21668399 has 4 divisors (see below), whose sum is σ = 22035720. Its totient is φ = 21301080.

The previous prime is 21668389. The next prime is 21668401. The reversal of 21668399 is 99386612.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21668399 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 (21668389) 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, 183572 + ... + 183689.

It is an arithmetic number, because the mean of its divisors is an integer number (5508930).

Almost surely, 2^21668399 is an apocalyptic number.

21668399 is a deficient number, since it is larger than the sum of its proper divisors (367321).

21668399 is an equidigital number, since it uses as much as digits as its factorization.

21668399 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 367320.

The product of its digits is 139968, while the sum is 44.

The square root of 21668399 is about 4654.9327599870. The cubic root of 21668399 is about 278.7889832668.

The spelling of 21668399 in words is "twenty-one million, six hundred sixty-eight thousand, three hundred ninety-nine".