Adding to 21440 its reverse (4412), we get a palindrome (25852).
21440 is an esthetic number in base 15, because in such base its adjacent digits differ by 1.
It is a congruent number.
It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21440.
It is an unprimeable number.
21440 is an untouchable number, because it is not equal to the sum of proper divisors of any number.
221440 is an apocalyptic number.
21440 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 21440, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25908).
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
21440 is an equidigital number, since it uses as much as digits as its factorization.
21440 is an evil number, because the sum of its binary digits is even.
The square root of 21440 is about 146.4240417418. The cubic root of 21440 is about 27.7805982321.
The spelling of 21440 in words is "twenty-one thousand, four hundred forty", and thus it is an iban number.