General Topology – Compact and Connected Intersection in Hausdorff Space

compactnessconnectednessgeneral-topology

Prove that if $X$ is Hausdorff and $\mathfrak{C}$ is a nonempty chain of compact and connected subsets of $X$, then $\bigcap \mathfrak{C}$ is compact and connected.

Here are the definitions which were provided:


A family $\mathfrak{F}$ of sets is a chain if for all $A,B \in \mathfrak{F}$ either $A \subset B$, or $B \subset A$. (If $\mathfrak{C}$ is a nonempty chain of nonempty compact sets, then $\bigcap \mathfrak{C} \neq \emptyset$.)

A space is Hausdorff (or $T_2$) if for all distinct $a, b \in X$ there are $U, V \in \mathcal{T}$ so that $a \in U$, $b \in V$ and $U \cap V = \emptyset$. Here, $\mathcal{T}$ is the topology on $X$.


My confusion starts in what seems to me like a contradiction between definitions. We are assuming that $X$ is Hausdorff, so there is a separation of $X$ so there are disjoint $U, V \in \mathcal{T}$ with $a \in U$ and $b \in V$. But we are also given that there is a chain of compact and connected sets $\mathfrak{C}$, which by definition, $U \subset V$ or $V \subset U$. But they are supposed to be disjoint according to the Hausdorff property.

Can somebody please explain this to me and assist me in solving this problem with a proper proof? Many thanks in advance for your time and patience, it is greatly appreciated.

Best Answer

It seems the following.

Fix any element $K\in\mathfrak{C}$. Then the set $C=\bigcap \mathfrak{C}$ is compact as an intersection $\bigcap\{ K\cap L:L\in\mathfrak{C}\}$ of a family $ \{ K\cap L:L\in\mathfrak{C}\}$ of closed subsets of a compact set $K$.

Assume that the set $C$ is not connected. This means that $C$ can be represented as a union $C_1\cup C_2$ of two its disjoint non-empty clopen (that is, closed and open) subsets. Since $C_1$ and $C_2$ are disjoint compact subsets of a Hausdorff space $X$, it is well known and easy to prove that they can be separated by disjoint open neighborhoods, that is there exist two disjoint open subsets $U_1$ and $U_2$ of the space $X$ such that $C_1\subset U_1$ and $C_2\subset U_2$. Put $U=U_1\cup U_2$.

Now we are going to find a set $L\in\frak C$ such that $L\subset U$. If $K\subset U$, we are done. In the opposite case consider a family ${\frak C}ā€™=\{(K\cap L)\setminus U:L\in\frak C\}$. Since $\bigcap{\frak C}=C\subset U$, $\bigcap \mathfrak{C}'=\varnothing$. Since $\frak Cā€™$ is a family of compact subsets of the compact space $K$, it contains an empty element $(K\cap L)\setminus U$. Since the family $\frak C $ is a chain, this implies that there exists a set $L\in\frak C$ such that $L\subset U$. Then $(L\cap U_1)\cup (L\cap U_2)$ is a partition of the set $L$ into two its disjoint clopen subsets. Each of these sets is non-empty. Indeed, since $\frak C$ is a chain, $L\supset \bigcap \mathfrak{C}=C$. Then $L\cap U_i\supset C\cap U_i\ne\varnothing$. So the set $L$ is not connected, a contradiction.

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