644550021 has 8 divisors (see below), whose sum is σ = 954888960. Its totient is φ = 429699996.

The previous prime is 644550007. The next prime is 644550037. The reversal of 644550021 is 120055446.

It is not a de Polignac number, because 644550021 - 2⁶ = 644549957 is a prime.

It is a super-2 number, since 2×644550021² = 830889459142200882, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27), and also a Moran number because the ratio is a prime number: 23872223 = 644550021 / (6 + 4 + 4 + 5 + 5 + 0 + 0 + 2 + 1).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (644550061) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 11936085 + ... + 11936138.

It is an arithmetic number, because the mean of its divisors is an integer number (119361120).

Almost surely, 2^644550021 is an apocalyptic number.

It is an amenable number.

644550021 is a deficient number, since it is larger than the sum of its proper divisors (310338939).

644550021 is a wasteful number, since it uses less digits than its factorization.

644550021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 23872232 (or 23872226 counting only the distinct ones).

The product of its (nonzero) digits is 4800, while the sum is 27.

The square root of 644550021 is about 25387.9896998561. The cubic root of 644550021 is about 863.8112893220.

The spelling of 644550021 in words is "six hundred forty-four million, five hundred fifty thousand, twenty-one".