644611021 has 4 divisors (see below), whose sum is σ = 644662044. Its totient is φ = 644560000.

The previous prime is 644611001. The next prime is 644611139. The reversal of 644611021 is 120116446.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 120116446 = 2 ⋅60058223.

It can be written as a sum of positive squares in 2 ways, for example, as 249197796 + 395413225 = 15786^2 + 19885^2 .

It is a cyclic number.

It is not a de Polignac number, because 644611021 - 2²⁷ = 510393293 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 (644611001) 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, 9021 + ... + 37021.

It is an arithmetic number, because the mean of its divisors is an integer number (161165511).

Almost surely, 2^644611021 is an apocalyptic number.

It is an amenable number.

644611021 is a deficient number, since it is larger than the sum of its proper divisors (51023).

644611021 is a wasteful number, since it uses less digits than its factorization.

644611021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 51022.

The product of its (nonzero) digits is 1152, while the sum is 25.

The square root of 644611021 is about 25389.1910268917. The cubic root of 644611021 is about 863.8385387336.

Adding to 644611021 its reverse (120116446), we get a palindrome (764727467).

The spelling of 644611021 in words is "six hundred forty-four million, six hundred eleven thousand, twenty-one".