644021 has 4 divisors (see below), whose sum is σ = 736032. Its totient is φ = 552012.

The previous prime is 644009. The next prime is 644029. The reversal of 644021 is 120446.

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: 120446 = 2 ⋅60223.

It is a cyclic number.

It is not a de Polignac number, because 644021 - 2⁶ = 643957 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (644029) 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, 45995 + ... + 46008.

It is an arithmetic number, because the mean of its divisors is an integer number (184008).

2⁶⁴⁴⁰²¹ is an apocalyptic number.

It is an amenable number.

644021 is a deficient number, since it is larger than the sum of its proper divisors (92011).

644021 is an equidigital number, since it uses as much as digits as its factorization.

644021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 92010.

The product of its (nonzero) digits is 192, while the sum is 17.

The square root of 644021 is about 802.5091899785. The cubic root of 644021 is about 86.3574897329.

Adding to 644021 its reverse (120446), we get a palindrome (764467).

The spelling of 644021 in words is "six hundred forty-four thousand, twenty-one".