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Re: Grad in VectorAnalysis`

• To: mathgroup at smc.vnet.net
• Subject: [mg103868] Re: Grad in VectorAnalysis`
• From: Albert Retey <awnl at gmx-topmail.de>
• Date: Fri, 9 Oct 2009 07:20:00 -0400 (EDT)
• References: <hakjlm$d72$1@smc.vnet.net>

Murray Eisenberg wrote:
> After...
> 
>    Needs["VectorAnalysis`"]
>    SetCoordinates[Cartesian[x, y, z]]
> 
>    f[x_, y_] = x/(1 - x^2 - y^2)
> 
> ... why does the following not actually return the formula for gradient...
> 
>    Grad[f[x, y, z]]  (* or even Grad[Evaluate[f[x,y,z]]]

I think this is just a typo, Grad[f[x,y]] works as expected, using
Grad[f[x,y,z]] doesn't work since you only defined f[x_,y_]...

> ... whereas the following does return it:
> 
>    Grad[x/(1 - x^2 - y^2)]
> 
> 
>