It is obvious geometrically, but how one proves with a few words, analytically, the statement above?Additionally, if one has a smaller open disc $D_\epsilon$ of radius $\epsilon$ centered at a point of $D$, how to conclude that $D-D_\epsilon$ is still a manifol with boundary $\partial D \cup\partial D_\epsilon$?
The Closed disc $D$ is a manifold with boundary
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0@Mercy the closed one – 2012-06-24