21556566 has 24 divisors (see below), whose sum is σ = 48737520. Its totient is φ = 6872976.

The previous prime is 21556559. The next prime is 21556567. The reversal of 21556566 is 66565512.

It is a happy number.

It is a self number, because there is not a number n which added to its sum of digits gives 21556566.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21556567) 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 11 ways as a sum of consecutive naturals, for example, 25621 + ... + 26448.

It is an arithmetic number, because the mean of its divisors is an integer number (2030730).

Almost surely, 2^21556566 is an apocalyptic number.

21556566 is an abundant number, since it is smaller than the sum of its proper divisors (27180954).

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

21556566 is a wasteful number, since it uses less digits than its factorization.

21556566 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 52100 (or 52097 counting only the distinct ones).

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

The square root of 21556566 is about 4642.9049096444. The cubic root of 21556566 is about 278.3085355256.

It can be divided in two parts, 21556 and 566, that added together give a palindrome (22122).

The spelling of 21556566 in words is "twenty-one million, five hundred fifty-six thousand, five hundred sixty-six".