644821 has 4 divisors (see below), whose sum is σ = 649600. Its totient is φ = 640044.

The previous prime is 644801. The next prime is 644837. The reversal of 644821 is 128446.

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, and also an emirpimes, since its reverse is a distinct semiprime: 128446 = 2 ⋅64223.

It is a cyclic number.

It is not a de Polignac number, because 644821 - 2⁵ = 644789 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (644801) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2181 + ... + 2458.

It is an arithmetic number, because the mean of its divisors is an integer number (162400).

2⁶⁴⁴⁸²¹ is an apocalyptic number.

It is an amenable number.

644821 is a deficient number, since it is larger than the sum of its proper divisors (4779).

644821 is a wasteful number, since it uses less digits than its factorization.

644821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4778.

The product of its digits is 1536, while the sum is 25.

The square root of 644821 is about 803.0074719453. The cubic root of 644821 is about 86.3932325661.

The spelling of 644821 in words is "six hundred forty-four thousand, eight hundred twenty-one".