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I'd like to look at some group transformation of curves in the plane. As a start, I'd like to try to do use the rotation group. Here's the code I tried to rotate a parabola so that I can watch the rotation:

x1[t_] := t*Cos[c] - t^2*Sin[c];
u1[t_] := t*Sin[c] + t^2*Cos[c];
Manipulate[
 ParametricPlot[{x1[t], u1[t]}, {t, -3, 3}, 
  PlotRange -> {{-9, 9}, {-9, 9}}], {c, 0, 2}]

If I fix a value for c, and remove the manipulate command, this works fine:

x1[t_] := t*Cos[c] - t^2*Sin[c];
u1[t_] := t*Sin[c] + t^2*Cos[c];
c=2;
 ParametricPlot[{x1[t], u1[t]}, {t, -3, 3}, 
  PlotRange -> {{-9, 9}, {-9, 9}}]

Are there any suggestions out there, or reasons why the above doesn't work? I'd like to manipulate the group parameter to watch the resulting transformations of the curve.

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3 Answers 3

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you can try also this:

x1[t_] := t*Cos[c] - t^2*Sin[c]; 
u1[t_] := t*Sin[c] + t^2*Cos[c]; 
Manipulate[
  ParametricPlot[{x1[t], u1[t]} /. c -> d, {t, -3, 3}, 
  PlotRange -> {{-9, 9}, {-9, 9}}], {d, 0, 2}]
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Manipulate[ ParametricPlot[RotationMatrix[c].{t, t^2}, {t, -2, 2}, 
            PlotRange -> {{-5, 5}, {-5, 5}}], {c, 0, 2 Pi}]

Mathematica graphics

Other transformations can be handled the same way. For example for SL2(R)

Manipulate[ ParametricPlot[LinearFractionalTransform[{{{a, b}, {c, (1 + b c)/a}}}][{t,t^2}], 
                          {t, -5, 5}, PlotRange -> 9{{-1, 1}, {-1, 1}}], 
           {a, .1, 5}, {b, 0, 5}, {c, 0, 5}]

Mathematica graphics

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Okay, if I let x1 and u1 be functions of c also, this seems to work:

x1[t_, c_] := t*Cos[c] - t^2*Sin[c];
u1[t_, c_] := t*Sin[c] + t^2*Cos[c];
Manipulate[
 ParametricPlot[{x1[t, c], u1[t, c]}, {t, -3, 3}, 
  PlotRange -> {{-9, 9}, {-9, 9}}], {c, 0, 2}]
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    $\begingroup$ You should correct your question itself. If you think there is no more issue and you found your own mistake, then you can delete the question. $\endgroup$
    – Nasser
    Sep 28, 2014 at 1:35
  • $\begingroup$ @Nasser Why should he delete it? Perhaps somebody comes up with a better solution! $\endgroup$ Sep 28, 2014 at 1:38
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    $\begingroup$ @Nasser Perhaps the question is interesting to others. I don't see a compelling reason to delete it. $\endgroup$
    – mathgeek
    Sep 28, 2014 at 1:40

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