640691 has 2 divisors, whose sum is σ = 640692. Its totient is φ = 640690.

The previous prime is 640687. The next prime is 640727. The reversal of 640691 is 196046.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 640691 - 2² = 640687 is a prime.

It is a Sophie Germain prime.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 640691.

It is not a weakly prime, because it can be changed into another prime (640621) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 320345 + 320346.

It is an arithmetic number, because the mean of its divisors is an integer number (320346).

2⁶⁴⁰⁶⁹¹ is an apocalyptic number.

640691 is a deficient number, since it is larger than the sum of its proper divisors (1).

640691 is an equidigital number, since it uses as much as digits as its factorization.

640691 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1296, while the sum is 26.

The square root of 640691 is about 800.4317584904. The cubic root of 640691 is about 86.2083913286.

It can be divided in two parts, 640 and 691, that added together give a palindrome (1331).

The spelling of 640691 in words is "six hundred forty thousand, six hundred ninety-one".