21643865 has 4 divisors (see below), whose sum is σ = 25972644. Its totient is φ = 17315088.

The previous prime is 21643849. The next prime is 21643877. The reversal of 21643865 is 56834612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 10601536 + 11042329 = 3256^2 + 3323^2 .

It is a cyclic number.

It is not a de Polignac number, because 21643865 - 2⁴ = 21643849 is a prime.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2164382 + ... + 2164391.

It is an arithmetic number, because the mean of its divisors is an integer number (6493161).

Almost surely, 2^21643865 is an apocalyptic number.

It is an amenable number.

21643865 is a deficient number, since it is larger than the sum of its proper divisors (4328779).

21643865 is an equidigital number, since it uses as much as digits as its factorization.

21643865 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4328778.

The product of its digits is 34560, while the sum is 35.

The square root of 21643865 is about 4652.2967446198. The cubic root of 21643865 is about 278.6837241135.

The spelling of 21643865 in words is "twenty-one million, six hundred forty-three thousand, eight hundred sixty-five".