21664709 has 4 divisors (see below), whose sum is σ = 23634240. Its totient is φ = 19695180.

The previous prime is 21664703. The next prime is 21664717. The reversal of 21664709 is 90746612.

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 21664709 - 2²⁰ = 20616133 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 (21664703) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 984749 + ... + 984770.

It is an arithmetic number, because the mean of its divisors is an integer number (5908560).

Almost surely, 2^21664709 is an apocalyptic number.

It is an amenable number.

21664709 is a deficient number, since it is larger than the sum of its proper divisors (1969531).

21664709 is a wasteful number, since it uses less digits than its factorization.

21664709 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1969530.

The product of its (nonzero) digits is 18144, while the sum is 35.

The square root of 21664709 is about 4654.5363893733. The cubic root of 21664709 is about 278.7731569975.

The spelling of 21664709 in words is "twenty-one million, six hundred sixty-four thousand, seven hundred nine".