21251808 has 84 divisors (see below), whose sum is σ = 62799408. Its totient is φ = 7076160.

The previous prime is 21251803. The next prime is 21251821. The reversal of 21251808 is 80815212.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21251803) by changing a digit.

It is a polite number, since it can be written in 13 ways as a sum of consecutive naturals, for example, 22873 + ... + 23783.

It is an arithmetic number, because the mean of its divisors is an integer number (747612).

Almost surely, 2^21251808 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 21251808, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (31399704).

21251808 is an abundant number, since it is smaller than the sum of its proper divisors (41547600).

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

21251808 is an frugal number, since it uses more digits than its factorization.

21251808 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 939 (or 916 counting only the distinct ones).

The product of its (nonzero) digits is 1280, while the sum is 27.

The square root of 21251808 is about 4609.9683296092. The cubic root of 21251808 is about 276.9907680676.

The spelling of 21251808 in words is "twenty-one million, two hundred fifty-one thousand, eight hundred eight".