Re: Forcing a Derivative

• To: mathgroup at smc.vnet.net
• Subject: [mg50774] Re: Forcing a Derivative
• From: p-valko at tamu.edu (Peter Valko)
• Date: Sun, 19 Sep 2004 21:39:44 -0400 (EDT)
• References: <cijej8\$hlp\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```"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

Help says:
"Whenever Derivative[n][f] is generated, Mathematica rewrites it as
D[f[#]&, {#, n}]. If Mathematica finds an explicit value for this
derivative, it returns this value. Otherwise, it returns the original
Derivative form."
Therefore, it is perhaps better to write staright:

D[f [x]* g[x],{x,2}]

and the result will be (after Simplify):

46 + 126*x + 60*x^3

Peter

```

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