21005808 has 80 divisors (see below), whose sum is σ = 57853440. Its totient is φ = 6552832.

The previous prime is 21005807. The next prime is 21005813. The reversal of 21005808 is 80850012.

It is a Harshad number since it is a multiple of its sum of digits (24).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21005807) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 58333 + ... + 58691.

It is an arithmetic number, because the mean of its divisors is an integer number (723168).

Almost surely, 2^21005808 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21005808, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (28926720).

21005808 is an abundant number, since it is smaller than the sum of its proper divisors (36847632).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21005808 is a wasteful number, since it uses less digits than its factorization.

21005808 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 446 (or 440 counting only the distinct ones).

The product of its (nonzero) digits is 640, while the sum is 24.

The square root of 21005808 is about 4583.2093558990. The cubic root of 21005808 is about 275.9178499470.

The spelling of 21005808 in words is "twenty-one million, five thousand, eight hundred eight".