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If $M \subset \mathbb R^n$ is a compact smooth manifold with boundary, and ${M_\varepsilon }$ is the closed $\varepsilon$-neighborhood of $M$ in $\mathbb R^n$, then whether for sufficiently small $\varepsilon$, ${M_\varepsilon }$ is a smooth manifold?

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Yes. This follows from the tubular neighborhood theorem, which you may find in many differential geometry/topology books. See e.g. http://www.google.com/search?q=tubular%20neighborhood%20theorem&um=1&ie=UTF-8&hl=en&tbo=u&tbm=bks

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    Wouldn't this be rather a comment than an answer?2011-11-13
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    @t.b.: what's the difference?2011-11-13
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    @Damian: look at an answer e.g. by Arturo Magidin and look at this which basically says: it is a consequence of the tubular neighborhood theorem, look, here's what Google gives you. Then you should see a fundamental difference (in quality, content, helpfulness, etc...)2011-11-13
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    I wouldn't have used such a strong word. The link didn't work for me, (it still doesn't), so I stripped out the 's' from `https` and the rest of the Google noise, sorry that I overlooked the `&tbm=bks` modifier.2011-11-13
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    Now it works. Thank you.2011-11-13
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    @t.b.: ok, thank you, I understand.2011-11-13
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    Maybe it is not so easy for this situation. The book I am reading says that when $M$ is boundaryless ${M_\varepsilon }$ is indeed a smooth manifold by tubular neighborhood for some $\varepsilon$. But when $M$ has boundary, we can only prove that ${M_\varepsilon }$ is $C^1$ without higher smoothness. That is where I cannot figure out how.2011-11-13