217680821 has 2 divisors, whose sum is σ = 217680822. Its totient is φ = 217680820.

The previous prime is 217680803. The next prime is 217680857. The reversal of 217680821 is 128086712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 215678596 + 2002225 = 14686^2 + 1415^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-217680821 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (217680871) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 108840410 + 108840411.

It is an arithmetic number, because the mean of its divisors is an integer number (108840411).

Almost surely, 2^217680821 is an apocalyptic number.

It is an amenable number.

217680821 is a deficient number, since it is larger than the sum of its proper divisors (1).

217680821 is an equidigital number, since it uses as much as digits as its factorization.

217680821 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 10752, while the sum is 35.

The square root of 217680821 is about 14754.0103361764. The cubic root of 217680821 is about 601.5522962380.

The spelling of 217680821 in words is "two hundred seventeen million, six hundred eighty thousand, eight hundred twenty-one".