666561664 has 16 divisors (see below), whose sum is σ = 1327916070. Its totient is φ = 333280768.

The previous prime is 666561647. The next prime is 666561677. The reversal of 666561664 is 466165666.

666561664 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It can be written as a sum of positive squares in only one way, i.e., 581774400 + 84787264 = 24120^2 + 9208^2 .

It is a tau number, because it is divible by the number of its divisors (16).

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 2603629 + ... + 2603884.

Almost surely, 2^666561664 is an apocalyptic number.

666561664 is a gapful number since it is divisible by the number (64) formed by its first and last digit.

It is an amenable number.

666561664 is a deficient number, since it is larger than the sum of its proper divisors (661354406).

666561664 is an equidigital number, since it uses as much as digits as its factorization.

666561664 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5207527 (or 5207515 counting only the distinct ones).

The product of its digits is 933120, while the sum is 46.

The square root of 666561664 is about 25817.8555267474. The cubic root of 666561664 is about 873.5345981890.

It can be divided in two parts, 666 and 561664, that added together give a triangular number (562330 = T₁₀₆₀).

The spelling of 666561664 in words is "six hundred sixty-six million, five hundred sixty-one thousand, six hundred sixty-four".