21684 has 24 divisors (see below), whose sum is σ = 54880. Its totient is φ = 6624.

The previous prime is 21683. The next prime is 21701. The reversal of 21684 is 48612.

It is a self number, because there is not a number n which added to its sum of digits gives 21684.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21683) 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, 87 + ... + 225.

2²¹⁶⁸⁴ 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 21684, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27440).

21684 is an abundant number, since it is smaller than the sum of its proper divisors (33196).

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

21684 is a wasteful number, since it uses less digits than its factorization.

21684 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 159 (or 157 counting only the distinct ones).

The product of its digits is 384, while the sum is 21.

The square root of 21684 is about 147.2548810736. The cubic root of 21684 is about 27.8855875511.

It can be divided in two parts, 216 and 84, that added together give a triangular number (300 = T₂₄).

The spelling of 21684 in words is "twenty-one thousand, six hundred eighty-four".