21810816 has 64 divisors (see below), whose sum is σ = 64382400. Its totient is φ = 7269120.

The previous prime is 21810809. The next prime is 21810821. The reversal of 21810816 is 61801812.

It is a tau number, because it is divible by the number of its divisors (64).

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

It is a self number, because there is not a number n which added to its sum of digits gives 21810816.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 301 + ... + 6611.

It is an arithmetic number, because the mean of its divisors is an integer number (1005975).

Almost surely, 2^21810816 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 21810816, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32191200).

21810816 is an abundant number, since it is smaller than the sum of its proper divisors (42571584).

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

21810816 is an equidigital number, since it uses as much as digits as its factorization.

21810816 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6334 (or 6316 counting only the distinct ones).

The product of its (nonzero) digits is 768, while the sum is 27.

The square root of 21810816 is about 4670.2051346809. The cubic root of 21810816 is about 279.3984363925.

The spelling of 21810816 in words is "twenty-one million, eight hundred ten thousand, eight hundred sixteen".