It is an iterative approach in finding the y value for a given x value starting from a 1st order ODE. It asks the user the ODE function and the initial values and increment value. It also lets the user choose what termination criterion to use, either a specified x value or a number of iterations. A 1st order RK method is the simple Euler's method; for a 2nd order RK equation, there are 3 choices available: Heun's method, Midpoint's method, and Ralston's method; for a 3rd order RK method, the classical 3rd order is used; for a 4th order RK method, the classical 4th order is used (RK4); for a 5th order, Butcher's 5th order is used.