21666209 has 2 divisors, whose sum is σ = 21666210. Its totient is φ = 21666208.

The previous prime is 21666191. The next prime is 21666247. The reversal of 21666209 is 90266612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 16581184 + 5085025 = 4072^2 + 2255^2 .

It is a cyclic number.

It is not a de Polignac number, because 21666209 - 2¹⁶ = 21600673 is a prime.

It is a super-2 number, since 2×21666209² = 938849224863362, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21666709) 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 as a sum of consecutive naturals, namely, 10833104 + 10833105.

It is an arithmetic number, because the mean of its divisors is an integer number (10833105).

Almost surely, 2^21666209 is an apocalyptic number.

It is an amenable number.

21666209 is a deficient number, since it is larger than the sum of its proper divisors (1).

21666209 is an equidigital number, since it uses as much as digits as its factorization.

21666209 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 7776, while the sum is 32.

The square root of 21666209 is about 4654.6975197106. The cubic root of 21666209 is about 278.7795906571.

The spelling of 21666209 in words is "twenty-one million, six hundred sixty-six thousand, two hundred nine".