21180209 has 4 divisors (see below), whose sum is σ = 21672816. Its totient is φ = 20687604.

The previous prime is 21180197. The next prime is 21180227. The reversal of 21180209 is 90208112.

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 21180209 - 2¹⁶ = 21114673 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21180239) 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, 246239 + ... + 246324.

It is an arithmetic number, because the mean of its divisors is an integer number (5418204).

Almost surely, 2^21180209 is an apocalyptic number.

It is an amenable number.

21180209 is a deficient number, since it is larger than the sum of its proper divisors (492607).

21180209 is an equidigital number, since it uses as much as digits as its factorization.

21180209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 492606.

The product of its (nonzero) digits is 288, while the sum is 23.

The square root of 21180209 is about 4602.1961062084. The cubic root of 21180209 is about 276.6793502354.

The spelling of 21180209 in words is "twenty-one million, one hundred eighty thousand, two hundred nine".