644991 has 8 divisors (see below), whose sum is σ = 865920. Its totient is φ = 427032.

The previous prime is 644977. The next prime is 644999. The reversal of 644991 is 199446.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 644991 - 2⁷ = 644863 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (644999) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 171 + ... + 1148.

It is an arithmetic number, because the mean of its divisors is an integer number (108240).

2⁶⁴⁴⁹⁹¹ is an apocalyptic number.

644991 is a deficient number, since it is larger than the sum of its proper divisors (220929).

644991 is a wasteful number, since it uses less digits than its factorization.

644991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1485.

The product of its digits is 7776, while the sum is 33.

The square root of 644991 is about 803.1133170356. The cubic root of 644991 is about 86.4008241091.

The spelling of 644991 in words is "six hundred forty-four thousand, nine hundred ninety-one".