Suppose that for every $n\in\mathbb{N}$ the $V_n$ is a non-empty closed subset of a compact space $X$ and suppose that $V_{n+1}\subset V_n$,I have to show that $\bigcap_{n=1}^{\infty}V_n\neq \emptyset$

I'm not sure about my solution as follows:

So suppose that the intersection is empty, $\bigcap_{n=1}^{\infty}V_n = \emptyset$.

Now let $U_n=X\setminus V_n$ then the $U_n$ would form a covering of $X$ as $$\bigcup U_n=\bigcup(X\setminus C_n)=X\setminus(\cap C_n)=X$$

Then we have a finite subcovering $\{U_1,U_2,.....U_n\}$

Am I on the right track with this?

Thanks for any help

  • 2
    $\begingroup$ I would say that yes, you are on the right track. Now the obvious question is, what are you going to do with your finite subcovering? $\endgroup$ – MJD Oct 30 '12 at 21:45
  • $\begingroup$ Yes, you're on the way to the solution. $\endgroup$ – Beni Bogosel Oct 30 '12 at 21:45
  • $\begingroup$ Note also that your finite subcovering is not $\{U_1, U_2,\ldots, U_n\}$ but rather $\{U_{a_1}, U_{a_2},\ldots, U_{a_n}\}$. $\endgroup$ – MJD Oct 30 '12 at 21:46
  • $\begingroup$ The subsets $U_i$ are ordered and there is one containing all the rest. $\endgroup$ – i. m. soloveichik Oct 30 '12 at 21:48
  • $\begingroup$ This is a special case of a more general fact: "If $X$ is compact and $F_i, \ i\in I$ a family of closed subsets of $X$ with the property that every finite intersection of them is nonempty then $\cap_{i \in I}F_i \neq \emptyset$ ". The proof is the same as the suggested one. $\endgroup$ – P.. Oct 30 '12 at 21:52

Yes you are on the right track.

You have a finite covering, $U_1,\ldots,U_n$, which in turn means that $$X=(X\setminus V_1)\cup(X\setminus V_2)\cup\ldots\cup(X\setminus V_n)=X\setminus(V_1\cap\ldots\cap V_n)$$

Therefore $V_1\cap\ldots\cap V_n=\varnothing$. However since $V_{k+1}\subseteq V_k$ we have that $V_n=\varnothing$, in contradiction to the assumption that $V_k\neq\varnothing$ for all $k$.


Since there are only finitely many subsets $U_i$ and they are ordered then there is one containing all the rest. So one of the $U_i$ is $X$ and hence $V_i=\emptyset$, a contradiction.


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