21805643 has 4 divisors (see below), whose sum is σ = 21897120. Its totient is φ = 21714168.

The previous prime is 21805639. The next prime is 21805649. The reversal of 21805643 is 34650812.

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

It is a cyclic number.

It is not a de Polignac number, because 21805643 - 2² = 21805639 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21805649) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 45380 + ... + 45857.

It is an arithmetic number, because the mean of its divisors is an integer number (5474280).

Almost surely, 2^21805643 is an apocalyptic number.

21805643 is a deficient number, since it is larger than the sum of its proper divisors (91477).

21805643 is an equidigital number, since it uses as much as digits as its factorization.

21805643 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 91476.

The product of its (nonzero) digits is 5760, while the sum is 29.

The square root of 21805643 is about 4669.6512717761. The cubic root of 21805643 is about 279.3763457871.

The spelling of 21805643 in words is "twenty-one million, eight hundred five thousand, six hundred forty-three".