21808217 has 2 divisors, whose sum is σ = 21808218. Its totient is φ = 21808216.

The previous prime is 21808177. The next prime is 21808231. The reversal of 21808217 is 71280812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20830096 + 978121 = 4564^2 + 989^2 .

It is a cyclic number.

It is not a de Polignac number, because 21808217 - 2⁶ = 21808153 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21808217.

It is not a weakly prime, because it can be changed into another prime (21808247) 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, 10904108 + 10904109.

It is an arithmetic number, because the mean of its divisors is an integer number (10904109).

Almost surely, 2^21808217 is an apocalyptic number.

It is an amenable number.

21808217 is a deficient number, since it is larger than the sum of its proper divisors (1).

21808217 is an equidigital number, since it uses as much as digits as its factorization.

21808217 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1792, while the sum is 29.

The square root of 21808217 is about 4669.9268730891. The cubic root of 21808217 is about 279.3873381469.

The spelling of 21808217 in words is "twenty-one million, eight hundred eight thousand, two hundred seventeen".