21664321 has 24 divisors (see below), whose sum is σ = 26922240. Its totient is φ = 17341632.

The previous prime is 21664297. The next prime is 21664327. The reversal of 21664321 is 12346612.

It is not a de Polignac number, because 21664321 - 2⁷ = 21664193 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21664327) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 52765 + ... + 53173.

It is an arithmetic number, because the mean of its divisors is an integer number (1121760).

Almost surely, 2^21664321 is an apocalyptic number.

It is an amenable number.

21664321 is a deficient number, since it is larger than the sum of its proper divisors (5257919).

21664321 is a wasteful number, since it uses less digits than its factorization.

21664321 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 493 (or 486 counting only the distinct ones).

The product of its digits is 1728, while the sum is 25.

The square root of 21664321 is about 4654.4947094180. The cubic root of 21664321 is about 278.7714927759.

The spelling of 21664321 in words is "twenty-one million, six hundred sixty-four thousand, three hundred twenty-one".