21804 has 24 divisors (see below), whose sum is σ = 53760. Its totient is φ = 6864.

The previous prime is 21803. The next prime is 21817. The reversal of 21804 is 40812.

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

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, 237 + ... + 315.

It is an arithmetic number, because the mean of its divisors is an integer number (2240).

2²¹⁸⁰⁴ 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 21804, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (26880).

21804 is an abundant number, since it is smaller than the sum of its proper divisors (31956).

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

21804 is a wasteful number, since it uses less digits than its factorization.

21804 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 109 (or 107 counting only the distinct ones).

The product of its (nonzero) digits is 64, while the sum is 15.

The square root of 21804 is about 147.6617756903. The cubic root of 21804 is about 27.9369328844.

The spelling of 21804 in words is "twenty-one thousand, eight hundred four".