Does the notion of a derivative of a complex function make sense? If so, how should it be defined, and what does it represent? These and other questions will be the focus of the next few sections. Using our imagination, we take our lead from elementary Calculus and define the derivate of f; at z[0];, written f '(z[0];), by f '(z[0];) = Limit((f(z)-f(z[0]))/(z-z[0]),z = z[0]); , provided that the limit exists. When this happens, we say that the function f; is differentiable at z[0];. If we write Delta*z = z-z[0];, then this definition can be expressed in the form f '(z[0];) = Limit((f(z[0]+Delta*z)-f(z[0]))/(Delta*z),Delta*z = 0) .