664728 is an abundant number, since it is smaller than the sum of its proper divisors (38812572).

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

21664728 is a wasteful number, since it uses less digits than its factorization.

21664728 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 273 (or 225 counting only the distinct ones).

The product of its digits is 32256, while the sum is 36.

The square root of 21664728 is about 4654.5384303924. The cubic root of 21664728 is about 278.7732384924.

The spelling of 21664728 in words is "twenty-one million, six hundred sixty-four thousand, seven hundred twenty-eight".