Creating a function that is a derivative of another function??

*To*: mathgroup at smc.vnet.net*Subject*: [mg86718] Creating a function that is a derivative of another function??*From*: Eric <eric.r.george at aero.org>*Date*: Wed, 19 Mar 2008 05:25:38 -0500 (EST)

I have a function: W[t_] := a*t + b*t^2 + c*t^3 I want the derivative of this funtion wrt t as another function. The following seems like it works: dW[t_] := D[W[t], t] dW[t] a + 2 b t + 3 c t^2 But something isn't right?? dW[0] General::ivar: 0 is not a valid variable. >> (output line has a partial derivative symbol, subscript 0 followed by a 0) The description of the General::ivar error isn't help either?? Any idea what's going on here?? How can I assign the derivative of a function as another funtion?! Thanks Eric