21604028209 has 2 divisors, whose sum is σ = 21604028210. Its totient is φ = 21604028208.

The previous prime is 21604028207. The next prime is 21604028231. The reversal of 21604028209 is 90282040612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15072963984 + 6531064225 = 122772^2 + 80815^2 .

It is a cyclic number.

It is not a de Polignac number, because 21604028209 - 2¹ = 21604028207 is a prime.

Together with 21604028207, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (21604028203) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10802014104 + 10802014105.

It is an arithmetic number, because the mean of its divisors is an integer number (10802014105).

Almost surely, 2^21604028209 is an apocalyptic number.

It is an amenable number.

21604028209 is a deficient number, since it is larger than the sum of its proper divisors (1).

21604028209 is an equidigital number, since it uses as much as digits as its factorization.

21604028209 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 13824, while the sum is 34.

The spelling of 21604028209 in words is "twenty-one billion, six hundred four million, twenty-eight thousand, two hundred nine".