217981 has 2 divisors, whose sum is σ = 217982. Its totient is φ = 217980.

The previous prime is 217979. The next prime is 218003. The reversal of 217981 is 189712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 122500 + 95481 = 350^2 + 309^2 .

It is a cyclic number.

It is not a de Polignac number, because 217981 - 2¹ = 217979 is a prime.

It is a super-2 number, since 2×217981² = 95031432722, which contains 22 as substring.

Together with 217979, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (217901) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 108990 + 108991.

It is an arithmetic number, because the mean of its divisors is an integer number (108991).

2²¹⁷⁹⁸¹ is an apocalyptic number.

It is an amenable number.

217981 is a deficient number, since it is larger than the sum of its proper divisors (1).

217981 is an equidigital number, since it uses as much as digits as its factorization.

217981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1008, while the sum is 28.

The square root of 217981 is about 466.8843539893. The cubic root of 217981 is about 60.1828680145.

The spelling of 217981 in words is "two hundred seventeen thousand, nine hundred eighty-one".