$$r=\sqrt{y^2+x^2}$$
$$ y = 13x² + 4 $$$$ x =r \cdot cos \phi $$$$ r = \sqrt{(13\, r^2 \cos^2\phi + 4)^2+r^2 \, cos^2 \phi }$$
$$ r = \sqrt{169 \, r^4\cos^4\phi + 104 \, r^2 \cos^2\phi+16+ r^2 \,\cos^2 \phi }$$
$$ r = \sqrt{169 \, r^4\cos^4\phi + 105 \, r^2 \cos^2\phi+16 }$$
$$ r^2 = 169 \, r^4\cos^4\phi + 105 \, r^2 \cos^2\phi+16 $$
$$ 0 = 169 \, r^4\cos^4\phi + 105 \, r^2 \cos^2\phi-r^2+16 $$
$$ 0 = 169 \, r^4\cos^4\phi + r^2\,( 105\cos^2\phi-1)+16 $$
$$z=r^2$$
$$ 0 = z^2 \,169 \,\cos^4\phi + z\,( 105\cos^2\phi-1)+16 $$
$$ z_{1,2} = \frac{ 1-105\cos^2\phi\pm\sqrt{( 105\cos^2\phi-1)^2-4\cdot \,169 \,\cos^4\phi \cdot 16}}{2 \cdot\,169 \,\cos^4\phi } $$
$$ z_{1,2} = \frac{ 1-105\cos^2\phi\pm\sqrt{( 105\cos^2\phi)^2 -210\cos^2\phi+1-64\cdot \,169 \,\cos^4\phi }}{2 \cdot\,169 \,\cos^4\phi } $$
$$ z_{1,2} = \frac{ 1-105\cos^2\phi\pm\sqrt{209 \,\,\cos^4\phi -210\cos^2\phi+1 }}{338 \,\cos^4\phi } $$
$$ r_{1,2,3,4} = \pm\sqrt{\frac{ 1-105\cos^2\phi\pm\sqrt{209 \,\,\cos^4\phi -210\cos^2\phi+1 }}{338 \,\cos^4\phi } } $$
$$ \vec V(\phi)= r \cdot \begin{pmatrix} sin \phi\\\cos \phi\end{pmatrix}$$
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