Equation of a stretched limaçon!

Question

I have some limaçon with the polar equation $r=a+b\cos(\theta)$. I want to strecth the limaçon along some small vector $W=(c,d)$. By scretching along a small vector I mean the center of the coordinate system remains inside the limaçon. Now, the question is

this new closed curve still will be a limaçon? In any case, what would be the equation of this new closed curve?

I will appreciate any comment or solutions.

Answer 1

Given a curve $\gamma$ of parametric equations:

$$ \begin{cases} x = x(t) \\ y = y(t) \\ \end{cases} \; \; \; \; \; \; \text{for} \; t \in I \subseteq \mathbb{R} $$

in order for $\gamma$ to be scaled with respect to a point $C\left(x_C,\,y_C\right)$ by a factor $s_1 > 0$ in counterclockwise direction $\theta \in [0,\,2\pi)$ and by a factor $s_2 > 0$ in counterclockwise direction $\theta + \frac{\pi}{2}$, it's necessary:

• translate $\gamma$ in such a way that $C$ coincides with the origin;

• rotate $\gamma$ by $-\theta$ with respect to the origin;

• scale $\gamma$ by a factor $s_1$ along the x-axis and by a factor $s_2$ along the y-axis with respect to the origin;

• rotate $\gamma$ by $+\theta$ with respect to the origin;

• translate $\gamma$ in such a way that $C$ returns to its initial position;

hence the curve $\gamma'$ of parametric equations:

$$ \small \begin{cases} x = x_C + \left(x(t) - x_C\right)\left(s_1\cos^2\theta + s_2\sin^2\theta\right) + \left(y(t) - y_C\right)\left(s_1 - s_2\right)\cos\theta\,\sin\theta \\ y = y_C + \left(y(t) - y_C\right)\left(s_1\sin^2\theta + s_2\cos^2\theta\right) + \left(x(t) - x_C\right)\left(s_1 - s_2\right)\cos\theta\,\sin\theta \\ \end{cases} \; \; \; \; \; \; \text{for} \; t \in I $$

that is, considering the versor $\hat{n}$ of components $n_1 = \cos\theta$ and $n_2 = \sin\theta$, we get:

$$ \small \begin{cases} x = x_C + \left(x(t) - x_C\right)\left(s_1\,n_1^2 + s_2\,n_2^2\right) + \left(y(t) - y_C\right)\left(s_1 - s_2\right)n_1\,n_2 \\ y = y_C + \left(y(t) - y_C\right)\left(s_1\,n_2^2 + s_2\,n_1^2\right) + \left(x(t) - x_C\right)\left(s_1 - s_2\right)n_1\,n_2 \\ \end{cases} \; \; \; \; \; \; \text{for} \; t \in I\,. $$

If $s_2 = 1/s_1$ there's a squeezing, i.e. the area enclosed by a closed curve is invariant.

---

In particular, considering a limaçon of parametric equations:

$$ \begin{cases} x(t) = \left(1 + 2\cos(t)\right)\cos(t) \\ y(t) = \left(1 + 2\cos(t)\right)\sin(t) \\ \end{cases} \; \; \; \; \; \; \text{for} \; t \in [0,\,2\pi) $$

whose centroid turns out to be $C\left(\frac{6}{5},\,0\right)$, if we want it to be scaled with respect to the centroid by a factor $s_1 = k$ in the anticlockwise direction $\frac{\pi}{3}$ and by a factor $s_2 = 1$ in the anticlockwise direction $\frac{5\pi}{6}$, we get:

$$ \begin{cases} x = \frac{3\,(1 - k)}{10} + \frac{k + 3}{4}\,x(t) + \frac{\sqrt{3}\,\left(k - 1\right)}{4}\,y(t) \\ y = \frac{3\sqrt{3}\,(1 - k)}{10} + \frac{3\,k + 1}{4}\,y(t) + \frac{\sqrt{3}\,\left(k - 1\right)}{4}\,x(t) \\ \end{cases} \; \; \; \; \; \; \text{for} \; t \in [0,\,2\pi) $$

where for $0 < k < 1$ a contraction is obtained, while for $k > 1$ a dilation is obtained.

