Integral Transformation from circle to unit sphere
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I want to show that $displaystyle frac{1}{2pi r}int_{partial B(x,r)}u(y),mathrm{d}s(y)=frac{1}{|S^1|}int_{S^1}u(x+rtheta),mathrm{d}s(theta)$ This is essentially a shift and dilation from (or) to the unit sphere. I defined a diffeomorphism $Phicolon S^1rightarrowpartial B(x,r)subsetmathbb{R}^2\ thetamapsto x+rtheta$ where $theta$ is a point on $S^1$ . It follows that $|det(DPhi(S^1))|= r$ and therefore $displaystyle frac{1}{2pi r}int_{partial B(x,r)}u(y),mathrm{d}s(y)=frac{1}{2pi r}int_{S^1}u(Phi(S^1))r,mathrm{d}s(theta)=frac{1}{2pi(=|S^1|)}int_{S^1}u(x+rtheta),mathrm{d}s(theta)$ Is this correct? If yes, I appreciate to have a look at solutions using a different approach.
integration lebesgue-integral integral-transforms
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