diff --git a/src/measure/measurable-maps/in-measure.tex b/src/measure/measurable-maps/in-measure.tex index 07b0d97..b275f65 100644 --- a/src/measure/measurable-maps/in-measure.tex +++ b/src/measure/measurable-maps/in-measure.tex @@ -18,6 +18,7 @@ and $f_\alpha \to f$ \textbf{locally in measure} if $f_\alpha \to f$ in measure on every set of finite measure. \end{definition} + \begin{definition}[Cauchy in Measure] \label{definition:cauchy-in-measure} Let $(X, \cm, \mu)$ be a measure space, $(Y, d)$ be a metric space, and $\fF$ be a filter of $(\cm, \cb_Y)$-measurable functions, then $\fF$ is \textbf{Cauchy in measure} if for every $\eps, \delta > 0$, there exists $A \in \fF$ such that $\mu(\bracs{d(f, g) > \delta}) < \eps$ for all $f, g \in A$. @@ -25,6 +26,35 @@ Alternatively, if $\net{f}$ is a net of $(\cm, \cb_Y)$-measurable functions, then $\net{f}$ is \textbf{Cauchy in measure} if for every $\eps, \delta > 0$, there exists $\alpha_0 \in A$ such that for each $\alpha, \beta \in A$ with $\alpha, \beta \ge \alpha_0$, $\mu(\bracs{d(f_\alpha, f_\beta) > \delta}) < \eps$. \end{definition} +\begin{proposition} +\label{proposition:convergence-in-measure} + Let $(X, \cm, \mu)$ be a semifinite measure space, $(Y, d)$ be a metric space, and $\fF$ be a filter of $(\cm, \cb_Y)$-measurable functions, then $\fF$ is Cauchy in measure if and only if: + \begin{enumerate} + \item[(L)] $\fF$ is locally Cauchy in measure. + \item[(T)] For each $\eps, \delta > 0$, there exists $F \in \fF$ and $A \in \cm$ with $\mu(A) < \infty$ such that + \[ + \sup_{f, g \in F}\mu(A^c \cap \bracs{d(f, g) > \delta}) < \eps + \] + \end{enumerate} +\end{proposition} +\begin{proof} + (L) + (T) $\Rightarrow$ (In Measure): Let $\eps, \delta > 0$. By (T) then there exists $F_1 \in \fF$ and $A \in \cm$ with $\mu(A) < \infty$ such that + \[ + \sup_{f, g \in F_1}\mu(A^c \cap \bracs{d(f, g) > \delta}) < \eps + \] + + By (L), there exists $F_2 \in \fF$ with $F_2 \subset F_1$ such that + \[ + \sup_{f, g \in F_2}\mu(A \cap \bracs{d(f, g) > \delta}) < \eps + \] + + Therefore + \[ + \sup_{f, g \in F_2}\mu\bracs{d(f, g) > \delta} < 2\eps + \] +\end{proof} + + \begin{lemma} \label{lemma:ae-in-measure} Let $(X, \cm, \mu)$ be a finite measure space, $(Y, d)$ be a metric space, and $\seq{f_n}$ and $f$ be Borel measurable functions from $X$ to $Y$ such that $f_n \to f$ almost everywhere, then $f_n \to f$ in measure.