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Re: Problems evaluating Grad
*To*: mathgroup at smc.vnet.net
*Subject*: [mg39410] Re: [mg39372] Problems evaluating Grad
*From*: Murray Eisenberg <murraye at attbi.com>
*Date*: Fri, 14 Feb 2003 03:20:45 -0500 (EST)
*Organization*: Mathematics & Statistics, Univ. of Mass./Amherst
*References*: <200302130952.EAA20607@smc.vnet.net>
*Reply-to*: murray at math.umass.edu
*Sender*: owner-wri-mathgroup at wolfram.com
There are two different problems with your attempt.
First, Grad from the Calculus`VectorAnalysis` package wants the function
to have 3 variables, not 2. Here's one way to do this (with a slightly
more interesting f):
f[x_, y_] := x^2 + Sin[y]
Drop[Grad[f[x, y], Cartesian[x, y, z]], -1]
{2 x,Cos[y]}
Second, if you use arguments "a" and "b" directly in the Grad
expression, instead of the {x,y,x} coordinates, you now have a constant
function, so that you correctly obtain:
Drop[Grad[f[a, b], Cartesian[x, y, z]], -1]
{0, 0}
You can get the answer you presumably want, namely {2a, Cos[b]}, in
either of two ways -- either respecifying the coordinate system or else
using a ReplaceAll ( /. ):
Drop[Grad[f[a, b], Cartesian[a, b, z]], -1]
{2 a, Cos[b]}
Drop[Grad[f[x, y], Cartesian[x, y, z]], -1] /. {x -> a, y -> b}
{2 a, Cos[b]}
Konrad Den Ende wrote:
> I'd like to detrmine Grad to a function so i go:
> <<Calculus`VectorAnalysis`
> f[x_,y_]:=x+y
>
> Grad[f[a,b]]
>
> I get no errors, but the answer is simply the same thing
> that i asked, as if no computation was done. What do i
> do wrong?
>
> Also - when i evaluated the examples in Help i got the same
> behavior. The answer that already is there gets exchanged
> to something else upon my evaluation. Namely, the question
> itself. What is that?!
>
> --
>
> Vänligen
> Konrad
> ---------------------------------------------------
> phone #1: (+46/0) 708 - 70 73 92
> phone #2: (+46/0) 704 - 79 96 95
> url: http://konrads.webbsida.com
> e-mail: chamsterkonrad at bigfoot.com
> -----------------------------------
>
> Sleep - thing used by ineffective people
> as a substitute for coffee
>
> Ambition - a poor excuse for not having
> enough sence to be lazy
> ---------------------------------------------------
>
>
>
>
>
>
>
>
--
Murray Eisenberg murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower phone 413 549-1020 (H)
University of Massachusetts 413 545-2859 (W)
710 North Pleasant Street
Amherst, MA 01375
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