643856 has 10 divisors (see below), whose sum is σ = 1247502. Its totient is φ = 321920.

The previous prime is 643849. The next prime is 643859. The reversal of 643856 is 658346.

It can be written as a sum of positive squares in only one way, i.e., 409600 + 234256 = 640^2 + 484^2 .

It is an Ulam number.

It is not an unprimeable number, because it can be changed into a prime (643859) by changing a digit.

643856 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 20105 + ... + 20136.

2⁶⁴³⁸⁵⁶ is an apocalyptic number.

It is an amenable number.

643856 is a deficient number, since it is larger than the sum of its proper divisors (603646).

643856 is a wasteful number, since it uses less digits than its factorization.

643856 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 40249 (or 40243 counting only the distinct ones).

The product of its digits is 17280, while the sum is 32.

The square root of 643856 is about 802.4063808321. The cubic root of 643856 is about 86.3501140921.

The spelling of 643856 in words is "six hundred forty-three thousand, eight hundred fifty-six".