666355981 has 2 divisors, whose sum is σ = 666355982. Its totient is φ = 666355980.

The previous prime is 666355979. The next prime is 666356017. The reversal of 666355981 is 189553666.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 649791081 + 16564900 = 25491^2 + 4070^2 .

It is a cyclic number.

It is not a de Polignac number, because 666355981 - 2¹ = 666355979 is a prime.

It is a super-2 number, since 2×666355981² = 888060586828944722, which contains 22 as substring.

Together with 666355979, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (666356981) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 333177990 + 333177991.

It is an arithmetic number, because the mean of its divisors is an integer number (333177991).

Almost surely, 2^666355981 is an apocalyptic number.

It is an amenable number.

666355981 is a deficient number, since it is larger than the sum of its proper divisors (1).

666355981 is an equidigital number, since it uses as much as digits as its factorization.

666355981 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1166400, while the sum is 49.

The square root of 666355981 is about 25813.8718715345. The cubic root of 666355981 is about 873.4447391856. Note that the first 3 decimals are identical.

The spelling of 666355981 in words is "six hundred sixty-six million, three hundred fifty-five thousand, nine hundred eighty-one".