218391 has 4 divisors (see below), whose sum is σ = 291192. Its totient is φ = 145592.

The previous prime is 218389. The next prime is 218401. The reversal of 218391 is 193812.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 218391 - 2¹ = 218389 is a prime.

It is an Ulam number.

It is a D-number.

218391 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (218371) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 36396 + ... + 36401.

It is an arithmetic number, because the mean of its divisors is an integer number (72798).

2²¹⁸³⁹¹ is an apocalyptic number.

218391 is a deficient number, since it is larger than the sum of its proper divisors (72801).

218391 is an equidigital number, since it uses as much as digits as its factorization.

218391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 72800.

The product of its digits is 432, while the sum is 24.

The square root of 218391 is about 467.3232286116. The cubic root of 218391 is about 60.2205769917.

The spelling of 218391 in words is "two hundred eighteen thousand, three hundred ninety-one".