The most important property of e is that the derivative of e x is itself, and the value of e is lim (n→∞) (1+1/n) n. After expanding e^ix Taylor, there are

E IX = cos (x)+I * sin (x), which is the basic formula of complex variable function. By assigning the value of X to x=π, we can get a very interesting formula: E (I * π)+ 1 = 0, which is called "the most beautiful formula" in mathematics.