218643721 has 2 divisors, whose sum is σ = 218643722. Its totient is φ = 218643720.

The previous prime is 218643701. The next prime is 218643739. The reversal of 218643721 is 127346812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 137733696 + 80910025 = 11736^2 + 8995^2 .

It is a cyclic number.

It is not a de Polignac number, because 218643721 - 2¹¹ = 218641673 is a prime.

It is not a weakly prime, because it can be changed into another prime (218643701) 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 as a sum of consecutive naturals, namely, 109321860 + 109321861.

It is an arithmetic number, because the mean of its divisors is an integer number (109321861).

Almost surely, 2^218643721 is an apocalyptic number.

It is an amenable number.

218643721 is a deficient number, since it is larger than the sum of its proper divisors (1).

218643721 is an equidigital number, since it uses as much as digits as its factorization.

218643721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 16128, while the sum is 34.

The square root of 218643721 is about 14786.6061352834. The cubic root of 218643721 is about 602.4379702329.

The spelling of 218643721 in words is "two hundred eighteen million, six hundred forty-three thousand, seven hundred twenty-one".