640921 has 4 divisors (see below), whose sum is σ = 644644. Its totient is φ = 637200.

The previous prime is 640919. The next prime is 640933. The reversal of 640921 is 129046.

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

It can be written as a sum of positive squares in 2 ways, for example, as 99225 + 541696 = 315^2 + 736^2 .

It is a cyclic number.

It is not a de Polignac number, because 640921 - 2¹ = 640919 is a prime.

It is an Ulam number.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 640892 and 640901.

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, 1590 + ... + 1951.

It is an arithmetic number, because the mean of its divisors is an integer number (161161).

2⁶⁴⁰⁹²¹ is an apocalyptic number.

It is an amenable number.

640921 is a deficient number, since it is larger than the sum of its proper divisors (3723).

640921 is a wasteful number, since it uses less digits than its factorization.

640921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3722.

The product of its (nonzero) digits is 432, while the sum is 22.

The square root of 640921 is about 800.5754180588. The cubic root of 640921 is about 86.2187060033.

Adding to 640921 its reverse (129046), we get a palindrome (769967).

The spelling of 640921 in words is "six hundred forty thousand, nine hundred twenty-one".