It is a nialpdrome in base 4, base 8 and base 16.
It is a zygodrome in base 4.
It is a self number, because there is not a number n which added to its sum of digits gives 21504.
It is a congruent number.
21504 is a Friedman number, since it can be written as 21*(4+0)^5, using all its digits and the basic arithmetic operations.
221504 is an apocalyptic number.
21504 is a gapful number since it is divisible by the number (24) 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 21504, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32752).
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
21504 is an equidigital number, since it uses as much as digits as its factorization.
21504 is an odious number, because the sum of its binary digits is odd.
The square root of 21504 is about 146.6424222386. The cubic root of 21504 is about 27.8082131591.
The spelling of 21504 in words is "twenty-one thousand, five hundred four".