21809 has 4 divisors (see below), whose sum is σ = 22116. Its totient is φ = 21504.

The previous prime is 21803. The next prime is 21817. The reversal of 21809 is 90812.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 2209 + 19600 = 47^2 + 140^2 .

It is a cyclic number.

It is not a de Polignac number, because 21809 - 2¹² = 17713 is a prime.

It is a Duffinian number.

It is a plaindrome in base 15.

It is a nialpdrome in base 16.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21809.

It is not an unprimeable number, because it can be changed into a prime (21803) 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, 17 + ... + 209.

It is an arithmetic number, because the mean of its divisors is an integer number (5529).

2²¹⁸⁰⁹ is an apocalyptic number.

It is an amenable number.

21809 is a deficient number, since it is larger than the sum of its proper divisors (307).

21809 is a wasteful number, since it uses less digits than its factorization.

21809 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 306.

The product of its (nonzero) digits is 144, while the sum is 20.

The square root of 21809 is about 147.6787053031. The cubic root of 21809 is about 27.9390681805.

The spelling of 21809 in words is "twenty-one thousand, eight hundred nine".