216781 has 2 divisors, whose sum is σ = 216782. Its totient is φ = 216780.

The previous prime is 216779. The next prime is 216787. The reversal of 216781 is 187612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 189225 + 27556 = 435^2 + 166^2 .

It is a cyclic number.

It is not a de Polignac number, because 216781 - 2¹ = 216779 is a prime.

It is a super-2 number, since 2×216781² = 93988003922, which contains 22 as substring.

Together with 216779, it forms a pair of twin primes.

It is an alternating number because its digits alternate between even and odd.

It is a nialpdrome in base 12.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (216787) 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, 108390 + 108391.

It is an arithmetic number, because the mean of its divisors is an integer number (108391).

2²¹⁶⁷⁸¹ is an apocalyptic number.

It is an amenable number.

216781 is a deficient number, since it is larger than the sum of its proper divisors (1).

216781 is an equidigital number, since it uses as much as digits as its factorization.

216781 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 672, while the sum is 25.

The square root of 216781 is about 465.5974656288. The cubic root of 216781 is about 60.0722278323.

The spelling of 216781 in words is "two hundred sixteen thousand, seven hundred eighty-one".