216666661 has 2 divisors, whose sum is σ = 216666662. Its totient is φ = 216666660.

The previous prime is 216666643. The next prime is 216666691. The reversal of 216666661 is 166666612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 190605636 + 26061025 = 13806^2 + 5105^2 .

It is a cyclic number.

It is not a de Polignac number, because 216666661 - 2²⁵ = 183112229 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (216666631) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 108333330 + 108333331.

It is an arithmetic number, because the mean of its divisors is an integer number (108333331).

Almost surely, 2^216666661 is an apocalyptic number.

It is an amenable number.

216666661 is a deficient number, since it is larger than the sum of its proper divisors (1).

216666661 is an equidigital number, since it uses as much as digits as its factorization.

216666661 is an evil number, because the sum of its binary digits is even.

The product of its digits is 93312, while the sum is 40.

The square root of 216666661 is about 14719.6012513926. The cubic root of 216666661 is about 600.6166447354.

The spelling of 216666661 in words is "two hundred sixteen million, six hundred sixty-six thousand, six hundred sixty-one".