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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
>