diff --git a/src/fa/norm/separable.tex b/src/fa/norm/separable.tex index 979cc64..6a47c36 100644 --- a/src/fa/norm/separable.tex +++ b/src/fa/norm/separable.tex @@ -31,31 +31,6 @@ (3): For any $A \subset E$, $A = \bigcup_{n \in \natp}A \cap nS$. By \autoref{proposition:separable-metric-space}, $A \cap nS$ is separable for each $n \in \natp$. Therefore $A$ is also separable. \end{proof} -\begin{proposition} -\label{proposition:separable-banach-borel-sigma-algebra} - Let $E$ be a separable normed vector space, then the Borel $\sigma$-algebra on $E$ is generated by the following families of sets: - \begin{enumerate} - \item Open sets in $E$ with respect to the strong topology. - \item $\bracs{B(x, r)|x \in E, r > 0}$. - \item $\bracsn{\ol{B(x, r)}|x \in E, r > 0}$. - \item Open sets in $E$ with respect to the weak topology. - \end{enumerate} - - -\end{proposition} -\begin{proof} - (1) $\Leftrightarrow$ (2) $\Leftrightarrow$ (3): By \autoref{proposition:separable-metric-borel-sigma-algebra}. - - (4) $\subset$ (1): Every weakly open set is strongly open. - - (2) $\subset$ (4): By \autoref{proposition:seminorm-lsc}, $\norm{\cdot}_E: E \to [0, \infty)$ is Borel measurable with respect to the weak topology. For any $x \in E$, let - \[ - \phi_x: E \to [0, \infty) \quad y \mapsto \norm{x - y}_E - \] - - then $\phi_x$ is Borel measurable with respect to the weak topology, so $B(x, r) = \bracs{\phi_x < r}$ is a Borel set with respect to the weak topology. -\end{proof} - \begin{lemma} \label{lemma:compact-embed} Let $E$ be a normed vector space over $K \in \RC$ and $A \subset [0, 1]$ be closed, then $C(A; E)$ embeds isometrically into $C([0, 1]; E)$. @@ -82,5 +57,59 @@ Thus there exists $y \in (x - \delta, x)$ with $\norm{Tf(z) - Tf(x)}_E < \eps$ for all $z \in (y, x) \cap [0, 1]$. Similarly, there exists $y' \in (x, x + \delta)$ with $\norm{Tf(z) - Tf(x)}_E < \eps$ for all $z \in (x, y') \cap [0, 1]$. Therefore $Tf$ is continuous at $x$. Since this holds for all $x \in U$ and $x \in A$, $Tf \in C([0, 1]; E)$. \end{proof} +\begin{theorem}[Banach-Mazur] +\label{theorem:banach-mazur} + Let $E$ be a separable normed vector space over $K \in \RC$, then there exists an isometric embedding $\iota \in L(E; C([0, 1]; K))$. +\end{theorem} +\begin{proof} + Let $B$ be the closed unit ball of $E^*$, equipped with the weak* topology. By the \hyperref[Hahn-Banach Theorem]{proposition:hahn-banach-utility}, the linear mapping + \[ + E \to C(B; K) \quad x(\phi) = \dpn{x, \phi}{E} + \] + + is an isometric embedding. By \autoref{proposition:separable-dual}, $B$ is a compact metric space. The \hyperref[Alexandroff-Hausdorff Theorem]{theorem:cantor-universality} then provides a continuous surjection $f: 2^{\natp} \to B$. Thus the composition map + \[ + C(B; K) \to C(2^{\natp}; K) \quad g \mapsto g \circ f + \] + + is a linear isometric embedding. Let $\mathcal{C} \subset [0, 1]$ be the Cantor set, then $\mathcal{C}$ is homeomorphic to $2^{\natp}$ through \autoref{proposition:cantor-space-embedding}. Hence $C(2^{\natp}; K)$ is isometrically isomorphic to $C(\mathcal{C}; K)$. + + Finally, \autoref{lemma:compact-embed} provides yet another linear isometric embedding $C(\mathcal{C}; K) \to C([0, 1]; K)$. Composing the above maps as follows + \[ + \xymatrix{ +E \ar@{->}[r] & C(B; K) \ar@{->}[r] & C(2^{\mathbb N^+}; K) \ar@{->}[r] & C(\mathcal{C}; K) \ar@{->}[r] & C([0, 1]; K) +} + \] + + yields the desired embedding. +\end{proof} + + + +\begin{proposition} +\label{proposition:separable-banach-borel-sigma-algebra} + Let $E$ be a separable normed vector space, then the Borel $\sigma$-algebra on $E$ is generated by the following families of