21639568 has 40 divisors (see below), whose sum is σ = 43803000. Its totient is φ = 10343424.

The previous prime is 21639559. The next prime is 21639571. The reversal of 21639568 is 86593612.

It can be written as a sum of positive squares in 4 ways, for example, as 589824 + 21049744 = 768^2 + 4588^2 .

It is a Smith number, since the sum of its digits (40) coincides with the sum of the digits of its prime factors.

It is a self number, because there is not a number n which added to its sum of digits gives 21639568.

It is a congruent number.

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, 68980 + ... + 69292.

It is an arithmetic number, because the mean of its divisors is an integer number (1095075).

Almost surely, 2^21639568 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 21639568, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (21901500).

21639568 is an abundant number, since it is smaller than the sum of its proper divisors (22163432).

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

21639568 is a wasteful number, since it uses less digits than its factorization.

21639568 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 499 (or 493 counting only the distinct ones).

The product of its digits is 77760, while the sum is 40.

The square root of 21639568 is about 4651.8349067868. The cubic root of 21639568 is about 278.6652803464.

The spelling of 21639568 in words is "twenty-one million, six hundred thirty-nine thousand, five hundred sixty-eight".