666505561 has 2 divisors, whose sum is σ = 666505562. Its totient is φ = 666505560.

The previous prime is 666505537. The next prime is 666505571. The reversal of 666505561 is 165505666.

666505561 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., 552955225 + 113550336 = 23515^2 + 10656^2 .

It is a cyclic number.

It is not a de Polignac number, because 666505561 - 2⁵ = 666505529 is a prime.

It is not a weakly prime, because it can be changed into another prime (666505501) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 333252780 + 333252781.

It is an arithmetic number, because the mean of its divisors is an integer number (333252781).

Almost surely, 2^666505561 is an apocalyptic number.

It is an amenable number.

666505561 is a deficient number, since it is larger than the sum of its proper divisors (1).

666505561 is an equidigital number, since it uses as much as digits as its factorization.

666505561 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 162000, while the sum is 40.

The square root of 666505561 is about 25816.7689883920. The cubic root of 666505561 is about 873.5100896855.

The spelling of 666505561 in words is "six hundred sixty-six million, five hundred five thousand, five hundred sixty-one".