Re: Forcing a Derivative

*To*: mathgroup at smc.vnet.net*Subject*: [mg50776] Re: [mg50753] Forcing a Derivative*From*: DrBob <drbob at bigfoot.com>*Date*: Sun, 19 Sep 2004 21:39:53 -0400 (EDT)*References*: <200409190756.DAA17973@smc.vnet.net>*Reply-to*: drbob at bigfoot.com*Sender*: owner-wri-mathgroup at wolfram.com

I thought the answer was easy, but the more I looked into it, the more strange behavior I found. For instance, I expected part of the problem to be that Derivative[2][f * g] doesn't mention x, and f*g isn't a pure function. The same applies to these derivatives, but they both work: f[x_]=x^2+7; g[x_]=3x^3+23; Derivative[2][f] Derivative[2][g] 2& 18 #1& This one works but works strangely: prod=Function[x,f[x]g[x]]; Derivative[2][prod] Function[x, 18*x*f[x] + 2*g[x] + 2*Derivative[1][f][x]* Derivative[1][g][x]] Mathematica knows f and g or it couldn't have derived that answer; yet it leaves much of it unevaluated. It is correct as far as it goes, however: Derivative[2][prod][x]; D[f[x]*g[x], {x, 2}] // Expand % == %% // Simplify 46 + 126*x + 60*x^3 True Bobby On Sun, 19 Sep 2004 03:56:01 -0400 (EDT), Scott Guthery <sguthery at mobile-mind.com> wrote: > 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 > > > -- DrBob at bigfoot.com www.eclecticdreams.net

**References**:**Forcing a Derivative***From:*"Scott Guthery" <sguthery@mobile-mind.com>

**mysql-mlink anyone? I need to make it run under Mac OS X**

**Re: Re: Re: How to simplify to a result that is real**

**Re: Re: Re: Forcing a Derivative**

**Re: Forcing a Derivative**