# How do I use Runge-Kutta Iterative method to produce a table of values for a function?

Describe how the fourth-order Runge-Kutta method can be used to produce a table of values for the function

$$f(x)=\int_0^x e^{-t^2}\ \mathsf dx$$

at $100$ equally spaced points in the unit interval.

Answer: Find an appropriate initial-value problem whose solution is f. Solve df =e^{−x^2}, f(0)=0.

The answer is a little confusing and can anyone write out the full answer? I suppose my question is how after i take the derivative and transform the intergral equation into a differential equation with initial value, how do i know the step size h? n how do i know the initial value? I know what runge kutta 4th order is The runge kutta method is

$$k_1 = f(t_0,x(t_0))$$ $$k_2 = f(t_0+h/2,x(t_0)+k_1/2)$$ $$k_3 = f(t_0+h/2,x(t_0)hk_2/2)$$ $$k_4 = f(t_0+h,x(t_0)+k_3)$$ Then $$x(t_0+h)= x(t_0) + h\frac{k_1+2k_2+2k_3+k_4}{6}$$

• Look for Simpson integration, no need to transform a simple quadrature into a differential equation. – Lutz Lehmann Apr 18 '18 at 11:56

Focus less on symbols and concentrate on the properties of the objects. A change of symbols might clarify matters. Let $y : \mathbb{R} \rightarrow \mathbb{R}$ be given by $$y(t) = \int_0^t e^{-s^2}ds.$$ Then $y(0) = 0$ and $$y'(t) = e^{-t^2}.$$ If we define $f : \mathbb{R} \times \mathbb{R}$ as follows, $$f(t,y) = e^{-t^2},$$ where there is no explicit dependence on the second variable, then we can write $$y'(t) = f(t,y).$$ You are now in a position to apply, say, the classical fourth order Runge-Kutta method.