644138591 has 4 divisors (see below), whose sum is σ = 644198784. Its totient is φ = 644078400.

The previous prime is 644138569. The next prime is 644138617. The reversal of 644138591 is 195831446.

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: 195831446 = 2 ⋅97915723.

It is a cyclic number.

It is not a de Polignac number, because 644138591 - 2¹⁴ = 644122207 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 (644131591) 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, 9215 + ... + 37056.

It is an arithmetic number, because the mean of its divisors is an integer number (161049696).

Almost surely, 2^644138591 is an apocalyptic number.

644138591 is a deficient number, since it is larger than the sum of its proper divisors (60193).

644138591 is a wasteful number, since it uses less digits than its factorization.

644138591 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 60192.

The product of its digits is 103680, while the sum is 41.

The square root of 644138591 is about 25379.8855592377. The cubic root of 644138591 is about 863.6274538226.

The spelling of 644138591 in words is "six hundred forty-four million, one hundred thirty-eight thousand, five hundred ninety-one".