21565656 has 128 divisors (see below), whose sum is σ = 72576000. Its totient is φ = 5792256.

The previous prime is 21565633. The next prime is 21565667. The reversal of 21565656 is 65656512.

It is a happy number.

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a self number, because there is not a number n which added to its sum of digits gives 21565656.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 25285 + ... + 26123.

It is an arithmetic number, because the mean of its divisors is an integer number (567000).

Almost surely, 2^21565656 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 21565656, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (36288000).

21565656 is an abundant number, since it is smaller than the sum of its proper divisors (51010344).

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

21565656 is a wasteful number, since it uses less digits than its factorization.

21565656 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 878 (or 868 counting only the distinct ones).

The product of its digits is 54000, while the sum is 36.

The square root of 21565656 is about 4643.8837194745. The cubic root of 21565656 is about 278.3476491933.

The spelling of 21565656 in words is "twenty-one million, five hundred sixty-five thousand, six hundred fifty-six".