Re: Creating a function that is a derivative of another

*To*: mathgroup at smc.vnet.net*Subject*: [mg86746] Re: [mg86718] Creating a function that is a derivative of another*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Thu, 20 Mar 2008 02:52:49 -0500 (EST)*Reply-to*: hanlonr at cox.net

W[t_] := a*t + b*t^2 + c*t^3 Use Set rather than SetDelayed dW[t_] = D[W[t], t]; dW[0] a Or just use W'[0] a Bob Hanlon ---- Eric <eric.r.george at aero.org> wrote: > 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 >