643993721 has 24 divisors (see below), whose sum is σ = 806966784. Its totient is φ = 510935040.

The previous prime is 643993717. The next prime is 643993723. The reversal of 643993721 is 127399346.

It is not a de Polignac number, because 643993721 - 2² = 643993717 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (643993723) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 25721 + ... + 44153.

It is an arithmetic number, because the mean of its divisors is an integer number (33623616).

Almost surely, 2^643993721 is an apocalyptic number.

It is an amenable number.

643993721 is a deficient number, since it is larger than the sum of its proper divisors (162973063).

643993721 is a wasteful number, since it uses less digits than its factorization.

643993721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 18501 (or 18494 counting only the distinct ones).

The product of its digits is 244944, while the sum is 44.

The square root of 643993721 is about 25377.0313669665. The cubic root of 643993721 is about 863.5627042528.

The spelling of 643993721 in words is "six hundred forty-three million, nine hundred ninety-three thousand, seven hundred twenty-one".