216605981 has 2 divisors, whose sum is σ = 216605982. Its totient is φ = 216605980.

The previous prime is 216605957. The next prime is 216605989. The reversal of 216605981 is 189506612.

216605981 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 208340356 + 8265625 = 14434^2 + 2875^2 .

It is a cyclic number.

It is not a de Polignac number, because 216605981 - 2¹⁰ = 216604957 is a prime.

It is a super-2 number, since 2×216605981² = 93836302009944722, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (216605989) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 108302990 + 108302991.

It is an arithmetic number, because the mean of its divisors is an integer number (108302991).

Almost surely, 2^216605981 is an apocalyptic number.

It is an amenable number.

216605981 is a deficient number, since it is larger than the sum of its proper divisors (1).

216605981 is an equidigital number, since it uses as much as digits as its factorization.

216605981 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 25920, while the sum is 38.

The square root of 216605981 is about 14717.5399099170. The cubic root of 216605981 is about 600.5605696249.

The spelling of 216605981 in words is "two hundred sixteen million, six hundred five thousand, nine hundred eighty-one".