21668809 has 4 divisors (see below), whose sum is σ = 21965716. Its totient is φ = 21371904.

The previous prime is 21668807. The next prime is 21668827. The reversal of 21668809 is 90886612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 16160400 + 5508409 = 4020^2 + 2347^2 .

It is a cyclic number.

It is not a de Polignac number, because 21668809 - 2¹ = 21668807 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21668807) 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, 148344 + ... + 148489.

It is an arithmetic number, because the mean of its divisors is an integer number (5491429).

Almost surely, 2^21668809 is an apocalyptic number.

It is an amenable number.

21668809 is a deficient number, since it is larger than the sum of its proper divisors (296907).

21668809 is an equidigital number, since it uses as much as digits as its factorization.

21668809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 296906.

The product of its (nonzero) digits is 41472, while the sum is 40.

The square root of 21668809 is about 4654.9767990829. The cubic root of 21668809 is about 278.7907416302.

The spelling of 21668809 in words is "twenty-one million, six hundred sixty-eight thousand, eight hundred nine".