640931 has 4 divisors (see below), whose sum is σ = 643632. Its totient is φ = 638232.

The previous prime is 640919. The next prime is 640933. The reversal of 640931 is 139046.

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 640931 - 2¹⁰ = 639907 is a prime.

It is a super-3 number, since 3×640931³ = 789869033364223473, which contains 333 as substring. Note that it is a super-d number also for d = 2.

It is a Duffinian number.

It is a plaindrome in base 12.

It is a junction number, because it is equal to n+sod(n) for n = 640897 and 640906.

It is not an unprimeable number, because it can be changed into a prime (640933) 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 in 3 ways as a sum of consecutive naturals, for example, 956 + ... + 1481.

It is an arithmetic number, because the mean of its divisors is an integer number (160908).

2⁶⁴⁰⁹³¹ is an apocalyptic number.

640931 is a deficient number, since it is larger than the sum of its proper divisors (2701).

640931 is a wasteful number, since it uses less digits than its factorization.

640931 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2700.

The product of its (nonzero) digits is 648, while the sum is 23.

The square root of 640931 is about 800.5816635422. The cubic root of 640931 is about 86.2191544114.

Adding to 640931 its reverse (139046), we get a palindrome (779977).

The spelling of 640931 in words is "six hundred forty thousand, nine hundred thirty-one".