It is a super Niven number, because it is divisible the sum of any subset of its (nonzero) digits.
It is an Ulam number.
It is a nialpdrome in base 10.
It is a zygodrome in base 5.
It is a congruent number.
21000 is an untouchable number, because it is not equal to the sum of proper divisors of any number.
221000 is an apocalyptic number.
21000 is a gapful number since it is divisible by the number (20) formed by its first and last digit.
21000 is the 84-th octagonal number.
It is an amenable number.
It is a practical number, because each smaller number is the sum of distinct divisors of 21000, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (37440).
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
21000 is a wasteful number, since it uses less digits than its factorization.
21000 is an evil number, because the sum of its binary digits is even.
The square root of 21000 is about 144.9137674619. The cubic root of 21000 is about 27.5892417638.
Adding to 21000 its reverse (12), we get a palindrome (21012).
The spelling of 21000 in words is "twenty-one thousand", and thus it is an iban number.