640644391 has 4 divisors (see below), whose sum is σ = 640719400. Its totient is φ = 640569384.

The previous prime is 640644383. The next prime is 640644397. The reversal of 640644391 is 193446046.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 640644391 - 2³ = 640644383 is a prime.

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 640644391.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (640644397) by changing a digit.

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 3 ways as a sum of consecutive naturals, for example, 22761 + ... + 42418.

It is an arithmetic number, because the mean of its divisors is an integer number (160179850).

Almost surely, 2^640644391 is an apocalyptic number.

640644391 is a deficient number, since it is larger than the sum of its proper divisors (75009).

640644391 is an equidigital number, since it uses as much as digits as its factorization.

640644391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 75008.

The product of its (nonzero) digits is 62208, while the sum is 37.

The square root of 640644391 is about 25310.9539725392. The cubic root of 640644391 is about 862.0630078135.

The spelling of 640644391 in words is "six hundred forty million, six hundred forty-four thousand, three hundred ninety-one".