Question: inflexion point with maple

hello, i have some problems with maple. i need to know an inflexionpoint of the following function: f:= x->((x^3-x+1+(6/30))/(x^2-1)); i derivate till f'''(x) with the following commands dv1: D(f)(x); dv2: (D@@2)(f)(x); dv3: (D@@3)(f)(x); and here starts my problems. maple seems to do nothing. for dervation f'(x) and f''(x) it shows me nothing for derivation f'''(x) just a 0. i suppose the mistake is in front of the computer :O. so what is wrong? here my exact commands: restart; f:= x->((x^3-x+1+(6/30))/(x^2-1)); df1:= D(f)(x): df2:= (D@@2)(f)(x): df3:= (D@@3)(f)(x): hope you can help with that. (sorry for my bad english...)
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