Munkres Topology Solutions Chapter 5 May 2026

Prove that $[0,1]^\mathbbR$ is compact in product topology.

Proof. Let $X_1,\dots, X_n$ be compact. We use induction. Base case $n=1$ trivial. Assume $\prod_i=1^n-1 X_i$ compact. Let $\mathcalA$ be an open cover of $X_1 \times \dots \times X_n$ by basis elements $U \times V$ where $U \subset X_1$ open, $V \subset \prod_i=2^n X_i$ open. Fix $x \in X_1$. The slice $x \times \prod_i=2^n X_i$ is homeomorphic to $\prod_i=2^n X_i$, hence compact. Finitely many basis elements cover it; project to $X_1$ to get $W_x$ open containing $x$ such that $W_x \times \prod_i=2^n X_i$ is covered. Vary $x$, cover $X_1$ by $W_x$, extract finite subcover, then combine covers. □ munkres topology solutions chapter 5

Proof. Take $J$ as the set of continuous functions $f: X \to [0,1]$. Define $F: X \to [0,1]^J$ by $F(x)(f) = f(x)$. $F$ is continuous (product topology). $F$ injective because $X$ completely regular (compact Hausdorff $\Rightarrow$ normal $\Rightarrow$ completely regular) so functions separate points. $F$ is a closed embedding since $X$ compact, $[0,1]^J$ Hausdorff. □ Setup: $X$ compact Hausdorff, $C(X)$ with sup metric $d(f,g)=\sup_x\in X|f(x)-g(x)|$. Prove that $[0,1]^\mathbbR$ is compact in product topology

Proof. By Tychonoff, since $[0,1]$ is compact (Heine-Borel) and $\mathbbR$ is any index set, the product is compact. (Note: In product topology, not in box topology.) □ We use induction

(subspace of product): Let $X$ be compact Hausdorff. Show $X$ is homeomorphic to a subspace of $[0,1]^J$ for some $J$ (this is a step toward Urysohn metrization).

Proof. Let $f_n$ be Cauchy in sup metric. Then for each $x$, $f_n(x)$ Cauchy in $Y$, converges to $f(x)$. Need $f$ continuous. Fix $\epsilon>0$, choose $N$ such that $d(f_n,f_m)<\epsilon/3$ for $n,m\ge N$. For each $x$, pick $n_x\ge N$ such that $d(f_n_x(x),f(x))<\epsilon/3$. By continuity of $f_n_x$, $\exists \delta>0$ with $d(x,x')<\delta \Rightarrow d(f_n_x(x),f_n_x(x'))<\epsilon/3$. Then for $d(x,x')<\delta$: $d(f(x),f(x')) \le d(f(x),f_n_x(x)) + d(f_n_x(x),f_n_x(x')) + d(f_n_x(x'),f(x')) < \epsilon$. So $f$ continuous, uniform convergence. □ Exercise 39.1: Prove Tychonoff using nets: A space is compact iff every net has a convergent subnet. Then show product of compact spaces has this property.