644681 has 4 divisors (see below), whose sum is σ = 646932. Its totient is φ = 642432.

The previous prime is 644671. The next prime is 644687. The reversal of 644681 is 186446.

644681 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

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 379456 + 265225 = 616^2 + 515^2 .

It is a cyclic number.

It is not a de Polignac number, because 644681 - 2⁶ = 644617 is a prime.

It is a super-2 number, since 2×644681² = 831227183522, which contains 22 as substring.

It is a pancake number, because a pancake can be divided into 644681 parts by 1135 straight cuts.

It is a Duffinian number.

It is a Curzon number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 644681.

It is not an unprimeable number, because it can be changed into a prime (644687) 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, 620 + ... + 1293.

It is an arithmetic number, because the mean of its divisors is an integer number (161733).

2⁶⁴⁴⁶⁸¹ is an apocalyptic number.

It is an amenable number.

644681 is a deficient number, since it is larger than the sum of its proper divisors (2251).

644681 is a wasteful number, since it uses less digits than its factorization.

644681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2250.

The product of its digits is 4608, while the sum is 29.

The square root of 644681 is about 802.9202949235. The cubic root of 644681 is about 86.3869797052.

The spelling of 644681 in words is "six hundred forty-four thousand, six hundred eighty-one".