21808 has 20 divisors (see below), whose sum is σ = 44640. Its totient is φ = 10304.

The previous prime is 21803. The next prime is 21817. The reversal of 21808 is 80812.

It is a happy number.

It is a plaindrome in base 15.

It is a nialpdrome in base 16.

It is a zygodrome in base 15.

It is not an unprimeable number, because it can be changed into a prime (21803) by changing a digit.

21808 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 441 + ... + 487.

It is an arithmetic number, because the mean of its divisors is an integer number (2232).

2²¹⁸⁰⁸ 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 21808, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (22320).

21808 is an abundant number, since it is smaller than the sum of its proper divisors (22832).

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

21808 is a wasteful number, since it uses less digits than its factorization.

21808 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 84 (or 78 counting only the distinct ones).

The product of its (nonzero) digits is 128, while the sum is 19.

The square root of 21808 is about 147.6753195358. The cubic root of 21808 is about 27.9386411474.

The spelling of 21808 in words is "twenty-one thousand, eight hundred eight".