21209 has 4 divisors (see below), whose sum is σ = 21504. Its totient is φ = 20916.

The previous prime is 21193. The next prime is 21211. The reversal of 21209 is 90212.

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, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21209 - 2⁴ = 21193 is a prime.

It is a Duffinian number.

It is a plaindrome in base 7.

It is a nialpdrome in base 13.

It is a junction number, because it is equal to n+sod(n) for n = 21193 and 21202.

It is not an unprimeable number, because it can be changed into a prime (21269) 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, 44 + ... + 210.

It is an arithmetic number, because the mean of its divisors is an integer number (5376).

2²¹²⁰⁹ is an apocalyptic number.

It is an amenable number.

21209 is a deficient number, since it is larger than the sum of its proper divisors (295).

21209 is a wasteful number, since it uses less digits than its factorization.

21209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 294.

The product of its (nonzero) digits is 36, while the sum is 14.

The square root of 21209 is about 145.6331006331. The cubic root of 21209 is about 27.6804660117.

The spelling of 21209 in words is "twenty-one thousand, two hundred nine".