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Here is a question which seems true to me but I can't rigorously show. Suppose $K$ is a compact subset of $\mathbb{R}^n$ such that $\mathbb{R}^n\setminus K$ is connected, does it follow that for any connected open set $U\subset \mathbb{R}^n$ such that $U\supset K$, $U\setminus K$ is also connected?

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closed as too localized by Denis Serre, Ryan Budney, Will Jagy, Matthew Daws, Joel David Hamkins Nov 7 '11 at 9:45

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of course not ! – Denis Serre Nov 7 '11 at 7:07
No. (Do you want a specific counterexample? This feels a bit like a homework question...) – Tom Smith Nov 7 '11 at 7:11
I am sorry to have omitted the condition $U\supset K$, this makes the question looks rather stupid. But I must say that this is not a homework question, this is a claim (without proof) in a proof of a paper I am reading. I don't think it is completely trivial, as this is false if we replace Rn with some other connected spaces, such as the torus. Therefore somehow we must use the property of $\mathbb{R}^n$ (e.g. Jordan's theorem), but I have no idea how to. Perhaps it's also interesting to see if we can replace $\mathbb{R}^n$ with other spaces, e.g. spheres. – Kwong Nov 7 '11 at 11:27
up vote 3 down vote accepted

Yes. Let $C$ be the closed complement of $U$, then by excision of $C$ we have $H_1(\mathbb{R}, \mathbb{R} - K) = H_1(U, U - K)$; since $H_1(\mathbb{R})=0$, you also have in fact $H_1(\mathbb{R}, \mathbb{R}-K)= H_1(U, U-K)= 0$ when $\mathbb{R}-K$ is connected.

So $H_0(U-K)$ injects into $H_0(U)$ and $U-K$ must be connected.

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I am not very familiar with relative homology, but I think we need more than that. I can only see that if $H_0(U, U-K)=0$, the inclusion induces a surjective map from $H_0(U-K)$ to $H_0(U)$, by the long exact sequence of relative homology. (Correct me if I am wrong. ) Also, if your argument is correct, doesn't it imply that if $K$ is the equator of the torus $T^2$, and $U$ is a tubular neighborhood of $K$, then $U-K$ is connected? – Kwong Nov 7 '11 at 10:24
you also have $H_1(\mathbb{R}, \mathbb{R} -K)=0$ using that $H_1(\mathbb{R})=0$ (crucially!), so by excision $H_1(U, U-K)=0$, too. The last part of my answer was incorrect, let me edit (sorry i answered too quickly). – Pierre Nov 7 '11 at 11:43
This answer has the advantage of answering his extension question (that is, on what manifolds this is still true), so bravo – Richard Rast Nov 7 '11 at 14:37
Thanks for the neat answer. So we can replace $\mathbb{R}^n$ by a connected space with vanishing $H_1$, and $K$ to be closed subset. Nice. – Kwong Nov 8 '11 at 5:19

No. $\;\;$ Let $n=2$, $\; K = [0,1]^2 \;$, $\;$ and $\; U = (0,1)\times (-\infty,\scriptsize+\normalsize\infty) \;$.

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$K$ is not a subset of $U$ (as e.g. $(0,0) \in K \setminus U$). – Henno Brandsma Nov 7 '11 at 18:12

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