21816 has 32 divisors (see below), whose sum is σ = 61200. Its totient is φ = 7200.

The previous prime is 21803. The next prime is 21817. The reversal of 21816 is 61812.

It is a Harshad number since it is a multiple of its sum of digits (18).

It is a nude number because it is divisible by every one of its digits.

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

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21817) by changing a digit.

21816 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 7 ways as a sum of consecutive naturals, for example, 166 + ... + 266.

2²¹⁸¹⁶ is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21816, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30600).

21816 is an abundant number, since it is smaller than the sum of its proper divisors (39384).

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

21816 is a wasteful number, since it uses less digits than its factorization.

21816 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 116 (or 106 counting only the distinct ones).

The product of its digits is 96, while the sum is 18.

The square root of 21816 is about 147.7024035011. The cubic root of 21816 is about 27.9420570468.

Adding to 21816 its product of digits (96), we get a palindrome (21912).

It can be divided in two parts, 2181 and 6, that added together give a 7-th power (2187 = 3⁷).

The spelling of 21816 in words is "twenty-one thousand, eight hundred sixteen".