Some obvious ones are the "I can guess your number" tricks, where you do hard math on a secret number and tell the magician the result, who then does easy math

Some obvious ones are the "I can guess your number" tricks, where you do hard math on a secret number and tell the magician the result, who then does easy math to tell you your number. Such a loop of operations containing only multiplication, addition, and subtraction would work on activity complex numbers. Square roots would not.