640831 has 4 divisors (see below), whose sum is σ = 642600. Its totient is φ = 639064.

The previous prime is 640793. The next prime is 640837. The reversal of 640831 is 138046.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-640831 is a prime.

It is a super-2 number, since 2×640831² = 821328741122, which contains 22 as substring.

It is an Ulam number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (640837) 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 in 3 ways as a sum of consecutive naturals, for example, 121 + ... + 1138.

It is an arithmetic number, because the mean of its divisors is an integer number (160650).

2⁶⁴⁰⁸³¹ is an apocalyptic number.

640831 is a deficient number, since it is larger than the sum of its proper divisors (1769).

640831 is a wasteful number, since it uses less digits than its factorization.

640831 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1768.

The product of its (nonzero) digits is 576, while the sum is 22.

The square root of 640831 is about 800.5192065154. The cubic root of 640831 is about 86.2146701202.

Adding to 640831 its reverse (138046), we get a palindrome (778877).

The spelling of 640831 in words is "six hundred forty thousand, eight hundred thirty-one".