21728921 has 4 divisors (see below), whose sum is σ = 21781200. Its totient is φ = 21676644.

The previous prime is 21728909. The next prime is 21728939. The reversal of 21728921 is 12982712.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21728921 - 2⁶ = 21728857 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21728981) 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 3 ways as a sum of consecutive naturals, for example, 25511 + ... + 26348.

It is an arithmetic number, because the mean of its divisors is an integer number (5445300).

Almost surely, 2^21728921 is an apocalyptic number.

It is an amenable number.

21728921 is a deficient number, since it is larger than the sum of its proper divisors (52279).

21728921 is an equidigital number, since it uses as much as digits as its factorization.

21728921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 52278.

The product of its digits is 4032, while the sum is 32.

The square root of 21728921 is about 4661.4290727201. The cubic root of 21728921 is about 279.0483037943.

The spelling of 21728921 in words is "twenty-one million, seven hundred twenty-eight thousand, nine hundred twenty-one".