644829721 has 4 divisors (see below), whose sum is σ = 650745700. Its totient is φ = 638913744.

The previous prime is 644829709. The next prime is 644829727. The reversal of 644829721 is 127928446.

644829721 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 166926400 + 477903321 = 12920^2 + 21861^2 .

It is a cyclic number.

It is not a de Polignac number, because 644829721 - 2⁹ = 644829209 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (644829727) 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, 2957826 + ... + 2958043.

It is an arithmetic number, because the mean of its divisors is an integer number (162686425).

Almost surely, 2^644829721 is an apocalyptic number.

It is an amenable number.

644829721 is a deficient number, since it is larger than the sum of its proper divisors (5915979).

644829721 is a wasteful number, since it uses less digits than its factorization.

644829721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5915978.

The product of its digits is 193536, while the sum is 43.

The square root of 644829721 is about 25393.4976125779. The cubic root of 644829721 is about 863.9362204470.

The spelling of 644829721 in words is "six hundred forty-four million, eight hundred twenty-nine thousand, seven hundred twenty-one".