644721905 has 8 divisors (see below), whose sum is σ = 785213952. Its totient is φ = 508079088.

The previous prime is 644721877. The next prime is 644721937. The reversal of 644721905 is 509127446.

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

It is a cyclic number.

It is not a de Polignac number, because 644721905 - 2¹⁴ = 644705521 is a prime.

It is a Duffinian number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 961937 + ... + 962606.

It is an arithmetic number, because the mean of its divisors is an integer number (98151744).

Almost surely, 2^644721905 is an apocalyptic number.

It is an amenable number.

644721905 is a deficient number, since it is larger than the sum of its proper divisors (140492047).

644721905 is a wasteful number, since it uses less digits than its factorization.

644721905 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1924615.

The product of its (nonzero) digits is 60480, while the sum is 38.

The square root of 644721905 is about 25391.3746181651. The cubic root of 644721905 is about 863.8880675076. Note that the first 3 decimals are identical.

The spelling of 644721905 in words is "six hundred forty-four million, seven hundred twenty-one thousand, nine hundred five".