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MathGroup Archive 2004

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Re: Forcing a Derivative

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50855] Re: Forcing a Derivative
  • From: "Steve Luttrell" <steve_usenet at _removemefirst_luttrell.org.uk>
  • Date: Fri, 24 Sep 2004 04:41:29 -0400 (EDT)
  • References: <cijej8$hlp$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

For some reason my reply (on 21 September 2004) to this posting vanished 
into the aether. Here it is again:

Here's one way to coerce Mathematica into doing what you want.

Use total derivative to evaluate derivatives of objects w.r.t. anything:

Dt[f g, x]

gives

g Dt[f,x]+f Dt[g,x]

and

Dt[f+g,x]

gives

Dt[f,x]+Dt[g,x]

Don't like the verbose notation? Use the notation package to restore things
back to the way you wanted them in the first place.

<< Utilities`Notation`

Define a new notation that bidirectionally relates your preferred primed
notation (typeset form) to the Mathematica Dt notation (internal form). Note
that the expression below has to be entered using the notation palette
because it has some hidden information in it.

Notation[x_' \[DoubleLongLeftRightArrow] Dt[x_]]

Now

(f+g)'

gives

f'+g'

and

(f g)'

gives

g f'+f g'

Steve Luttrell

"Scott Guthery" <sguthery at mobile-mind.com> wrote in message 
news:cijej8$hlp$1 at smc.vnet.net...
> How does one force Derivative[n] to actually take the derivative?
>
> For example if ...
>
> f[x_] = x^2 + 7
>
> g[x_]=3x^3 + 23
>
> then
>
> Derivative[2][f * g]
>
> just puts a couple of primes on the product rather than actually computing 
> the dervative.
>
> Thanks for any insight.
>
> Cheers, Scott
> 



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