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Re: Partial Derivatives

• To: mathgroup at smc.vnet.net
• Subject: [mg7821] Re: Partial Derivatives
• From: Paul Abbott <paul at physics.uwa.edu.au>
• Date: Mon, 14 Jul 1997 03:01:07 -0400
• Organization: University of Western Australia
• Sender: owner-wri-mathgroup at wolfram.com

Kesh Govinder wrote:

> Does anyone else think that the partial derivative implementation in
> Mma is a little weird?   I am trying to find partial derivatives of
> an equation, eg y''[x] + y'[x] + x^2 ==0.
> What I would like is to say something like pD[eqn,y[x]] and get back 0
> while pD[eqn,y''[x]] will give me 1.  Certainly D does this.  However,
> I also want pD[eqn,x] to give me 2x and not y'''[x] + y''[x] + 2 x.
> Can anyone please suggest a way to get around this?
> 
> Note that entering the equation as y'' + y' + x^2 does not work.  Here
> D[eqn,y] gives me the strange result y''' + y'' which I really cannot
> understand.

D is perfectly well implemented and works fine!  However, I assume that
you are trying to solve a variational problem and need variational
derivatives?  If so then have a look at the Calculus`VariationalMethods`
package which includes VariationalD and other related operators. This
should do what you want.

Cheers,
	Paul 

____________________________________________________________________ 
Paul Abbott                                   Phone: +61-8-9380-2734
Department of Physics                           Fax: +61-8-9380-1014
The University of Western Australia           
Nedlands WA  6907                     mailto:paul at physics.uwa.edu.au 
AUSTRALIA                              http://www.pd.uwa.edu.au/Paul

            God IS a weakly left-handed dice player
____________________________________________________________________