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[parent] universal nets in compact spaces are convergent (Theorem)

Theorem - A universal net $ (x_{\alpha})_{\alpha \in \mathcal{A}}$ in a compact space $ X$ is convergent.

Proof : Suppose by contradiction that $ (x_{\alpha})_{\alpha \in \mathcal{A}}$ was not convergent. Then for every $ x \in X$ we would find neighborhoods $ U_x$ such that

$\displaystyle \forall_{\alpha \in \mathcal{A}}\;\;\; \exists_{\alpha \leq \alpha_0} \;\;\; x_{\alpha_0} \notin U_x $

The collection of all this neighborhoods cover $ X$, and as $ X$ is compact, a finite number $ U_{x_1}, U_{x_2}, \dots, U_{x_n}$ also cover $ X$.

The net $ (x_{\alpha})_{\alpha \in \mathcal{A}}$ is not eventually in $ U_{x_k}$ so it must be eventually in $ X-U_{x_k}$ (because it is a universal net). Therefore we can find $ \alpha_k \in \mathcal{A}$ such that

$\displaystyle \forall_{\alpha_k \leq \alpha} \;\;\; x_{\alpha} \in X-U_{x_k} $

Because we have a finite number $ \alpha_1, \alpha_2 \dots, \alpha_n \in \mathcal{A}$ we can find $ \gamma \in \mathcal{A}$ such that $ \alpha_k \leq \gamma$ for each $ 1 \leq k \leq n$.

Then $ x_{\gamma} \in X-U_{x_k}$ for all $ k$, i.e. $ x_{\gamma} \notin U_{x_k}$ for all $ k$. But $ U_{x_1}, U_{x_2}, \dots, U_{x_n}$ cover $ X$ and thus we have a contradiction. $ \square$



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Cross-references: eventually, net, number, finite, cover, collection, neighborhoods, contradiction, proof, convergent, compact, universal net
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This is version 1 of universal nets in compact spaces are convergent, born on 2007-09-04.
Object id is 9918, canonical name is UniversalNetsInCompactSpacesAreConvergent.
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Classification:
AMS MSC54A20 (General topology :: Generalities :: Convergence in general topology )

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