640909 has 4 divisors (see below), whose sum is σ = 643104. Its totient is φ = 638716.

The previous prime is 640907. The next prime is 640919. The reversal of 640909 is 909046.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 640909 - 2¹ = 640907 is a prime.

It is a Duffinian number.

640909 is a modest number, since divided by 909 gives 64 as remainder.

640909 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (640901) 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, 577 + ... + 1270.

It is an arithmetic number, because the mean of its divisors is an integer number (160776).

2⁶⁴⁰⁹⁰⁹ is an apocalyptic number.

It is an amenable number.

640909 is a deficient number, since it is larger than the sum of its proper divisors (2195).

640909 is a wasteful number, since it uses less digits than its factorization.

640909 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2194.

The product of its (nonzero) digits is 1944, while the sum is 28.

The square root of 640909 is about 800.5679234144. The cubic root of 640909 is about 86.2181679074.

The spelling of 640909 in words is "six hundred forty thousand, nine hundred nine".