Complex number
With exponentiation, there is a difficulty to be overcome. There is no real number that gives −1 when it is squared. In other words, −1 (or any other negative number) has no real square root. For example, there is no real number  that solves the equation . To solve this problem, mathematicians introduced a symbol i and called it the imaginary unit. This is the imaginary number that will give −1 when it is squared.