21605643 has 8 divisors (see below), whose sum is σ = 32008400. Its totient is φ = 14403744.

The previous prime is 21605627. The next prime is 21605659. The reversal of 21605643 is 34650612.

21605643 is a `hidden beast` number, since 2 + 16 + 0 + 5 + 643 = 666.

It is an interprime number because it is at equal distance from previous prime (21605627) and next prime (21605659).

It is not a de Polignac number, because 21605643 - 2⁴ = 21605627 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27), and also a Moran number because the ratio is a prime number: 800209 = 21605643 / (2 + 1 + 6 + 0 + 5 + 6 + 4 + 3).

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21605663) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 400078 + ... + 400131.

It is an arithmetic number, because the mean of its divisors is an integer number (4001050).

Almost surely, 2^21605643 is an apocalyptic number.

21605643 is a deficient number, since it is larger than the sum of its proper divisors (10402757).

21605643 is an equidigital number, since it uses as much as digits as its factorization.

21605643 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 800218 (or 800212 counting only the distinct ones).

The product of its (nonzero) digits is 4320, while the sum is 27.

The square root of 21605643 is about 4648.1870659430. The cubic root of 21605643 is about 278.5195802068.

The spelling of 21605643 in words is "twenty-one million, six hundred five thousand, six hundred forty-three".