From 76107709a8604077e3b1a91f26b5b979872d1437 Mon Sep 17 00:00:00 2001 From: Bokuan Li Date: Tue, 16 Jun 2026 13:42:54 -0400 Subject: [PATCH] Added a description of weak-* convergence in the case of bounded Borel functions. --- refs.bib | 14 +++++++------- src/measure/vector/fin.tex | 21 ++++++++++++++++++++- 2 files changed, 27 insertions(+), 8 deletions(-) diff --git a/refs.bib b/refs.bib index 16abb18..05b13af 100644 --- a/refs.bib +++ b/refs.bib @@ -216,11 +216,11 @@ doi = {10.1007/BF02771592} } -@MISC {StackRadonDual, - TITLE = {How to understand C(X)'' = bounded Borel measurable functions?}, - AUTHOR = {GEdgar (https://math.stackexchange.com/users/442/gedgar)}, - HOWPUBLISHED = {Mathematics Stack Exchange}, - NOTE = {URL:https://math.stackexchange.com/q/392719 (version: 2013-05-15)}, - EPRINT = {https://math.stackexchange.com/q/392719}, - URL = {https://math.stackexchange.com/q/392719} +@misc {StackRadonDual, + title = {How to understand C(X)'' = bounded Borel measurable functions?}, + author = {GEdgar (https://math.stackexchange.com/users/442/gedgar)}, + howpublished = {Mathematics Stack Exchange}, + note = {URL:https://math.stackexchange.com/q/392719 (version: 2013-05-15)}, + eprint = {https://math.stackexchange.com/q/392719}, + url = {https://math.stackexchange.com/q/392719} } \ No newline at end of file diff --git a/src/measure/vector/fin.tex b/src/measure/vector/fin.tex index b229793..5c8bbfb 100644 --- a/src/measure/vector/fin.tex +++ b/src/measure/vector/fin.tex @@ -93,9 +93,28 @@ In any case, the above example shows that a linear functional on $M(X, \cm; \com \] \end{proof} + +Despite the fact that it does not cover the full dual space, the bounded Borel functions still forms a subspace where weak-* convergence has a convenient description. + \begin{proposition} \label{proposition:measure-l-infinity-dominated-convergence} - Let $ + Let $(X, \cm)$ be a measurable space, $E$ be a normed vector space over $K \in \RC$, and $\mathscr{M} \subset M(X, \cm; E)$ be a closed subspace such that: + \begin{enumerate} + \item[(P)] For each $x \in X$, $\bracs{x} \in \cm$, and the delta mass $\delta_x$ is in $\mathscr{M}$. + \end{enumerate} + + then for any sequence $f_n: X \to E^*$ of bounded measurable functions and $f: X \to E^*$ be a bounded measurable function, the following are equivalent: + \begin{enumerate} + \item For each $\mu \in \mathscr{M}$, $\limv{n}\int f_n d\mu = \int f d\mu$. + \item For each $x \in X$, $\limv{n}f_n(x) = f(x)$, and $\sup_{n \in \natp}\norm{f_n}_u < \infty$. + \end{enumerate} \end{proposition} +\begin{proof} + (1) $\Rightarrow$ (2): By (P), for each $x \in X$, $\limv{n}f_n(x) = f(x)$. By the \hyperref[Uniform Boundedness Principle]{theorem:uniform-boundedness}, + \[ + \sup_{n \in \natp}\norm{f_n}_u \le \sup_{n \in \natp}\norm{f_n}_{\mathscr{M}^*} < \infty + \] + (2) $\Rightarrow$ (1): By the \hyperref[Dominated Convergence Theorem]{theorem:dct-bochner-vector}. +\end{proof}