21218 has 6 divisors (see below), whose sum is σ = 32139. Its totient is φ = 10506.

The previous prime is 21211. The next prime is 21221. The reversal of 21218 is 81212.

It can be written as a sum of positive squares in only one way, i.e., 10609 + 10609 = 103^2 + 103^2 .

It is an ABA number since it can be written as A⋅B^A, here for A=2, B=103.

It is an Ulam number.

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

It is a Duffinian number.

It is a Curzon number.

It is a nialpdrome in base 13.

It is not an unprimeable number, because it can be changed into a prime (21211) by changing a digit.

21218 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 2 ways as a sum of consecutive naturals, for example, 155 + ... + 257.

2²¹²¹⁸ is an apocalyptic number.

21218 is a deficient number, since it is larger than the sum of its proper divisors (10921).

21218 is an equidigital number, since it uses as much as digits as its factorization.

21218 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 208 (or 105 counting only the distinct ones).

The product of its digits is 32, while the sum is 14.

The square root of 21218 is about 145.6639969244. The cubic root of 21218 is about 27.6843808429.

Multiplying 21218 by its product of digits (32), we get a square (678976 = 824²).

The spelling of 21218 in words is "twenty-one thousand, two hundred eighteen".