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I have some function $\psi$, I compute the transformation $T = \vec{x} - \nabla \psi$. I'd like to visualize the action of $T$ on a square centered at the origin.

The mathematica website shows some examples of how to compute transformations.

https://reference.wolfram.com/language/ref/TransformedRegion.html

I have two questions:

  1. If I want to subimpose the initial region (perhaps in red) under the final region, how would I do this? Also I'd like to add coordinate axes.

  2. Below is included a sketch of code. How do I get the coordinates of the gradient I compute into the transformation function. Below is my best guess, but I'm totally lost on how to accomplish this.

Clear["Global`*"]
psi = -4 (x^2 + y^2);
gradPsi = Grad[psi, {x, y}]; /. {Indexed[#, 1] -> x , 
  Indexed[#, 2] -> y}
R = 
  TransformedRegion[
   Rectangle[], {Indexed[#, 1] - Indexed[gradPsi, 1], 
     Indexed[#, 2] + Indexed[gradPsi, 2]} &];
Region[R]
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1 Answer 1

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psi = -4 (x^2 + y^2);

gradPsi = Grad[psi, {x, y}]

R = TransformedRegion[Rectangle[], {Indexed[#, 1] - First@gradPsi /. x -> #[[1]], 
     Indexed[#, 2] + Last@gradPsi /. y -> #[[2]]} &];

Show[Graphics[{Opacity[.5], Blue, Rectangle[]}], 
 RegionPlot[R, PlotStyle -> Opacity[.5, Red]], 
 PlotRange -> All, Frame -> True, FrameTicks -> All]

enter image description here

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  • $\begingroup$ Almost there, thanks!! I meant to say the center of the square is centered at the origin. I think this is the bottom left corner. I'm having a little trouble interpreting the picture. For the example I showed, the Transformation should blow up a box whose center is at the origin by a factor of 9. $T = \langle x - (-8x), y - (-8y) \rangle$ $\endgroup$
    – yoshi
    Dec 21, 2020 at 17:36
  • 1
    $\begingroup$ Ah got it, thanks! There's a typo in the formula, and the box can be recentered using documentation. $\endgroup$
    – yoshi
    Dec 21, 2020 at 17:46

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