218721 has 4 divisors (see below), whose sum is σ = 291632. Its totient is φ = 145812.

The previous prime is 218719. The next prime is 218723. The reversal of 218721 is 127812.

218721 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is an interprime number because it is at equal distance from previous prime (218719) and next prime (218723).

It is not a de Polignac number, because 218721 - 2¹ = 218719 is a prime.

It is a D-number.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a plaindrome in base 14.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 218721.

It is not an unprimeable number, because it can be changed into a prime (218723) 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, 36451 + ... + 36456.

It is an arithmetic number, because the mean of its divisors is an integer number (72908).

2²¹⁸⁷²¹ is an apocalyptic number.

It is an amenable number.

218721 is a deficient number, since it is larger than the sum of its proper divisors (72911).

218721 is an equidigital number, since it uses as much as digits as its factorization.

218721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 72910.

The product of its digits is 224, while the sum is 21.

The square root of 218721 is about 467.6761700151. The cubic root of 218721 is about 60.2508938534.

Subtracting from 218721 its reverse (127812), we obtain a palindrome (90909).

It can be divided in two parts, 218 and 721, that added together give a palindrome (939).

The spelling of 218721 in words is "two hundred eighteen thousand, seven hundred twenty-one".