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MathGroup Archive 2003

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Re: Re: Derivative of a funtion evaluated at a point in 3D

  • To: mathgroup at smc.vnet.net
  • Subject: [mg44473] Re: [mg44468] Re: Derivative of a funtion evaluated at a point in 3D
  • From: christopherpurcell <christopherpurcell at mac.com>
  • Date: Wed, 12 Nov 2003 08:01:24 -0500 (EST)
  • References: <boign7$oj1$1@smc.vnet.net> <200311110055.TAA25212@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

The question is ill-posed - you have not provided a complete enough 
description
for anyone to help you. The answer may well be 0, but without "stuff" 
there is no way to know.
If in fact you really want to evaluate D[F,p1,p2,p3,p4] then the answer 
could well be 0 (p1,p2,p3,p4 were most likely taken to be scalars not 
vectors by D).
Consider this trivial example:
F = ({x1, x2, x3} + {y1, y2, y3} + {z1, z2, z3})/3;
D[F, x1, x2, x3] (* mixed partials vanish *)
I suspect what you want is something more like:
{{D[F, x1], D[F, x2], D[F, x3]},{D[F, y1], D[F, y2], D[F, y3]},{D[F, 
z1], D[F, z2], D[F, z3]}}
You should evaluate ?D and follow the links to the help browser entries 
for D to see what this expression (D[F,p1,p2,p3,p4] ) means.


On Nov 10, 2003, at 8:55 PM, mroc wrote:

> mroc_1000 at hotmail.com (mroc) wrote in message 
> news:<boign7$oj1$1 at smc.vnet.net>...
>> Hello, I am totally new to Mathematica and trying to program a simple
>> FEM-type problem. I am trying to take the partial derivative
>> (symbolically) of an expression that is a function of four points in
>> 3D. All I can think of to do is F[p1,p2,p3,p4] = stuff then
>> D[F,p1,p2,p3,p4] . But I keep getting a zero expression as a result.
>> (where stuff is a nasty combination of these points) Any thoughts?
>
> Anyone? What am I missing? Is this question too easy or too hard?
>
>
Dr Christopher Purcell
Sensors & Actuators Group
DRDC-Atlantic, 9 Grove St., PO Box 1012,
Dartmouth NS B2Y 3Z7 Canada
Tel 902-426-3100 x389
Fax 902-426-9654


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