21287808 has 96 divisors (see below), whose sum is σ = 64920960. Its totient is φ = 6672384.

The previous prime is 21287797. The next prime is 21287831. The reversal of 21287808 is 80878212.

It is a tau number, because it is divible by the number of its divisors (96).

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 19041 + ... + 20127.

It is an arithmetic number, because the mean of its divisors is an integer number (676260).

Almost surely, 2^21287808 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 21287808, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32460480).

21287808 is an abundant number, since it is smaller than the sum of its proper divisors (43633152).

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

21287808 is a wasteful number, since it uses less digits than its factorization.

21287808 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1124 (or 1109 counting only the distinct ones).

The product of its (nonzero) digits is 14336, while the sum is 36.

The square root of 21287808 is about 4613.8712595823. The cubic root of 21287808 is about 277.1470848441.

21287808 divided by its sum of digits (36) gives a triangular number (591328 = T₁₀₈₇).

The spelling of 21287808 in words is "twenty-one million, two hundred eighty-seven thousand, eight hundred eight".