headbam

Thanks for both answers! I had no problems understanding the analytical method Preben submitted. I didnt get exactly what was done in the numerical approach (I am not into the Proc stuff yet) but i dont want to bother you with teaching a newbie :D

@PatrickT:

I understood the first three paragraphs of your answer although i dont know how to convert the boundary conditions into equivalent initial conditions (meaning that the IC will provide a function which will satisfy the BC).