21909921 has 8 divisors (see below), whose sum is σ = 31869024. Its totient is φ = 13278720.

The previous prime is 21909917. The next prime is 21909941. The reversal of 21909921 is 12990912.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21909921 - 2² = 21909917 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (33), and also a Moran number because the ratio is a prime number: 663937 = 21909921 / (2 + 1 + 9 + 0 + 9 + 9 + 2 + 1).

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21909911) 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 7 ways as a sum of consecutive naturals, for example, 331936 + ... + 332001.

It is an arithmetic number, because the mean of its divisors is an integer number (3983628).

Almost surely, 2^21909921 is an apocalyptic number.

It is an amenable number.

21909921 is a deficient number, since it is larger than the sum of its proper divisors (9959103).

21909921 is a wasteful number, since it uses less digits than its factorization.

21909921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 663951.

The product of its (nonzero) digits is 2916, while the sum is 33.

The square root of 21909921 is about 4680.8034566728. The cubic root of 21909921 is about 279.8209782515.

The spelling of 21909921 in words is "twenty-one million, nine hundred nine thousand, nine hundred twenty-one".