640666 has 16 divisors (see below), whose sum is σ = 1061928. Its totient is φ = 288000.

The previous prime is 640663. The next prime is 640667. The reversal of 640666 is 666046.

640666 is nontrivially palindromic in base 5.

640666 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

640666 is an esthetic number in base 8, because in such base its adjacent digits differ by 1.

It can be written as a sum of positive squares in 4 ways, for example, as 385641 + 255025 = 621^2 + 505^2 .

640666 is a modest number, since divided by 666 gives 640 as remainder.

It is not an unprimeable number, because it can be changed into a prime (640663) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 766 + ... + 1366.

2⁶⁴⁰⁶⁶⁶ is an apocalyptic number.

640666 is a deficient number, since it is larger than the sum of its proper divisors (421262).

640666 is a wasteful number, since it uses less digits than its factorization.

640666 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 657.

The product of its (nonzero) digits is 5184, while the sum is 28.

The square root of 640666 is about 800.4161417663. The cubic root of 640666 is about 86.2072700195.

The spelling of 640666 in words is "six hundred forty thousand, six hundred sixty-six".