643891 has 4 divisors (see below), whose sum is σ = 677800. Its totient is φ = 609984.

The previous prime is 643889. The next prime is 643919. The reversal of 643891 is 198346.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 198346 = 2 ⋅99173.

It is a cyclic number.

It is not a de Polignac number, because 643891 - 2¹ = 643889 is a prime.

It is a Duffinian number.

643891 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (643691) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 16926 + ... + 16963.

It is an arithmetic number, because the mean of its divisors is an integer number (169450).

2⁶⁴³⁸⁹¹ is an apocalyptic number.

643891 is the 508-th centered pentagonal number.

643891 is a deficient number, since it is larger than the sum of its proper divisors (33909).

643891 is a wasteful number, since it uses less digits than its factorization.

643891 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 33908.

The product of its digits is 5184, while the sum is 31.

The square root of 643891 is about 802.4281899335. The cubic root of 643891 is about 86.3516787272.

The spelling of 643891 in words is "six hundred forty-three thousand, eight hundred ninety-one".