217988821 has 2 divisors, whose sum is σ = 217988822. Its totient is φ = 217988820.

The previous prime is 217988819. The next prime is 217988831. The reversal of 217988821 is 128889712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 149181796 + 68807025 = 12214^2 + 8295^2 .

It is a cyclic number.

It is not a de Polignac number, because 217988821 - 2¹ = 217988819 is a prime.

Together with 217988819, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (217988831) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 108994410 + 108994411.

It is an arithmetic number, because the mean of its divisors is an integer number (108994411).

Almost surely, 2^217988821 is an apocalyptic number.

It is an amenable number.

217988821 is a deficient number, since it is larger than the sum of its proper divisors (1).

217988821 is an equidigital number, since it uses as much as digits as its factorization.

217988821 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 129024, while the sum is 46.

The square root of 217988821 is about 14764.4444866713. Note that the first 4 decimals coincide. The cubic root of 217988821 is about 601.8358777845.

The spelling of 217988821 in words is "two hundred seventeen million, nine hundred eighty-eight thousand, eight hundred twenty-one".