X a compact Hausdorff space. The intersection of subsets that are open and closed is connected.

compactnessconnectednessgeneral-topology

it's me again with yet another topology question.

So this is the problem:
Let (X, $\mathcal{T}$) be a compact Hausdorff space. For every p $\in$ X, we define:

$$Q_p= \cap\{A \subset X: A \in \mathcal{T} \cap \mathcal {F}, p \in A\}$$

Show that $Q_p$ is connected for every p in X.

So here's my attempt:

We're told that $Q_p$ is the intersection of every set that's open and closed that contains p, we need to prove that said intersection is connected, I tried to do it by contradiction, assume that there is a separation for $Q_p$, such that $Q_p= A \cup B$ and $A \cap B \neq \emptyset$. But I couldn't find how the fact that $X$ is compact and Hausdorff helps me stablish a contradiction.

Help?

Best Answer

Note that $Q_p$ is an intersection of closed sets, so it is closed, and hence compact, because $X$ is compact. Furthermore, the family which we intersect to get $Q_p$ forms an open cover of $Q_p$, so by compactness it has a finite subcover $A_1,\ldots A_n$. Since the topology is closed under finite intersections, we get that $Q_p$ is also open. Therefore the family of its open subsets in the relative topology $\mathcal{T}_{Q_p}$ is a subset of $\mathcal{T}$, same for closed subsets.

Now assume that there $Q_p$ has a non-trivial clopen subset $C$. Note that it may not contain $p$ by definition of $Q_p$. However, we also get that $Q_p\setminus C$ is clopen in $C$ and since both $\mathcal{T}_{Q_p}\subset\mathcal{T}$ and $\mathcal{F}_{Q_p}\subset \mathcal{F}$, we find that $Q_p\setminus C$ is clopen in $X$ and contains $p$, which contradicts the definition of $Q_p$.