Re: Creating a function that is a derivative of another function??
- To: mathgroup at smc.vnet.net
- Subject: [mg86753] Re: Creating a function that is a derivative of another function??
- From: Helen Read <read at math.uvm.edu>
- Date: Thu, 20 Mar 2008 02:54:08 -0500 (EST)
- References: <email@example.com>
Eric 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 > 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?! The problem is with the set delayed in the definition of dW. When you substitute 0 for t, it is substituted in the definition of dW, and you wind up with the nonsensical D[W[t],0] Try set instead: dw[t_]=D[W[t],t] and all is well. However, there really isn't any need to do this to begin with, since having defined W[t], you can simply use W'[t] W' etc. -- Helen Read University of Vermont