Find parametrization of $x^2y^2+y^2z^2+z^2x^2=xyz$, $x,y,z>0$. I've checked it's manifold but I can't find parametrization(s).
Find parametrization of a manifold
1 Answers
So, $$\frac {xy}z+\frac{yz}x+\frac{zx}y=1$$
As $x,y,z>0$
we can write $\frac {xy}z+\frac {yz}x=\cos^2\theta$ and $\frac{zx}y=\sin^2\theta$
So, $\frac {xy}{z\cos^2\theta}+\frac {yz}{x\cos^2\theta}=1$
So, we can write $\frac {xy}{z\cos^2\theta}=\cos^2\phi$ and $\frac {yz}{x\cos^2\theta}=\sin^2\phi$ as $\frac {xy}{z\cos^2\theta},\frac {yz}{x\cos^2\theta}>0$
So, $\frac {xy}z=\cos^2\theta\cos^2\phi--->(1)$
$\frac {yz}x=\cos^2\theta\sin^2\phi--->(2)$
$\frac{zx}y=\sin^2\theta--->(3)$
On multiplication, $xyz=\cos^4\theta\cos^2\phi \sin^2\phi \sin^2\theta-->(4) $
$(4)/(1),z^2=\cos^2\theta\sin^2\phi \sin^2\theta\implies z=\cos\theta\sin\phi \sin\theta=\frac{\sin\phi \sin2\theta}2$
Similarly, $(4)/(2)$ and $(4)/(3)$ will supply
$x=\frac{\cos\phi \sin2\theta}2$ and $y=\cos\phi\sin\phi \cos^2\theta$
As $x,y,z>0$
either $\sin2\theta, \sin\phi,\cos\phi>0\implies 0<\phi<\frac{\pi}2$ and $0<2\theta<\pi$ with $\theta\ne \frac{\pi}2$
or, $\sin2\theta, \sin\phi,\cos\phi<0\implies \pi<\phi<\frac{3\pi}2$ and $\pi<2\theta<2\pi$ with $\theta\ne \frac{3\pi}2$
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0How did you find this $\cos^2\theta\cos^2\phi$? – 2012-12-01
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0@Berci, if $X^2+Y^2+Z^2=1,$ we can write $X^2+Y^2=\cos^2\theta,Z^2=\sin^2\theta$ So, $(\frac{X}{\cos\theta})^2+(\frac{Y}{\sin\theta})^2=1$ apply the above method again. – 2012-12-01
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0@Berci, it's possible as $x,y,z>0; so,\frac{xy}z>0$ can be expressed as $Z^2$ where $Z$ is a real number – 2012-12-01
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0Thank You lab bhattacharjee! Bu i still have a problem, why this function $\phi : \begin{cases} x= x=\frac{\cos \phi \sin 2 \theta}{2} \\ y= \cos \phi \sin \phi \cos ^2 \theta \\ z= \cos \theta \sin \phi \sin \theta \end{cases}$ is bijective? – 2012-12-01
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0@pawel_now, to you, what this function $\phi$ should have been? If we think parametric form of an ellipse $\frac {x^2}{a^2}+\frac {y^2}{b^2}=1, (x=a\cos\phi,y=b\sin \phi)$ which is clearly one-to-one wrt $\phi$ – 2012-12-02