21664309 has 4 divisors (see below), whose sum is σ = 21677760. Its totient is φ = 21650860.

The previous prime is 21664297. The next prime is 21664327. The reversal of 21664309 is 90346612.

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 21664309 - 2¹⁵ = 21631541 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21664339) 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 3 ways as a sum of consecutive naturals, for example, 3919 + ... + 7660.

It is an arithmetic number, because the mean of its divisors is an integer number (5419440).

Almost surely, 2^21664309 is an apocalyptic number.

It is an amenable number.

21664309 is a deficient number, since it is larger than the sum of its proper divisors (13451).

21664309 is a wasteful number, since it uses less digits than its factorization.

21664309 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 13450.

The product of its (nonzero) digits is 7776, while the sum is 31.

The square root of 21664309 is about 4654.4934203413. The cubic root of 21664309 is about 278.7714413048.

The spelling of 21664309 in words is "twenty-one million, six hundred sixty-four thousand, three hundred nine".