21109921 has 16 divisors (see below), whose sum is σ = 24745600. Its totient is φ = 17635968.

The previous prime is 21109901. The next prime is 21109927. The reversal of 21109921 is 12990112.

It is not a de Polignac number, because 21109921 - 2⁵ = 21109889 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21109927) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 55510 + ... + 55888.

It is an arithmetic number, because the mean of its divisors is an integer number (1546600).

Almost surely, 2^21109921 is an apocalyptic number.

21109921 is the 2653-rd octagonal number.

It is an amenable number.

21109921 is a deficient number, since it is larger than the sum of its proper divisors (3635679).

21109921 is a wasteful number, since it uses less digits than its factorization.

21109921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 568.

The product of its (nonzero) digits is 324, while the sum is 25.

The square root of 21109921 is about 4594.5534059362. The cubic root of 21109921 is about 276.3729511460.

The spelling of 21109921 in words is "twenty-one million, one hundred nine thousand, nine hundred twenty-one".