@leouam I really miss the old-school Maple without these mostly useless modern input features..

@Carl LoveIt was just an example of a simple program. Perhaps I exaggerated a bit with "Nothing." Some commands do work, like diff(sin(x),x), but most of my code has become useless, and I am trying to figure out what the problem is.

@mmcdara Well, I think we don't neet the initial conditions for higher order derivates, since we know the solution already. The initial condition is needed to get x(t). 

@mmcdara This problem describes an oscillation of a non-linear spring, so the acceleration is -1 when the amplitude x(0)=1 and the initial velocity x'(0)=0 are given.  Anyway Y(0) is not given explicitly in the script, is it? 

The first reply gives the wrong values of the second and third derivatives, but the second one seems to be ok.