640891 has 2 divisors, whose sum is σ = 640892. Its totient is φ = 640890.

The previous prime is 640873. The next prime is 640901. The reversal of 640891 is 198046.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 640891 - 2⁵ = 640859 is a prime.

It is not a weakly prime, because it can be changed into another prime (640691) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 320445 + 320446.

It is an arithmetic number, because the mean of its divisors is an integer number (320446).

2⁶⁴⁰⁸⁹¹ is an apocalyptic number.

640891 is a deficient number, since it is larger than the sum of its proper divisors (1).

640891 is an equidigital number, since it uses as much as digits as its factorization.

640891 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1728, while the sum is 28.

The square root of 640891 is about 800.5566813162. The cubic root of 640891 is about 86.2173607509.

The spelling of 640891 in words is "six hundred forty thousand, eight hundred ninety-one".