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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 $\partial D \cup\partial D_\epsilon$?

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    @Mercy the closed one2012-06-24

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