640660681 has 4 divisors (see below), whose sum is σ = 646648272. Its totient is φ = 634673092.

The previous prime is 640660673. The next prime is 640660687. The reversal of 640660681 is 186066046.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 186066046 = 2 ⋅93033023.

It is a cyclic number.

It is not a de Polignac number, because 640660681 - 2³ = 640660673 is a prime.

It is a super-2 number, since 2×640660681² = 820892216358767522, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 640660681.

It is not an unprimeable number, because it can be changed into a prime (640660687) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2993635 + ... + 2993848.

It is an arithmetic number, because the mean of its divisors is an integer number (161662068).

Almost surely, 2^640660681 is an apocalyptic number.

It is an amenable number.

640660681 is a deficient number, since it is larger than the sum of its proper divisors (5987591).

640660681 is a wasteful number, since it uses less digits than its factorization.

640660681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5987590.

The product of its (nonzero) digits is 41472, while the sum is 37.

The square root of 640660681 is about 25311.2757679260. The cubic root of 640660681 is about 862.0703144606.

The spelling of 640660681 in words is "six hundred forty million, six hundred sixty thousand, six hundred eighty-one".