218391721 has 6 divisors (see below), whose sum is σ = 226183022. Its totient is φ = 210860160.

The previous prime is 218391703. The next prime is 218391727. The reversal of 218391721 is 127193812.

It can be written as a sum of positive squares in 3 ways, for example, as 1860496 + 216531225 = 1364^2 + 14715^2 .

It is not a de Polignac number, because 218391721 - 2⁵ = 218391689 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (218391727) 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 in 5 ways as a sum of consecutive naturals, for example, 129000 + ... + 130681.

Almost surely, 2^218391721 is an apocalyptic number.

It is an amenable number.

218391721 is a deficient number, since it is larger than the sum of its proper divisors (7791301).

218391721 is an equidigital number, since it uses as much as digits as its factorization.

218391721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 259739 (or 259710 counting only the distinct ones).

The product of its digits is 6048, while the sum is 34.

The square root of 218391721 is about 14778.0824534173. The cubic root of 218391721 is about 602.2064326273.

The spelling of 218391721 in words is "two hundred eighteen million, three hundred ninety-one thousand, seven hundred twenty-one".