644839781615 has 4 divisors (see below), whose sum is σ = 773807737944. Its totient is φ = 515871825288.

The previous prime is 644839781491. The next prime is 644839781617. The reversal of 644839781615 is 516187938446.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 644839781615 - 2¹⁰ = 644839780591 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 (644839781617) 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, 64483978157 + ... + 64483978166.

It is an arithmetic number, because the mean of its divisors is an integer number (193451934486).

Almost surely, 2^644839781615 is an apocalyptic number.

644839781615 is a deficient number, since it is larger than the sum of its proper divisors (128967956329).

644839781615 is a wasteful number, since it uses less digits than its factorization.

644839781615 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 128967956328.

The product of its digits is 34836480, while the sum is 62.

The spelling of 644839781615 in words is "six hundred forty-four billion, eight hundred thirty-nine million, seven hundred eighty-one thousand, six hundred fifteen".