21209209 has 12 divisors (see below), whose sum is σ = 24988800. Its totient is φ = 17945928.

The previous prime is 21209207. The next prime is 21209231. The reversal of 21209209 is 90290212.

21209209 = T₄₆₂ + T₄₆₃ + ... + T₆₀₈.

It is not a de Polignac number, because 21209209 - 2¹ = 21209207 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21209207) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 1132 + ... + 6610.

It is an arithmetic number, because the mean of its divisors is an integer number (2082400).

Almost surely, 2^21209209 is an apocalyptic number.

It is an amenable number.

21209209 is a deficient number, since it is larger than the sum of its proper divisors (3779591).

21209209 is an equidigital number, since it uses as much as digits as its factorization.

21209209 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5572 (or 5565 counting only the distinct ones).

The product of its (nonzero) digits is 648, while the sum is 25.

The square root of 21209209 is about 4605.3456982077. The cubic root of 21209209 is about 276.8055693537.

The spelling of 21209209 in words is "twenty-one million, two hundred nine thousand, two hundred nine".