Adding to 21000000 its reverse (12), we get a palindrome (21000012).
It is a super Niven number, because it is divisible the sum of any subset of its (nonzero) digits.
It is a nialpdrome in base 10.
It is a congruent number.
It is an unprimeable number.
Almost surely, 221000000 is an apocalyptic number.
21000000 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 21000000, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (39686992).
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
21000000 is an frugal number, since it uses more digits than its factorization.
21000000 is an odious number, because the sum of its binary digits is odd.
The square root of 21000000 is about 4582.5756949558. The cubic root of 21000000 is about 275.8924176381.