21009 has 8 divisors (see below), whose sum is σ = 28800. Its totient is φ = 13616.

The previous prime is 21001. The next prime is 21011. The reversal of 21009 is 90012.

It is a happy number.

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

It is a cyclic number.

It is not a de Polignac number, because 21009 - 2³ = 21001 is a prime.

It is a Curzon number.

It is a nialpdrome in base 13 and base 16.

It is a junction number, because it is equal to n+sod(n) for n = 20985 and 21003.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21001) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 67 + ... + 215.

It is an arithmetic number, because the mean of its divisors is an integer number (3600).

2²¹⁰⁰⁹ is an apocalyptic number.

It is an amenable number.

21009 is a deficient number, since it is larger than the sum of its proper divisors (7791).

21009 is a wasteful number, since it uses less digits than its factorization.

21009 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 199.

The product of its (nonzero) digits is 18, while the sum is 12.

The square root of 21009 is about 144.9448170857. The cubic root of 21009 is about 27.5931825212.

The spelling of 21009 in words is "twenty-one thousand, nine".