If $M$ is a smooth $n$-manifold, the famous Whitney embedding theorems show that we can view $M$ as an embedded submanifold of some Euclidean space $\mathbb{R}^N$.

Does the Heine-Borel theorem still hold for subsets of $M$? I.e., is $S\subseteq M$ compact iff $S$ is closed in $M$, and bounded in the induced metric from $\mathbb{R}^n$?


1 Answer 1


No. For example, if $M=(0,1)$, then $M$ is closed in itself and embeds boundedly in $\Bbb{R}$, but is not compact.

If $M$ were embedded as a closed submanifold of $\Bbb{R}^N$, the result you want would follow immediately. But there's no guarantee that $M$ has a closed embedding in $\Bbb{R}^N$ just because it has an embedding in $\Bbb{R}^N$.


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