644721 has 16 divisors (see below), whose sum is σ = 1072128. Its totient is φ = 334800.

The previous prime is 644717. The next prime is 644729. The reversal of 644721 is 127446.

It is not a de Polignac number, because 644721 - 2² = 644717 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 644691 and 644700.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (644729) 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 15 ways as a sum of consecutive naturals, for example, 1165 + ... + 1626.

It is an arithmetic number, because the mean of its divisors is an integer number (67008).

2⁶⁴⁴⁷²¹ is an apocalyptic number.

It is an amenable number.

644721 is a deficient number, since it is larger than the sum of its proper divisors (427407).

644721 is a wasteful number, since it uses less digits than its factorization.

644721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2812.

The product of its digits is 1344, while the sum is 24.

The square root of 644721 is about 802.9452036098. The cubic root of 644721 is about 86.3887663292.

The spelling of 644721 in words is "six hundred forty-four thousand, seven hundred twenty-one".