21387618 has 96 divisors (see below), whose sum is σ = 56635200. Its totient is φ = 5963328.

The previous prime is 21387617. The next prime is 21387629. The reversal of 21387618 is 81678312.

21387618 is a `hidden beast` number, since 2 + 1 + 38 + 7 + 618 = 666.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21387617) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 156046 + ... + 156182.

It is an arithmetic number, because the mean of its divisors is an integer number (589950).

Almost surely, 2^21387618 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21387618, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (28317600).

21387618 is an abundant number, since it is smaller than the sum of its proper divisors (35247582).

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

21387618 is a wasteful number, since it uses less digits than its factorization.

21387618 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 221 (or 208 counting only the distinct ones).

The product of its digits is 16128, while the sum is 36.

The square root of 21387618 is about 4624.6749074935. The cubic root of 21387618 is about 277.5795535246.

The spelling of 21387618 in words is "twenty-one million, three hundred eighty-seven thousand, six hundred eighteen".