21665864 has 16 divisors (see below), whose sum is σ = 44316720. Its totient is φ = 9848080.

The previous prime is 21665827. The next prime is 21665867. The reversal of 21665864 is 46856612.

It is not an unprimeable number, because it can be changed into a prime (21665867) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 123014 + ... + 123189.

It is an arithmetic number, because the mean of its divisors is an integer number (2769795).

Almost surely, 2^21665864 is an apocalyptic number.

It is an amenable number.

21665864 is an abundant number, since it is smaller than the sum of its proper divisors (22650856).

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

21665864 is a wasteful number, since it uses less digits than its factorization.

21665864 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 246220 (or 246216 counting only the distinct ones).

The product of its digits is 69120, while the sum is 38.

The square root of 21665864 is about 4654.6604602269. The cubic root of 21665864 is about 278.7781109417.

The spelling of 21665864 in words is "twenty-one million, six hundred sixty-five thousand, eight hundred sixty-four".