216991 has 2 divisors, whose sum is σ = 216992. Its totient is φ = 216990.

The previous prime is 216973. The next prime is 217001. The reversal of 216991 is 199612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 216991 - 2¹¹ = 214943 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (216901) 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, 108495 + 108496.

It is an arithmetic number, because the mean of its divisors is an integer number (108496).

2²¹⁶⁹⁹¹ is an apocalyptic number.

216991 is a deficient number, since it is larger than the sum of its proper divisors (1).

216991 is an equidigital number, since it uses as much as digits as its factorization.

216991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 972, while the sum is 28.

The square root of 216991 is about 465.8229277311. The cubic root of 216991 is about 60.0916192865.

The spelling of 216991 in words is "two hundred sixteen thousand, nine hundred ninety-one".