21391808 has 14 divisors (see below), whose sum is σ = 42449496. Its totient is φ = 10695872.

The previous prime is 21391801. The next prime is 21391819. The reversal of 21391808 is 80819312.

It is a Harshad number since it is a multiple of its sum of digits (32).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21391801) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 167060 + ... + 167187.

Almost surely, 2^21391808 is an apocalyptic number.

It is an amenable number.

21391808 is a deficient number, since it is larger than the sum of its proper divisors (21057688).

21391808 is an equidigital number, since it uses as much as digits as its factorization.

21391808 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 334259 (or 334249 counting only the distinct ones).

The product of its (nonzero) digits is 3456, while the sum is 32.

The square root of 21391808 is about 4625.1278901237. The cubic root of 21391808 is about 277.5976790024.

The spelling of 21391808 in words is "twenty-one million, three hundred ninety-one thousand, eight hundred eight".