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$\DeclareMathOperator{\int}{int}$Let $(X,(e_\alpha)_{\alpha \in I})$ be a finite cell complex, i.e. $I$ finite. If $X$ is Hausdorff then the closures of the cells $\bar e_\alpha$ are compact hence $X$ is compact as a finite union of compact spaces.

What happens if $X$ is not Hausdorff? Are the closures of the cells still compact?

If $X$ is not Hausdorff there could be dense $0$-cells. But all finite examples I found were still compact. So is there an example of a finite and non-compact cell complex?

Definition of cell complex:

A cell complex is a pair $(X,(e_\alpha)_{\alpha \in I})$ where $X$ is a topological space and $(e_\alpha)_{\alpha \in I}$ is a cell decomposition, i.e. $e_\alpha\subset X$ and $X = \bigcup_{\alpha \in I} e_\alpha$ disjoint union and there is a map $n: I \to \mathbb{N_0}$ and characteristic maps $\Phi_\alpha: D^{n(\alpha)} \to X$ such that $\Phi_\alpha|_{\int{D}^{n(\alpha)}}$ is a topological embedding with $e_\alpha = \Phi_\alpha(\int{D^{n(\alpha)}})$ and $\Phi_\alpha(\partial D^{n(\alpha)})\subseteq \bigcup_{n(\beta) < n(\alpha)} e_\beta$ for all $\alpha \in I$.

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    $\begingroup$ A cell complex is always Hausdorff, so the question is void. ...unless "cell complex" means something different to you (which I assume it doesn't, since you tagged this with cw-complexes), in which case you owe us a definition. $\endgroup$ Commented Oct 17 at 10:53
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    $\begingroup$ Can someone explain what is still unclear? Or is it group dynamics that someone downvoted and now all other also downvote for fun (without comment or explanation)? It is a serious question. Anyway... $\endgroup$ Commented Oct 17 at 11:22
  • $\begingroup$ Where did you find this definition of a cell-complex? $\endgroup$ Commented Oct 19 at 14:31
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    $\begingroup$ @KritikerderElche It is basically the "intrinsic" definition. If one adds the closure-finiteness and weak topology condition one gets a non-Hausdorff or Hausdorff CW-complex. $\endgroup$ Commented Oct 19 at 19:01

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Every cell is contained in the image of its characteristic map, which is compact. Thus, a finite cell complex is a finite union of compacta and thus compact.

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    $\begingroup$ Oh, yes indeed. Nice and simple: $X = \bigcup e_\alpha \subseteq \bigcup \Phi_\alpha(D^{n(\alpha)}) \subseteq X \implies X= \bigcup_{\alpha \in I} \Phi_\alpha(D^{n(\alpha)})$ and since $\Phi_\alpha(D^{n(\alpha)})$ are compact and $I$ is finite $X$ is compact. Thanks. $\endgroup$ Commented Oct 17 at 11:58
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    $\begingroup$ Still I don't understand the downvotes... But anyway. $\endgroup$ Commented Oct 17 at 11:58

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