diff --git a/src/measure/measurable-maps/approx.tex b/src/measure/measurable-maps/approx.tex index fa3199a..1cb85ff 100644 --- a/src/measure/measurable-maps/approx.tex +++ b/src/measure/measurable-maps/approx.tex @@ -23,9 +23,13 @@ \begin{definition}[Approximation of the Identity] \label{definition:approximation-id-measure} - Let $X$ be a topological space, $\mathcal{A}: X \to 2^X$ be an \hyperref[admissible approximant function]{definition:admissible-approximant-function} and $\net{I} \subset X^X$ be a net, then $\net{I}$ is an \textbf{$\mathcal{A}$-admissible approximation of the identity} if: + Let $X$ be a topological space and $\net{I} \subset X^X$ be a net, then $\net{I}$ is an \textbf{approximation of the identity} if: \begin{enumerate}[label=(AI\arabic*)] \item For each $x \in X$, $I_\alpha(x) \to x$. + \end{enumerate} + + For any \hyperref[admissible approximant function]{definition:admissible-approximant-function} $\mathcal{A}: X \to 2^X$, $\net{I}$ is \textbf{$\mathcal{A}$-admissible} if: + \begin{enumerate}[label=(AI\arabic*), start=1] \item For each $x \in X$ and $\alpha \in A$, $I_\alpha(x) \in \mathcal{A}(x)$. \end{enumerate} @@ -36,14 +40,12 @@ \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: \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 $I_N(X) \subset \bracsn{x_n|1 \le n \le N}$. + \item $\seq{I_n}$ is a Borel measurable, $\mathcal{A}$-admissible \hyperref[approximation of the identity]{definition:approximation-id-measure}. + \item For each $N \in \natp$, $I_N(X) \subset \bracsn{x_n|1 \le n \le N}$. \item For each $N \in \natp$ and $x \in X$, \[ d(x, I_N(x)) = \min\bracs{d(x, x_n)| 1 \le n \le N, x_n \in \mathcal{A}(x)} \] - - In particular, $I_N(x) \to x$ as $N \to \infty$. \end{enumerate} \end{lemma} \begin{proof} @@ -59,7 +61,21 @@ 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)} \] - be the minimum $n \in C_N(x)$ on which the minimal distance from $x$ to $\bracs{x_m|m \in C_N(x)}$ is achieved. Then, for each $n \in \natp$, + be the minimum $n \in C_N(x)$ on which the minimal distance from $x$ to $\bracs{x_m|m \in C_N(x)}$ is achieved. Define + \[ + I_N: X \to X \quad x \mapsto x_{k_N(x)} + \] + + (2): For each $x \in X$, $k_N(x) \in [N]$, so $I_N(x) \in \bracsn{x_n|1 \le n \le N}$. + + (3): Let $x \in X$, then by definition of $k_N$ and $C_N$, + \begin{align*} + d(x, I_N(x))& = d(x, x_{k_N(x)}) = \min_{n \in C_N(x)}d(x, x_n) \\ + &= \min\bracsn{d(x, x_n)|1 \le n \le N, x_n \in \mathcal{A}(x)} + \end{align*} + + + (1, Borel Measurable): Fix $N \in \natp$, then for each $n \in \natp$, \begin{align*} \bracs{k_N \le n} &= \bigcup_{j = 1}^n \bracs{x \in X \bigg | j \in C_N(x), d(x, x_j) = \min_{m \in C_N(x)}d(x, x_m)} \\ &= \bigcup_{j = 1}^n \bracs{j \in C_N} \cap \bracs{x \in X \bigg | d(x, x_j) = \min_{m \in C_N(x)}d(x, x_m)} \\ @@ -76,23 +92,17 @@ I_N: X \to \bracsn{x_n|1 \le n \le N} \quad x \mapsto x_{k_N(x)} \] - then for each $1 \le n \le N$, $\bracs{I_N = x_n} = \bracs{k_N = n}$ is a Borel set. Thus $I_N$ is Borel measurable. + By assumption that $\seq{x_n}$ are distinct, $\bracs{I_N = x_n} = \bracs{k_N = n}$ is a Borel set for each $1 \le n \le N$. Therefore $I_N$ is Borel measurable. - In addition, for each $x \in X$, $k_N(x) \in C_N(x)$, so + (1, $\mathcal{A}$-Admissible): Let $N \in \natp$ and $x \in X$, then \[ I_N(x) = x_{k_N(x)} \in \bracs{x_n|n \in C_N(x)} \subset \mathcal{A}(x) \] - and $\seq{I_N}$ satisfies (AI2). - - Finally, let $x \in X$, then by definition of $k_N$, + (1, Approximation): Let $x \in X$ and $\eps > 0$. Since $x \in \ol{\mathcal{A}(x)^o}$ and $\seq{x_n}$ is dense in $X$, there exists $N_0 \in \natp$ such that $x_{N_0} \in \mathcal{A}(x)$ and $d(x, x_{N_0}) < \eps$. By (3), \[ - d(x, I_N(x)) = d(x, x_{k_N(x)}) = \min\bracs{d(x, x_n)| 1 \le n \le N, x_n \in \mathcal{A}(x)} + \limv{N}d(x, I_N(x)) = \limv{N}\min\bracs{d(x, x_n)| 1 \le n \le N, x_n \in \mathcal{A}(x)} = 0 \] - - For any $\eps > 0$, since $x \in \ol{\mathcal{A}(x)^o}$, there exists $N_0 \in \natp$ such that $x_{N_0} \in \mathcal{A}(x)$ and $d(x, x_{N_0}) < \eps$. In which case, for any $N \ge N_0$, $N_0 \in C_N(x)$ and $d(x, I_N(x)) \le d(x, x_{N_0}) < \eps$. Thus $I_N(x) \to x$ as $N \to \infty$, and $\seq{I_N}$ satisfies (AI1). - - Therefore $\seq{I_N}$ is an approximation of the identity satisfying (1) and (2). \end{proof} \begin{remark}