21624 is nontrivially palindromic in base 7.
It is a nude number because it is divisible by every one of its digits.
It is a congruent number.
It is an unprimeable number.
221624 is an apocalyptic number.
21624 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 21624, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29160).
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
21624 is a wasteful number, since it uses less digits than its factorization.
21624 is an odious number, because the sum of its binary digits is odd.
The square root of 21624 is about 147.0510115572. The cubic root of 21624 is about 27.8598438256.
Subtracting from 21624 its sum of digits (15), we obtain a square (21609 = 1472).
Subtracting from 21624 its product of digits (96), we obtain a triangular number (21528 = T207).
The spelling of 21624 in words is "twenty-one thousand, six hundred twenty-four".