diff --git a/content/Math4201/Math4201_L29.md b/content/Math4201/Math4201_L29.md new file mode 100644 index 0000000..df336c2 --- /dev/null +++ b/content/Math4201/Math4201_L29.md @@ -0,0 +1,118 @@ +# Math4201 Topology I (Lecture 29) + +## Compact and connected spaces + +### Compact spaces + +#### Theorem of uncountable compact Hausdorff spaces without isolated points + +Any non-empty compact Hausdorff space $X$ without any isolated points is uncountable. + +
+Proof + +By contradiction, let $X=\{x_n\}_{n\in\mathbb{N}}$ be a countable set. + +We construct inductively a sequence of open non-empty subspaces $\{V_i\}$ of $X$ such that + +$$ +\overline{V_1}\supseteq \overline{V_2}\supseteq \overline{V_3}\supseteq \dots +$$ + +where $x_i$ not in $\overline{V_i}$. + +This could imply that for any $j\leq i$, $x_j$ is not in $\overline{V_j}$ but $\overline{V_j}\supseteq \overline{V_i}$. This contradicts the fact that $x_j$ not in $\overline{V_i}$, that is $x_1, ..., x_i$ are not in $\overline{V_i}$. + +This is a contradiction because $\{\overline{V_i}\}$ satisfies the finite intersection property. + +$$ +\bigcap_{i=1}^\infty \overline{V_i} \neq \emptyset +$$ + +But in this case, $\bigcap_{i=1}^\infty \overline{V_i} = \emptyset$ because $x_i$ not in $\overline{V_i}$. + +To construct such $\{V_i\}$, we can start with $V_1,\dots, V_{k-1}$ are constructed, then there is a point $y_k$ in $V_{k-1}$ which isn't same as $x_k$. + +So $x_k$ not in $\overline{V_k}$ because $U_k$ is an open neighborhood of $x_k$ that don't intersect with $V_k$. + +Since $X$ is Hausdorff, there exists an open neighborhood $U_k$ of $x_k$ and $U_y$ of $y_k$ such that $U_k\cap U_y=\emptyset$. + +Let $x_k\in U_k$ and $y_k\in W_k\subseteq X$ that is open, and $U_k\cap W_k=\emptyset$. + +Let $V_k=W_k\cap V_{k-1}$. Then this is open and is contained in $V_{k-1}$ since $\overline{V_k}\subseteq \overline{V_{k-1}}$. + +Case 1: $x_k\notin V_{k-1}$. There is such $y_k$ because $V_{k-1}$ is not empty. + +Case 2: $x_k\in V_{k-1}$. Since $x_k$ is not isolated points, any open neighborhood of $x_k$ including $V_{k-1}$ contains another point. + +Therefore, $X$ is uncountable. + +
+ +#### Definition of limit point compact + +A space $X$ is limit point compact if any infinite subset of $X$ has a limit point. + +#### Definition of sequentially compact + +A space $X$ is sequentially compact if any sequence has a convergent subsequence. i.e. If $\{x_n\}_{n\in\mathbb{N}}$ is a sequence in $X$, then there are $n_1 +Example of limit point compact spaces but not compact + +Let $X'=\{a,b\}$ with trivial topology, and $X=\mathbb{N}\times X'$ with the product topology where we use the discrete topology on $\mathbb{N}$. + +$X$ isn't compact because $\{\{i\}\times X':i\in\mathbb{N}\}$ is an open cover of $X$ that doesn't have a finite subcover. + +because these open sets are disjoint, $X$ is limit point compact. + +Let $A\subseteq X$ be an infinite subset of $X$. In particular, it contains a point of the form $(i,a)$ or $(i,b)$ for $i\in\mathbb{N}$. Let $(i,a)\in A$. Then $(i,b)$ is a limit point of $A$, since any open neighborhood ($\{i\}\times X'$) of $(i,b)$ contains a point of the form $(i,a)$ or $(i,b)$. + +$X$ is not sequentially compact because the sequence $\{(n,a)\}_{n\in\mathbb{N}}$ has no convergent subsequence. + + + +
+Proof + +First, we show that 1. implies 2. + +Let $X$ be compact and $A\subseteq X$ be an infinite subset of $X$ that doesn't have any limit points. + +Then $X-A$ is open because any $x\in X-A$ isn't in the closure of $A$ otherwise it would be a limit point for $A$, and hence $x$ has an open neighborhood contained in the complement of $A$. + +Next, let $x\in A$. Since $x$ isn't a limit point of $A$, there is an open neighborhood $U_x$ of $x$ in $X$ that $U_x\cap A=\{x\}$. Now consider the open covering of $X$ given as + +$$ +\{X-A\}\cup \{U_x:x\in A\} +$$ + +This is an open cover because either $x\in X-A$ or $x\in A$ and in the latter case, $x\in U_x$ since $X$ is compact, this should have a finite subcover. Any such subcover should contain $U_x$ for any $x$ because $U_x$ is the element in the subcover for $x$. + +This implies that our finite cover contains infinite open sets, which is a contradiction. + +--- + +Continue with the proof that 2. implies 3. next time. +
+ +#### Corollary of compact spaces + +1. If $X$ is a compact topological space, then it is limit point compact. +2. If $X$ is a sequentially compact topological space, then it is limit point compact. + +
+Proof + +Proof of 1. follows from the theorem of limit point compact spaces. + + +
diff --git a/content/Math4201/_meta.js b/content/Math4201/_meta.js index 47a0510..308e7d9 100644 --- a/content/Math4201/_meta.js +++ b/content/Math4201/_meta.js @@ -32,4 +32,5 @@ export default { Math4201_L26: "Topology I (Lecture 26)", Math4201_L27: "Topology I (Lecture 27)", Math4201_L28: "Topology I (Lecture 28)", + Math4201_L29: "Topology I (Lecture 29)", }