It is a tau number, because it is divible by the number of its divisors (80).
It is a nialpdrome in base 4 and base 16.
It is a zygodrome in base 4.
It is a congruent number.
221840 is an apocalyptic number.
21840 is a gapful number since it is divisible by the number (20) formed by its first and last digit.
It is an amenable number.
It is a practical number, because each smaller number is the sum of distinct divisors of 21840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (41664).
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
21840 is a wasteful number, since it uses less digits than its factorization.
21840 is an evil number, because the sum of its binary digits is even.
The square root of 21840 is about 147.7836256153. The cubic root of 21840 is about 27.9522997373.
Adding to 21840 its product of nonzero digits (64), we get a square (21904 = 1482).
The spelling of 21840 in words is "twenty-one thousand, eight hundred forty".