666661381 has 2 divisors, whose sum is σ = 666661382. Its totient is φ = 666661380.

The previous prime is 666661363. The next prime is 666661403. The reversal of 666661381 is 183166666.

666661381 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 621704356 + 44957025 = 24934^2 + 6705^2 .

It is a cyclic number.

It is not a de Polignac number, because 666661381 - 2⁵ = 666661349 is a prime.

It is a super-2 number, since 2×666661381² = 888874793833654322, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (666661321) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 333330690 + 333330691.

It is an arithmetic number, because the mean of its divisors is an integer number (333330691).

Almost surely, 2^666661381 is an apocalyptic number.

It is an amenable number.

666661381 is a deficient number, since it is larger than the sum of its proper divisors (1).

666661381 is an equidigital number, since it uses as much as digits as its factorization.

666661381 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 186624, while the sum is 43.

The square root of 666661381 is about 25819.7866180184. The cubic root of 666661381 is about 873.5781560026.

The spelling of 666661381 in words is "six hundred sixty-six million, six hundred sixty-one thousand, three hundred eighty-one".