-21 is an odd number.
-21 is a Harshad number, which means it is divisible by the sum of its digits (2 + 1 = 3, and -21 is divisible by 3).
In binary, -21 is represented as 1111111111111111111111111111111111111111111111111111111111101011.
In hex, -21 is represented as ffffffffffffffeb.