640641981 has 16 divisors (see below), whose sum is σ = 1027596800. Its totient is φ = 346813488.

The previous prime is 640641973. The next prime is 640641983. The reversal of 640641981 is 189146046.

It is not a de Polignac number, because 640641981 - 2³ = 640641973 is a prime.

It is a super-2 number, since 2×640641981² = 820844295639208722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (640641983) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 802411 + ... + 803208.

It is an arithmetic number, because the mean of its divisors is an integer number (64224800).

Almost surely, 2^640641981 is an apocalyptic number.

It is an amenable number.

640641981 is a deficient number, since it is larger than the sum of its proper divisors (386954819).

640641981 is a wasteful number, since it uses less digits than its factorization.

640641981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1605648.

The product of its (nonzero) digits is 41472, while the sum is 39.

The square root of 640641981 is about 25310.9063646484. The cubic root of 640641981 is about 862.0619268319.

The spelling of 640641981 in words is "six hundred forty million, six hundred forty-one thousand, nine hundred eighty-one".