Wanting to view everything through a gif, in Wolfram Mathematica 12.2 it's sufficient to write:

x[t_] := (1 + 2 Cos[t]) Cos[t]
y[t_] := (1 + 2 Cos[t]) Sin[t]
γ[t_] := {{x[t]}, {y[t]}}
Lγ = Integrate[Norm[γ'[t]], {t, 0, 2π}];
xC = Integrate[x[t] Norm[γ'[t]], {t, 0, 2π}] / Lγ;
yC = Integrate[y[t] Norm[γ'[t]], {t, 0, 2π}] / Lγ;
Cγ = {{xC}, {yC}};
R[θ_] := {{Cos[θ], -Sin[θ]}, {Sin[θ], Cos[θ]}}
S[k_] := {{k, 0}, {0, 1}}
γp[k_, t_] := Cγ + R[π/3].S[k].R[-π/3].(γ[t] - Cγ)
frames1 = Table[ParametricPlot[{γ[t], γp[k, t]}, {t, 0, 2π},
                AxesLabel -> {"x", "y"}, 
                PlotStyle -> {Blue, Red},
                PlotRange -> {{-2, 4}, {-4, 4}},
                PlotLegends -> TextString@Row@{"k = ", k}],
               {k, 1, 0, -0.1}];
frames2 = Table[ParametricPlot[{γ[t], γp[k, t]}, {t, 0, 2π},
                AxesLabel -> {"x", "y"},
                PlotStyle -> {Blue, Red},
                PlotRange -> {{-2, 4}, {-4, 4}},
                PlotLegends -> TextString@Row@{"k = ", k}],
               {k, 1, 2, +0.1}];
frames = Table[Magnify[Grid[{{frames1[[i]], frames2[[i]]}}], 1.2], {i, 11}];
Export["image.gif", frames, "AnimationRepetitions" -> ∞, "DisplayDurations" -> 1];

from which I would say that the purpose has been achieved! ^_^

---

About the last request in the comments, i.e. wanting to scale with respect to the centroid:

• by a factor $1.5$ in the anticlockwise direction $\frac{\pi}{3}$;

• by a factor $0.8$ in the anticlockwise direction $\frac{5\pi}{6}$;

• by a factor $0.9$ in the anticlockwise direction $\frac{4\pi}{3}$;

it's necessary to divide the curve $\gamma$ into the parts placed above and below the line passing through the centroid $C_{\gamma}$ and of anticlockwise direction $\frac{5\pi}{6}$:

x[t_] := (3 + 2 Cos[t]) Cos[t]
y[t_] := (3 + 2 Cos[t]) Sin[t]
γ[t_] := {{x[t]}, {y[t]}}
Lγ = Integrate[Norm[γ'[t]], {t, 0, 2π}];
xC = Integrate[x[t] Norm[γ'[t]], {t, 0, 2π}] / Lγ;
yC = Integrate[y[t] Norm[γ'[t]], {t, 0, 2π}] / Lγ;
Cγ = Rationalize[N[{{xC}, {yC}}], 10^-10];
r[u_] := Cγ + {Cos[5π/6], Sin[5π/6]} u
tsol1 = Reduce[{γ[t] > r[u], 0 < t < 2π}][[All, 1]];
tsol2 = Reduce[{γ[t] < r[u], 0 < t < 2π}][[All, 1]];
γ1[t_] := Piecewise[{{γ[t], tsol1}, {"", True}}]
γ2[t_] := Piecewise[{{γ[t], tsol2}, {"", True}}]
R[θ_] := {{Cos[θ], -Sin[θ]}, {Sin[θ], Cos[θ]}}
S[k_] := {{k, 0}, {0, 0.8}}
γp1[k_, t_] := Cγ + R[π/3].S[k].R[-π/3].(γ1[t] - Cγ)
γp2[k_, t_] := Cγ + R[π/3].S[k].R[-π/3].(γ2[t] - Cγ)
ParametricPlot[{γ[t], γp1[1.5, t], γp2[0.9, t]}, 
               {t, 0, 2π}, AxesLabel -> {"x", "y"}, 
               PlotStyle -> {Blue, Red, Red}]

That's all.