21009618 has 80 divisors (see below), whose sum is σ = 54641664. Its totient is φ = 5909760.

The previous prime is 21009617. The next prime is 21009623. The reversal of 21009618 is 81690012.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a junction number, because it is equal to n+sod(n) for n = 21009591 and 21009600.

It is not an unprimeable number, because it can be changed into a prime (21009613) by changing a digit.

It is a polite number, since it can be written in 39 ways as a sum of consecutive naturals, for example, 109903 + ... + 110093.

Almost surely, 2^21009618 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21009618, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27320832).

21009618 is an abundant number, since it is smaller than the sum of its proper divisors (33632046).

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

21009618 is a wasteful number, since it uses less digits than its factorization.

21009618 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 309 (or 300 counting only the distinct ones).

The product of its (nonzero) digits is 864, while the sum is 27.

The square root of 21009618 is about 4583.6249846601. The cubic root of 21009618 is about 275.9345307853.

The spelling of 21009618 in words is "twenty-one million, nine thousand, six hundred eighteen".