644905 has 4 divisors (see below), whose sum is σ = 773892. Its totient is φ = 515920.

The previous prime is 644899. The next prime is 644909. The reversal of 644905 is 509446.

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 501264 + 143641 = 708^2 + 379^2 .

It is not a de Polignac number, because 644905 - 2¹⁵ = 612137 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (644909) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 64486 + ... + 64495.

It is an arithmetic number, because the mean of its divisors is an integer number (193473).

2⁶⁴⁴⁹⁰⁵ is an apocalyptic number.

It is an amenable number.

644905 is a deficient number, since it is larger than the sum of its proper divisors (128987).

644905 is a wasteful number, since it uses less digits than its factorization.

644905 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 128986.

The product of its (nonzero) digits is 4320, while the sum is 28.

The square root of 644905 is about 803.0597736159. The cubic root of 644905 is about 86.3969838482.

The spelling of 644905 in words is "six hundred forty-four thousand, nine hundred five".