21909 has 8 divisors (see below), whose sum is σ = 29920. Its totient is φ = 14256.

The previous prime is 21893. The next prime is 21911. The reversal of 21909 is 90912.

21909 = 26² + 27² + ... + 43².

It is a happy number.

21909 is nontrivially palindromic in base 8.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21909 - 2⁴ = 21893 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21929) 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, 147 + ... + 255.

It is an arithmetic number, because the mean of its divisors is an integer number (3740).

2²¹⁹⁰⁹ is an apocalyptic number.

It is an amenable number.

21909 is a deficient number, since it is larger than the sum of its proper divisors (8011).

21909 is a wasteful number, since it uses less digits than its factorization.

21909 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 179.

The product of its (nonzero) digits is 162, while the sum is 21.

The square root of 21909 is about 148.0168909280. The cubic root of 21909 is about 27.9817057368.

The spelling of 21909 in words is "twenty-one thousand, nine hundred nine".