21721808 has 20 divisors (see below), whose sum is σ = 42393120. Its totient is φ = 10781664.

The previous prime is 21721789. The next prime is 21721813. The reversal of 21721808 is 80812712.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2660 + ... + 7107.

It is an arithmetic number, because the mean of its divisors is an integer number (2119656).

Almost surely, 2^21721808 is an apocalyptic number.

It is an amenable number.

21721808 is a deficient number, since it is larger than the sum of its proper divisors (20671312).

21721808 is a wasteful number, since it uses less digits than its factorization.

21721808 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9914 (or 9908 counting only the distinct ones).

The product of its (nonzero) digits is 1792, while the sum is 29.

The square root of 21721808 is about 4660.6660468221. Note that the first 3 decimals coincide. The cubic root of 21721808 is about 279.0178514890.

The spelling of 21721808 in words is "twenty-one million, seven hundred twenty-one thousand, eight hundred eight".