666644521 has 2 divisors, whose sum is σ = 666644522. Its totient is φ = 666644520.

The previous prime is 666644519. The next prime is 666644551. The reversal of 666644521 is 125446666.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 665898025 + 746496 = 25805^2 + 864^2 .

It is a cyclic number.

It is not a de Polignac number, because 666644521 - 2¹ = 666644519 is a prime.

Together with 666644519, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (666644501) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 333322260 + 333322261.

It is an arithmetic number, because the mean of its divisors is an integer number (333322261).

Almost surely, 2^666644521 is an apocalyptic number.

It is an amenable number.

666644521 is a deficient number, since it is larger than the sum of its proper divisors (1).

666644521 is an equidigital number, since it uses as much as digits as its factorization.

666644521 is an evil number, because the sum of its binary digits is even.

The product of its digits is 207360, while the sum is 40.

The square root of 666644521 is about 25819.4601221637. The cubic root of 666644521 is about 873.5707916183.

The spelling of 666644521 in words is "six hundred sixty-six million, six hundred forty-four thousand, five hundred twenty-one".