Families of optimal derivative-free two- and three-point iterative methods for solving nonlinear equations

Necessary and sufficient conditions for derivative-free two- and three-point iterative methods to have the optimal convergence order are obtained. These conditions can be effectively used not only for determining the order of convergence of iterative methods but also for designing new methods. Furthermore, the use of the method of generating functions makes it possible to construct a wide class of optimal derivative-free two- and three-point methods that includes many well-known methods as particular cases. An analytical formula for the optimal choice of the parameter of iterations improving the order of convergence is derived.