21681618 has 48 divisors (see below), whose sum is σ = 52202880. Its totient is φ = 5978784.

The previous prime is 21681599. The next prime is 21681619. The reversal of 21681618 is 81618612.

It is not an unprimeable number, because it can be changed into a prime (21681619) 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 23 ways as a sum of consecutive naturals, for example, 7255 + ... + 9797.

It is an arithmetic number, because the mean of its divisors is an integer number (1087560).

Almost surely, 2^21681618 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21681618, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (26101440).

21681618 is an abundant number, since it is smaller than the sum of its proper divisors (30521262).

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

21681618 is a wasteful number, since it uses less digits than its factorization.

21681618 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2591 (or 2584 counting only the distinct ones).

The product of its digits is 4608, while the sum is 33.

The square root of 21681618 is about 4656.3524351149. The cubic root of 21681618 is about 278.8456643104.

The spelling of 21681618 in words is "twenty-one million, six hundred eighty-one thousand, six hundred eighteen".