218991 has 4 divisors (see below), whose sum is σ = 291992. Its totient is φ = 145992.

The previous prime is 218989. The next prime is 218993. The reversal of 218991 is 199812.

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

It is an interprime number because it is at equal distance from previous prime (218989) and next prime (218993).

It is not a de Polignac number, because 218991 - 2¹ = 218989 is a prime.

It is a D-number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (218993) 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 3 ways as a sum of consecutive naturals, for example, 36496 + ... + 36501.

It is an arithmetic number, because the mean of its divisors is an integer number (72998).

2²¹⁸⁹⁹¹ is an apocalyptic number.

218991 is a deficient number, since it is larger than the sum of its proper divisors (73001).

218991 is an equidigital number, since it uses as much as digits as its factorization.

218991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 73000.

The product of its digits is 1296, while the sum is 30.

The square root of 218991 is about 467.9647422616. The cubic root of 218991 is about 60.2756758847.

The spelling of 218991 in words is "two hundred eighteen thousand, nine hundred ninety-one".