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

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86745] Re: Creating a function that is a derivative of another function??
  • From: "David Park" <djmpark at comcast.net>
  • Date: Thu, 20 Mar 2008 02:52:38 -0500 (EST)
  • References: <frqpr4$55a$1@smc.vnet.net>

When Mathematica evaluates your dW statement with 0 it uses D[W[0],0], which 
makes no sense for the D statement. Use the Derivative construction.

W[t_] := a t + b t^2 + c t^3
dW[t_] := Derivative[1][W][t]

dW[t]
a + 2 b t + 3 c t^2

dW[0]
a

-- 
David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/


"Eric" <eric.r.george at aero.org> wrote in message 
news:frqpr4$55a$1 at smc.vnet.net...
>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
> 



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