644851763681 has 4 divisors (see below), whose sum is σ = 646885996692. Its totient is φ = 642817530672.

The previous prime is 644851763641. The next prime is 644851763683. The reversal of 644851763681 is 186367158446.

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

It can be written as a sum of positive squares in 2 ways, for example, as 56838851281 + 588012912400 = 238409^2 + 766820^2 .

It is a cyclic number.

It is not a de Polignac number, because 644851763681 - 2⁶ = 644851763617 is a prime.

It is a super-2 number, since 2×644851763681² (a number of 24 digits) contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 644851763681.

It is not an unprimeable number, because it can be changed into a prime (644851763683) 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, 1017116030 + ... + 1017116663.

It is an arithmetic number, because the mean of its divisors is an integer number (161721499173).

Almost surely, 2^644851763681 is an apocalyptic number.

It is an amenable number.

644851763681 is a deficient number, since it is larger than the sum of its proper divisors (2034233011).

644851763681 is a wasteful number, since it uses less digits than its factorization.

644851763681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2034233010.

The product of its digits is 23224320, while the sum is 59.

The spelling of 644851763681 in words is "six hundred forty-four billion, eight hundred fifty-one million, seven hundred sixty-three thousand, six hundred eighty-one".