sets: + \begin{enumerate} + \item Open sets in $E$ with respect to the strong topology. + \item $\bracs{B(x, r)|x \in E, r > 0}$. + \item $\bracsn{\ol{B(x, r)}|x \in E, r > 0}$. + \item Open sets in $E$ with respect to the weak topology. + \end{enumerate} + + +\end{proposition} +\begin{proof} + (1) $\Leftrightarrow$ (2) $\Leftrightarrow$ (3): By \autoref{proposition:separable-metric-borel-sigma-algebra}. + + (4) $\subset$ (1): Every weakly open set is strongly open. + + (2) $\subset$ (4): By \autoref{proposition:seminorm-lsc}, $\norm{\cdot}_E: E \to [0, \infty)$ is Borel measurable with respect to the weak topology. For any $x \in E$, let + \[ + \phi_x: E \to [0, \infty) \quad y \mapsto \norm{x - y}_E + \] + + then $\phi_x$ is Borel measurable with respect to the weak topology, so $B(x, r) = \bracs{\phi_x < r}$ is a Borel set with respect to the weak topology. +\end{proof} + diff --git a/src/measure/radon/c0.tex b/src/measure/radon/c0.tex index 42cf621..a769f6d 100644 --- a/src/measure/radon/c0.tex +++ b/src/measure/radon/c0.tex @@ -125,7 +125,7 @@ As such a $\phi \in C_0(X; E)$ exists for all $\eps > 0$ and $\seqf{A_j}$, $\norm{I_\mu}_{C_0(X; E)^*} \ge \norm{\mu}_{\text{var}}$. Therefore the map $\mu \mapsto I_\mu$ is isometric. - (Surjective): Let $B = \bracsn{\phi \in E^*|\norm{\phi}_{E^*} \le 1}$ and equip it with the weak*-topology and + (Surjective): Let $B = \bracsn{\phi \in E^*|\norm{\phi}_{E^*} \le 1}$ and equip it with the weak* topology and \[ T: C_0(X; E) \to C_0(X \times B; K) \quad (Tf)(x, \phi) = \dpn{f(x), \phi}{E} \] diff --git a/src/topology/dst/zero.tex b/src/topology/dst/zero.tex index 914ce6f..746e1b7 100644 --- a/src/topology/dst/zero.tex +++ b/src/topology/dst/zero.tex @@ -22,6 +22,17 @@ Let $2 = \bracs{0, 1}$ be equipped with the discrete topology, then $2^{\natp}$ is the \textbf{Cantor space}. \end{definition} +\begin{proposition} +\label{proposition:cantor-space-embedding} + The mapping + \[ + 2^{\natp} \to [0, 1] \quad \seq{x_n} \mapsto 2\sum_{n \in \natp} \frac{x_n}{3^n} + \] + + is an embedding. +\end{proposition} + + \begin{definition}[Baire Space] \label{definition:the-baire-space} Let $\natp$ be equipped with the discrete topology, then $\mathscr{N} = (\natp)^{\natp}$ is the \textbf{Baire space}. @@ -57,11 +68,11 @@ Finally, suppose that $\bracsn{C(n_1, \cdots, n_N)| \bracsn{n_k}_1^N \subset \natp, N \in \natp} \subset 2^X$ has been constructed to satisfy (i)-(iv). Since $X$ is complete, for each $\seq{n_k} \in \mathscr{N}$, there exists a unique $f(\seq{n_k}) \in X$ such that $\bracs{f(\seq{n_k})} = \bigcap_{N \in \natp}C(n_1, \cdots, n_N)$. For each $x \in X$, (iii) and (iv) imply that there exists $\seq{n_k} \in \mathscr{N}$ such that $x = f(\seq{n_k})$. As such, $f: \mathscr{N} \to X$ is a surjective mapping. For any $\seq{m_k}, \seq{n_k} \in \mathscr{N}$ and $N \in \natp$ with $m_k = n_k$ for each $1 \le k \le N$, $d(f(\seq{m_k}), f(\seq{n_k})) \le 1/N$. Therefore $f \in C(\mathscr{N}; X)$. \end{proof} -\begin{proposition} -\label{proposition:cantor-universality} +\begin{theorem}[Alexandroff-Hausdorff] +\label{theorem:cantor-universality} Let $X$ be a non-empty compact metrisable space, then there exists a surjective mapping $f \in C(2^{\natp}; X)$. -\end{proposition} -\begin{proof} +\end{theorem} +\begin{proof}[Proof, adapted from {{\cite[Proposition 8.2.7]{CohnMeasure}}}. ] Let $d: X \times X \to [0, \infty)$ be a metric on $X$. Since $X$ is compact, for each $N \in \natp$, there exists $K_N \in \natp$ and $\seqf{x_{N, k}|1 \le k \le K_N} \subset X$ such that $X = \bigcup_{k = 1}^{K_N}B(x_{N, k}, 1/N)$. To construct the desired map, it is sufficient to construct