640821 has 8 divisors (see below), whose sum is σ = 859104. Its totient is φ = 424880.

The previous prime is 640793. The next prime is 640837. The reversal of 640821 is 128046.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 640821 - 2¹³ = 632629 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (640891) 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 7 ways as a sum of consecutive naturals, for example, 211 + ... + 1151.

It is an arithmetic number, because the mean of its divisors is an integer number (107388).

2⁶⁴⁰⁸²¹ is an apocalyptic number.

It is an amenable number.

640821 is a deficient number, since it is larger than the sum of its proper divisors (218283).

640821 is a wasteful number, since it uses less digits than its factorization.

640821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1171.

The product of its (nonzero) digits is 384, while the sum is 21.

The square root of 640821 is about 800.5129605447. The cubic root of 640821 is about 86.2142216654.

Adding to 640821 its reverse (128046), we get a palindrome (768867).

The spelling of 640821 in words is "six hundred forty thousand, eight hundred twenty-one".