21812 has 24 divisors (see below), whose sum is σ = 47040. Its totient is φ = 8640.

The previous prime is 21803. The next prime is 21817.

21812 is nontrivially palindromic in base 10.

It is a Harshad number since it is a multiple of its sum of digits (14).

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

It is a zygodrome in base 5.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21812.

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

21812 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, 512 + ... + 552.

It is an arithmetic number, because the mean of its divisors is an integer number (1960).

2²¹⁸¹² 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 21812, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (23520).

21812 is an abundant number, since it is smaller than the sum of its proper divisors (25228).

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

21812 is a wasteful number, since it uses less digits than its factorization.

21812 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 71 (or 69 counting only the distinct ones).

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

The square root of 21812 is about 147.6888621393. The cubic root of 21812 is about 27.9403492015.

The spelling of 21812 in words is "twenty-one thousand, eight hundred twelve".