Question #173909

Let (X,T) be a Hausdorff space and K = {A ⊂ X| A∩K is open ∀K ⊂ X compact}. Show that K is a finer topology than K, and that (X,K) is compact.



1
Expert's answer
2021-03-31T14:08:45-0400

The topology is finer because, if UXU\subseteq X is open then, by definition it is open in subspace topology, infact open for evry compact subset of X,X, i.e. UAU\cap A is open in A.A. Hence UK.U\in K. Hence KK has a finer topology. But its not compact if XX is not.

Counterexample : Take the real line and endow it with the the standard topology. Then it is Hausdorff, but real line is non-compact. In particular take, R=n=1(n,n)\mathbb{R}=\cup_{n=1}^{\infty} (-n,n) . This is a cover in KK since it has finer topology. But it has no finite cover. Infact take finitely many and consider the largest n.n. Then n+1n+1 remains uncovered in that collection.



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