It is a sphenic number, since it is the product of 3 distinct primes.
It is a Curzon number.
It is a plaindrome in base 11 and base 16.
It is a zygodrome in base 16.
It is a self number, because there is not a number n which added to its sum of digits gives 21998.
It is a congruent number.
221998 is an apocalyptic number.
21998 is a wasteful number, since it uses less digits than its factorization.
21998 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 666.
The square root of 21998 is about 148.3172275901. The cubic root of 21998 is about 28.0195441780.
Subtracting from 21998 its product of digits (1296), we obtain a palindrome (20702).
The spelling of 21998 in words is "twenty-one thousand, nine hundred ninety-eight".