-81 is an odd number.
-81 is a Harshad number, which means it is divisible by the sum of its digits (8 + 1 = 9, and -81 is divisible by 9).
In binary, -81 is represented as 1111111111111111111111111111111111111111111111111111111110101111.
In hex, -81 is represented as ffffffffffffffaf.