21566556 has 54 divisors (see below), whose sum is σ = 59934056. Its totient is φ = 6534000.

The previous prime is 21566551. The next prime is 21566593. The reversal of 21566556 is 65566512.

It is a happy number.

It is a super-2 number, since 2×21566556² = 930232675402272, which contains 22 as substring.

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 21566556.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21566551) 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, 1881 + ... + 6831.

Almost surely, 2^21566556 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 21566556, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29967028).

21566556 is an abundant number, since it is smaller than the sum of its proper divisors (38367500).

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

21566556 is a wasteful number, since it uses less digits than its factorization.

21566556 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4983 (or 4967 counting only the distinct ones).

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

The square root of 21566556 is about 4643.9806201146. The cubic root of 21566556 is about 278.3515212357.

It can be divided in two parts, 21566 and 556, that added together give a palindrome (22122).

The spelling of 21566556 in words is "twenty-one million, five hundred sixty-six thousand, five hundred fifty-six".