216821809 has 2 divisors, whose sum is σ = 216821810. Its totient is φ = 216821808.

The previous prime is 216821807. The next prime is 216821921. The reversal of 216821809 is 908128612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 180821809 + 36000000 = 13447^2 + 6000^2 .

It is a cyclic number.

It is not a de Polignac number, because 216821809 - 2¹ = 216821807 is a prime.

Together with 216821807, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (216821807) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 108410904 + 108410905.

It is an arithmetic number, because the mean of its divisors is an integer number (108410905).

Almost surely, 2^216821809 is an apocalyptic number.

It is an amenable number.

216821809 is a deficient number, since it is larger than the sum of its proper divisors (1).

216821809 is an equidigital number, since it uses as much as digits as its factorization.

216821809 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 13824, while the sum is 37.

The square root of 216821809 is about 14724.8704238781. The cubic root of 216821809 is about 600.7599712590.

The spelling of 216821809 in words is "two hundred sixteen million, eight hundred twenty-one thousand, eight hundred nine".