21918 has 16 divisors (see below), whose sum is σ = 47376. Its totient is φ = 6720.

The previous prime is 21911. The next prime is 21929. The reversal of 21918 is 81912.

It is a plaindrome in base 16.

It is a junction number, because it is equal to n+sod(n) for n = 21894 and 21903.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21911) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 63 + ... + 218.

It is an arithmetic number, because the mean of its divisors is an integer number (2961).

2²¹⁹¹⁸ is an apocalyptic number.

21918 is an abundant number, since it is smaller than the sum of its proper divisors (25458).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

It is a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (23688).

21918 is a wasteful number, since it uses less digits than its factorization.

21918 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 299.

The product of its digits is 144, while the sum is 21.

The square root of 21918 is about 148.0472897422. The cubic root of 21918 is about 27.9855367481.

It can be divided in two parts, 21 and 918, that added together give a palindrome (939).

The spelling of 21918 in words is "twenty-one thousand, nine hundred eighteen".