21855209 has 2 divisors, whose sum is σ = 21855210. Its totient is φ = 21855208.

The previous prime is 21855191. The next prime is 21855227. The reversal of 21855209 is 90255812.

It is a balanced prime because it is at equal distance from previous prime (21855191) and next prime (21855227).

It can be written as a sum of positive squares in only one way, i.e., 15960025 + 5895184 = 3995^2 + 2428^2 .

It is a cyclic number.

It is not a de Polignac number, because 21855209 - 2¹² = 21851113 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21855259) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10927604 + 10927605.

It is an arithmetic number, because the mean of its divisors is an integer number (10927605).

Almost surely, 2^21855209 is an apocalyptic number.

It is an amenable number.

21855209 is a deficient number, since it is larger than the sum of its proper divisors (1).

21855209 is an equidigital number, since it uses as much as digits as its factorization.

21855209 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 7200, while the sum is 32.

The square root of 21855209 is about 4674.9555078097. The cubic root of 21855209 is about 279.5878673180.

The spelling of 21855209 in words is "twenty-one million, eight hundred fifty-five thousand, two hundred nine".