# Runge-Kutta convergence [closed]

I am facing a problem solving a ODE with a Runge-Kutta 4th order method:

The expression in order to solve is :

$$Ay^{''}+By^{'}+Cy= Cu$$

$$y =OUTPUT$$

$$u=INPUT$$

We are using a sample time of 0.01s (with 0.001s sample time it does work), but the solver with some combinations of A,B and C does not converge. Then, we would like to know these combinations of A,B and C that make the method fail before start to using it.

## closed as off-topic by Joonas Ilmavirta, Chris Godsil, Alex Degtyarev, Douglas Zare, Christian RemlingJul 28 '15 at 20:02

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question does not appear to be about research level mathematics within the scope defined in the help center." – Joonas Ilmavirta, Chris Godsil, Alex Degtyarev, Douglas Zare, Christian Remling
If this question can be reworded to fit the rules in the help center, please edit the question.

• Whenever you have $B^2-4AC<0$ you'd have better luck with a predictor corrector method. The short answer is because your solution is "oscillating". – Jeb Jul 28 '15 at 12:01