diff --git a/src/measure/measurable-maps/approx.tex b/src/measure/measurable-maps/approx.tex index 2317052..0e6ff97 100644 --- a/src/measure/measurable-maps/approx.tex +++ b/src/measure/measurable-maps/approx.tex @@ -34,7 +34,7 @@ \begin{lemma}[Existence of Simple Approximations of the Identity] \label{lemma:separable-metric-space-approx-identity} - Let $X$ be a separable metric space, $\mathcal{A}: X \to 2^X$ be a Borel measurable \hyperref[admissible approximant function]{definition:admissible-approximant-function}, and $\seq{x_n} \subset X$ be a dense subset with $x_1 \in \bigcap_{x \in X}\mathcal{A}(x)$, then there exists $\seq{I_n} \subset X^X$ such that: + Let $X$ be a separable and metrisable topological space, $\mathcal{A}: X \to 2^X$ be a Borel measurable \hyperref[admissible approximant function]{definition:admissible-approximant-function}, and $\seq{x_n} \subset X$ be a dense subset with $x_1 \in \bigcap_{x \in X}\mathcal{A}(x)$, then there exists $\seq{I_n} \subset X^X$ such that: \begin{enumerate} \item $\seq{I_n}$ is an $\mathcal{A}$-admissible \hyperref[approximation of the identity]{definition:approximation-id-measure}. \item For each $N \in \natp$, $I_N$ is Borel measurable with $I_N(X) \subset \bracsn{x_n|1 \le n \le N}$. @@ -50,7 +50,7 @@ Since $x_1 \in \bigcap_{y \in X}\mathcal{A}(y)$, $1 \in C_N(x)$ and $C_N(x) \ne \emptyset$. - From here, let + Fix a metric $d: Y \times Y \to [0, \infty)$ and let \[ k_N(x) = \min\bracs{n \in C_N(x) \bigg | d(x, x_n) = \min_{m \in C_N(x)}d(x, x_m)} \] @@ -90,7 +90,7 @@ \begin{corollary} \label{corollary:measurable-simple-separable} - Let $(X, \cm)$ be a measurable space, $Y$ be a separable metric space, and $\mathcal{A}: Y \to 2^Y$ be a Borel measurable \hyperref[admissible approximant function]{definition:admissible-approximant-function}, then for any $f: X \to Y$, the following are equivalent: + Let $(X, \cm)$ be a measurable space, $Y$ be a separable and metrisable topological space, and $\mathcal{A}: Y \to 2^Y$ be a Borel measurable \hyperref[admissible approximant function]{definition:admissible-approximant-function}, then for any $f: X \to Y$, the following are equivalent: \begin{enumerate} \item $f$ is $(\cm, \cb_Y)$-measurable. \item For any dense subset $\seq{y_n} \subset Y$ with $y_1 \in \bigcap_{y \in Y}\mathcal{A}(y)$, there exists a sequence $\seq{f_n}$ of $(\cm, \cb_Y)$-measurable simple functions such that diff --git a/src/measure/measure/localisable.tex b/src/measure/measure/localisable.tex index 87cb741..326f428 100644 --- a/src/measure/measure/localisable.tex +++ b/src/measure/measure/localisable.tex @@ -158,12 +158,41 @@ \end{align*} is a null set. Therefore $f|_A = f_A$ almost everywhere. - \item For all $A \in \cf$, $f|_A = g|_A$ almost everywhere. Since $\mu$ is semifinite, $f = g$ almost everywhere. + \item[(U)] For all $A \in \cf$, $f|_A = g|_A$ almost everywhere. Since $\mu$ is semifinite, $f = g$ almost everywhere. \end{enumerate} Therefore $f$ is the desired function. - Now suppose that $Y$ is arbitrary. Let $\seq{y_n} \subset Y$ be a dense subset of $Y$. By \autoref{corollary:measurable-simple-separable}, for each + Now suppose that $Y$ is an arbitrary separable metrisable space. By \autoref{lemma:admissible-approximant-existence}, there exists $\seq{I_n} \subset Y^Y$ such that: + \begin{enumerate}[label=(\roman*)] + \item $I_n \to \text{Id}$ pointwise as $n \to \infty$. + \item For each $n \in \natp$, $I_n(Y)$ is finite and Borel measurable. + \end{enumerate} + + For each $n \in \natp$, let $f_{A, n} = I_n \circ f_A$, then + \begin{enumerate}[label=(\alph*)] + \item For each $A \in \cf$, $f_{A, n} \in \mathcal{L}^0(A; Y)$. + \item For each $A, B \in \cf$, $f_{A, n}|_{A \cap B} = f_{B, n}|_{A \cap B}$ almost everywhere. + \item For each $A \in \cf$, $f_{A, n}(A) \subset I_n(Y)$. + \end{enumerate} + + By the finite case, there exists $f_n: X \to Y$ such that: + \begin{enumerate} + \item $f_n \in \mathcal{L}^0(X; Y)$. + \item For each $A \in \cf$, $f_n|_A = f_{A, n}$ almost everywhere. + \end{enumerate} + + Since $Y$ is Polish, \autoref{proposition:metric-measurable-limit} implies that $\bracsn{\limv{n}f_n \text{ exists}} \in \cm$. For each $A \in \cf$, + \[ + \mu\paren{\bracs{\limv{n}f_n \text{ does not exist}} \cap A} = \mu\bracs{\limv{n}f_{A, n} \ne f_A} = 0 + \] + + As $\mu$ is semifinite, $\mu\bracsn{\limv{n}f_n \text{ does not exist}} = 0$, so there exists $f \in \mathcal{L}^0(X; Y)$ such that $f = \limv{n}f_n$ almost everywhere. In which case, + \begin{enumerate} + \item $f \in \mathcal{L}^0(X; Y)$. + \item For each $A \in \cf$, $f|_A = \limv{n}f_n|A = \limv{n}f_{A, n} = f_A$ almost everywhere. + \item[(U)] For all $A \in \cf$, $f|_A = g|_A$ almost everywhere. Since $\mu$ is semifinite, $f = g$ almost everywhere. + \end{enumerate} \end{proof}