640661988 has 96 divisors (see below), whose sum is σ = 1628605440. Its totient is φ = 195350400.

The previous prime is 640661963. The next prime is 640661999. The reversal of 640661988 is 889166046.

It is a super-2 number, since 2×640661988² = 820895565736224288, which contains 22 as substring.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 989881 + ... + 990527.

It is an arithmetic number, because the mean of its divisors is an integer number (16964640).

Almost surely, 2^640661988 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 640661988, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (814302720).

640661988 is an abundant number, since it is smaller than the sum of its proper divisors (987943452).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

640661988 is a wasteful number, since it uses less digits than its factorization.

640661988 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 817 (or 815 counting only the distinct ones).

The product of its (nonzero) digits is 497664, while the sum is 48.

The square root of 640661988 is about 25311.3015864455. The cubic root of 640661988 is about 862.0709006914.

The spelling of 640661988 in words is "six hundred forty million, six hundred sixty-one thousand, nine hundred eighty-eight".