21561856 has 40 divisors (see below), whose sum is σ = 44979264. Its totient is φ = 10306560.

The previous prime is 21561851. The next prime is 21561887. The reversal of 21561856 is 65816512.

It is a happy number.

It is not an unprimeable number, because it can be changed into a prime (21561851) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 10861 + ... + 12691.

Almost surely, 2^21561856 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 21561856, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (22489632).

21561856 is an abundant number, since it is smaller than the sum of its proper divisors (23417408).

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

21561856 is an equidigital number, since it uses as much as digits as its factorization.

21561856 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1872 (or 1856 counting only the distinct ones).

The product of its digits is 14400, while the sum is 34.

The square root of 21561856 is about 4643.4745611449. The cubic root of 21561856 is about 278.3312993824.

The spelling of 21561856 in words is "twenty-one million, five hundred sixty-one thousand, eight hundred fifty-six".