666405781 has 2 divisors, whose sum is σ = 666405782. Its totient is φ = 666405780.

The previous prime is 666405757. The next prime is 666405823. The reversal of 666405781 is 187504666.

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., 455139556 + 211266225 = 21334^2 + 14535^2 .

It is a cyclic number.

It is not a de Polignac number, because 666405781 - 2⁷ = 666405653 is a prime.

It is a super-2 number, since 2×666405781² = 888193329900439922, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (666405581) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 333202890 + 333202891.

It is an arithmetic number, because the mean of its divisors is an integer number (333202891).

Almost surely, 2^666405781 is an apocalyptic number.

It is an amenable number.

666405781 is a deficient number, since it is larger than the sum of its proper divisors (1).

666405781 is an equidigital number, since it uses as much as digits as its factorization.

666405781 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 241920, while the sum is 43.

The square root of 666405781 is about 25814.8364511573. The cubic root of 666405781 is about 873.4664975578.

The spelling of 666405781 in words is "six hundred sixty-six million, four hundred five thousand, seven hundred eighty-one".