21231209 has 4 divisors (see below), whose sum is σ = 21591120. Its totient is φ = 20871300.

The previous prime is 21231173. The next prime is 21231241. The reversal of 21231209 is 90213212.

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 21231209 - 2¹² = 21227113 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21231269) 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, 179867 + ... + 179984.

It is an arithmetic number, because the mean of its divisors is an integer number (5397780).

Almost surely, 2^21231209 is an apocalyptic number.

It is an amenable number.

21231209 is a deficient number, since it is larger than the sum of its proper divisors (359911).

21231209 is an equidigital number, since it uses as much as digits as its factorization.

21231209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 359910.

The product of its (nonzero) digits is 216, while the sum is 20.

The square root of 21231209 is about 4607.7336077512. The cubic root of 21231209 is about 276.9012450599.

The spelling of 21231209 in words is "twenty-one million, two hundred thirty-one thousand, two hundred nine".