21305556 has 72 divisors (see below), whose sum is σ = 58915584. Its totient is φ = 6462720.

The previous prime is 21305551. The next prime is 21305563. The reversal of 21305556 is 65550312.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21305551) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 18411 + ... + 19533.

It is an arithmetic number, because the mean of its divisors is an integer number (818272).

Almost surely, 2^21305556 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 21305556, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29457792).

21305556 is an abundant number, since it is smaller than the sum of its proper divisors (37610028).

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

21305556 is a wasteful number, since it uses less digits than its factorization.

21305556 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1181 (or 1176 counting only the distinct ones).

The product of its (nonzero) digits is 4500, while the sum is 27.

The square root of 21305556 is about 4615.7941895193. The cubic root of 21305556 is about 277.2240841627.

Adding to 21305556 its reverse (65550312), we get a palindrome (86855868).

The spelling of 21305556 in words is "twenty-one million, three hundred five thousand, five hundred fifty-six".