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Neighborhood Retract of Boundary

  1. Aug 22, 2012 #1


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    Here is the problem: If M is a manifold with boundary, then find a retraction r:U→∂M where U is a neighborhood of ∂M.

    I realize the Collar Neighborhood Theorem essentially provides the desired map, but I am actually using this result to prove the aforementioned theorem. My thought on how to prove the theorem is to show local existence, show that you can locally extend a retraction, and then use Zorn's Lemma to construct a neighborhood retraction of the boundary. The only difficulty I run into here is showing local extendability. Can anyone help me with this step?
  2. jcsd
  3. Aug 29, 2012 #2
    I don't remember needing Zorn's Lemma when I had to do this exercise, but I think we were allowed to assume the boundary was compact.

    Can you be more specific about what you mean by locally extending a retraction?
  4. Aug 29, 2012 #3
    i think the open sets on the boundary are isomorphic to half spaces, if that helps any.
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