21309 has 4 divisors (see below), whose sum is σ = 28416. Its totient is φ = 14204.

The previous prime is 21283. The next prime is 21313. The reversal of 21309 is 90312.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21309 - 2⁵ = 21277 is a prime.

It is a D-number.

21309 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21319) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3549 + ... + 3554.

It is an arithmetic number, because the mean of its divisors is an integer number (7104).

2²¹³⁰⁹ is an apocalyptic number.

It is an amenable number.

21309 is a deficient number, since it is larger than the sum of its proper divisors (7107).

21309 is an equidigital number, since it uses as much as digits as its factorization.

21309 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7106.

The product of its (nonzero) digits is 54, while the sum is 15.

The square root of 21309 is about 145.9760254288. The cubic root of 21309 is about 27.7239020929.

The spelling of 21309 in words is "twenty-one thousand, three hundred nine".