643721 has 8 divisors (see below), whose sum is σ = 699720. Its totient is φ = 592176.

The previous prime is 643703. The next prime is 643723. The reversal of 643721 is 127346.

It can be written as a sum of positive squares in 4 ways, for example, as 60025 + 583696 = 245^2 + 764^2 .

It is a de Polignac number, because none of the positive numbers 2^k-643721 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (643723) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 2051 + ... + 2343.

It is an arithmetic number, because the mean of its divisors is an integer number (87465).

2⁶⁴³⁷²¹ is an apocalyptic number.

It is an amenable number.

643721 is a deficient number, since it is larger than the sum of its proper divisors (55999).

643721 is an equidigital number, since it uses as much as digits as its factorization.

643721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 332 (or 306 counting only the distinct ones).

The product of its digits is 1008, while the sum is 23.

The square root of 643721 is about 802.3222544589. The cubic root of 643721 is about 86.3440785395.

It can be divided in two parts, 64 and 3721, that multiplied together give a square (238144 = 488²).

The spelling of 643721 in words is "six hundred forty-three thousand, seven hundred twenty-one".