644981 has 4 divisors (see below), whose sum is σ = 658752. Its totient is φ = 631212.

The previous prime is 644977. The next prime is 644999. The reversal of 644981 is 189446.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 189446 = 2 ⋅94723.

It is a cyclic number.

It is not a de Polignac number, because 644981 - 2² = 644977 is a prime.

It is a super-2 number, since 2×644981² = 832000980722, which contains 22 as substring.

It is a Curzon number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 644981.

It is not an unprimeable number, because it can be changed into a prime (644911) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 6815 + ... + 6908.

It is an arithmetic number, because the mean of its divisors is an integer number (164688).

2⁶⁴⁴⁹⁸¹ is an apocalyptic number.

It is an amenable number.

644981 is a deficient number, since it is larger than the sum of its proper divisors (13771).

644981 is a wasteful number, since it uses less digits than its factorization.

644981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 13770.

The product of its digits is 6912, while the sum is 32.

The square root of 644981 is about 803.1070912400. The cubic root of 644981 is about 86.4003775847.

Subtracting from 644981 its reverse (189446), we obtain a triangular number (455535 = T₉₅₄).

The spelling of 644981 in words is "six hundred forty-four thousand, nine hundred eighty-one".