thanks for your reply. i think i understood what you wrote. but i didn't help me to solve my problem. here my maple code: 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): df4:= (D@@4)(f)(x): df5:= (D@@5)(f)(x): df6:= (D@@6)(f)(x): df7:= (D@@7)(f)(x): with( RealDomain ): ws1:= evalf(solve(df1=0.0,x)); ws2:= evalf(solve(df2=0.0,x)); ws3:= evalf(solve(df3=0.0,x)); ws4:= evalf(solve(df4=0.0,x)); ws5:= evalf(solve(df5=0.0,x)); ws6:= evalf(solve(df6=0.0,x)); ws7:= evalf(solve(df7=0.0,x)); i just tested a bit to get used to differentiation in maple. i'm just interested in the real numbers solutions. maple gives me the following output: ws1 := 0.3305756764, 1.745494243 ws2 := ws3 := 0. ws4 := ws5 := 0. ws6 := ws7 := 0. what exactly does maple say me? the nullpoints for 2nd, 4th, 6th differentiation doesn't exist? and for the 3rd, 5th und 7th it is 0 ?!? my target is still to get the inflexion point. that require that the 2nd differentiation has at least 1 nullpoint. the nullpoints of the 2nd differentiation are used for x in the 3rd differentiation. does this mean there is no inflexion point?! regards

woops....thx. next question. if i try to get the null points for a function that doesn't have null points does maple give me anything or just "blank"?