21643 has 4 divisors (see below), whose sum is σ = 22608. Its totient is φ = 20680.

The previous prime is 21617. The next prime is 21647. The reversal of 21643 is 34612.

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 21643 - 2⁵ = 21611 is a prime.

It is an Ulam number.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21647) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 448 + ... + 493.

It is an arithmetic number, because the mean of its divisors is an integer number (5652).

2²¹⁶⁴³ is an apocalyptic number.

21643 is a gapful number since it is divisible by the number (23) formed by its first and last digit.

21643 is a deficient number, since it is larger than the sum of its proper divisors (965).

21643 is an equidigital number, since it uses as much as digits as its factorization.

21643 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 964.

The product of its digits is 144, while the sum is 16.

The square root of 21643 is about 147.1156008043. The cubic root of 21643 is about 27.8680011520.

The spelling of 21643 in words is "twenty-one thousand, six hundred forty-three".