218931721 has 2 divisors, whose sum is σ = 218931722. Its totient is φ = 218931720.

The previous prime is 218931719. The next prime is 218931743. The reversal of 218931721 is 127139812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 179694025 + 39237696 = 13405^2 + 6264^2 .

It is a cyclic number.

It is not a de Polignac number, because 218931721 - 2¹ = 218931719 is a prime.

Together with 218931719, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (218931751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 109465860 + 109465861.

It is an arithmetic number, because the mean of its divisors is an integer number (109465861).

Almost surely, 2^218931721 is an apocalyptic number.

It is an amenable number.

218931721 is a deficient number, since it is larger than the sum of its proper divisors (1).

218931721 is an equidigital number, since it uses as much as digits as its factorization.

218931721 is an evil number, because the sum of its binary digits is even.

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

The square root of 218931721 is about 14796.3414734859. The cubic root of 218931721 is about 602.7023668971.

The spelling of 218931721 in words is "two hundred eighteen million, nine hundred thirty-one thousand, seven hundred twenty-one".