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January 28th, 2017, 07:37 PM  #1 
Newbie Joined: Jan 2017 From: Toronto Posts: 2 Thanks: 0  Finding the Radial Shadow that a manifold casts on a sphere centered at $(a,b,c)$
Hello everyone, I am new to this forum and as such, I extend my warmest greetings. I have a question about this problem and I want to understand it, but I'm not sure if my logic is solid. The question is: Let M be compact, connected, oriented surface with boundary in $\mathbb{R}^3$, and $(a,b,c)$ not in M. Define:$$\Omega_{a,b,c} = \int_{M}\omega_{a,b,c}$$ where:$$\omega_{a,b,c} = \frac{(xa)dy\wedge dz+(yb)dz\wedge dx+(zc)dx\wedge dy}{[(xa)^2+(yb)^2+(zc)^2]^{3/2}}$$Show there is a path $j(t)$ crossing $M$ transversally, such that if $\mu$ is an orientation form on $M$ and $j'(t)\wedge \mu$ is orientation form on $\mathbb{R}^3$ (for $j'(t)$, using the metric to identify tangent and cotangent bundles), then: $$\lim_{t_{1}\rightarrow0^{}, t_2\rightarrow0^{+}}\Omega(j(t_2)j(t_1))=4\pi$$ The question ends here. Now reading a solution manual to Spivak's Calculus on Manifolds, I found that if $M$ is a boundary surface in $\mathbb{R}^3$ of a manifold $N$, then the integral on $M$ of $\omega_{a,b,c}$ is also $4\pi$. Now, the way I think I should approach this is by letting $N=M\cup M'$, where $M'$ is a 2dimensional Manifold. Then, let $t_2 = (a,b,c)$ and $t_1 = (a',b',c')$. Then $t_2\notin N$, $t_1 \in N\partial N$. Then: $$\Omega(j(t_2)) +\int_{M'}\omega_{a,b,c}=0,\Omega(j(t_1))+\int_{M' }\omega_{a,b,c}=4\pi$$ Subtracting these two and taking the limit as $t_2\rightarrow0^{+}, t_1\rightarrow0^{}$ gives the desired limit. Is this a fine answer for this question? Or am I missing something that could be included in this solution? Thanks for your time. Last edited by mathandeconstudent; January 28th, 2017 at 07:49 PM. 

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casts, centered, finding, manifold, radial, shadow, sphere 
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