Help: Improved Euler Algorithm

The Improved Euler (or Heun) algorithm for the differential equation Y' = f(X,Y) (where X is the independent variable) is

            
            y₁ = Y(X) + h f(X,Y(X))
            Y(X+h) = Y(X) + h/2 (f(X,Y(X)) + f(X+h,y₁))

The step size is

            
             h = (X_F - X_I)/N

where X_I is the initial value of X, X_F the final value and N the number of steps, as set in the Main Menu.