216891721 has 2 divisors, whose sum is σ = 216891722. Its totient is φ = 216891720.

The previous prime is 216891683. The next prime is 216891733. The reversal of 216891721 is 127198612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 162205696 + 54686025 = 12736^2 + 7395^2 .

It is a cyclic number.

It is not a de Polignac number, because 216891721 - 2⁹ = 216891209 is a prime.

It is not a weakly prime, because it can be changed into another prime (216892721) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 108445860 + 108445861.

It is an arithmetic number, because the mean of its divisors is an integer number (108445861).

Almost surely, 2^216891721 is an apocalyptic number.

It is an amenable number.

216891721 is a deficient number, since it is larger than the sum of its proper divisors (1).

216891721 is an equidigital number, since it uses as much as digits as its factorization.

216891721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 12096, while the sum is 37.

The square root of 216891721 is about 14727.2441753371. The cubic root of 216891721 is about 600.8245339797.

The spelling of 216891721 in words is "two hundred sixteen million, eight hundred ninety-one thousand, seven hundred twenty-one".