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'''9.9.''' Let <math>f \in L^{1}([0,1])</math> and let <math> F(x)=\int_{0}^{x}f(t)dt </math>. If <math> ~E  </math> is a measurable subset of <math>~[0,1]  </math>, show that
 
'''9.9.''' Let <math>f \in L^{1}([0,1])</math> and let <math> F(x)=\int_{0}^{x}f(t)dt </math>. If <math> ~E  </math> is a measurable subset of <math>~[0,1]  </math>, show that
 
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'''(a)'''  <math> F(E)=\{y: \exist ~x \in E </math> with <math> ~y=F(x)\} </math> is measurable.
 
'''(a)'''  <math> F(E)=\{y: \exist ~x \in E </math> with <math> ~y=F(x)\} </math> is measurable.
 
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'''Proof.'''
 
'''Proof.'''
  
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'''(b)'''  <math> m(F(E)) \leq \int_{E}|f(t)| dt </math>.
 
'''(b)'''  <math> m(F(E)) \leq \int_{E}|f(t)| dt </math>.
 
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'''Proof.'''
 
'''Proof.'''
  
 
'''(Step 1)''' <math> ~E=(a,b) </math>
 
'''(Step 1)''' <math> ~E=(a,b) </math>
  
First assume that <math>f \geq 0 </math>. Since <math> ~F</math> is monotone and continuous, <math> m(F(E))=|F(b)-F(a)|=\left|\int_{a}^{b}f(t) dt \right| \leq \int_{E}|f(t)| dt </math>.
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Let <math>~ f=f^{+}-f^{-} </math> and let <math> F^{+}(x)=\int_{0}^{x}f^{+}(t)dt, ~F^{-}(x)=\int_{0}^{x}f^{-}(t)dt </math>, so that <math>~F^{+}</math> and <math>~F^{-}</math> are increasing.
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Then, <math>~F(x)=\int_{0}^{x}f(t)dt=F^{+}(x)-F^{-}(x)</math>, and <math>\int_{0}^{x}|f(t)|dt=F^{+}(x)+F^{-}(x) </math>.
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<math> m(F(E)) = \max_{x \in E}F(x)- \min_{x \in E}F(x) \leq \left( \max_{x \in E}F^{+}(x) - \min_{x \in E}F^{-}(t) \right)-\left( \min_{x \in E} F^{+}(x)-\max_{x \in E}F^{-}(x) \right) </math>
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<math>=(F^{+}(b)-F^{-}(a))-(F^{+}(a)-F^{-}(b))=(F^{+}(b)+F^{-}(b))-(F^{+}(a)+F^{-}(a))=\int_{a}^{b}|f(t)|dt =\int_{E}|f(t)|dt </math>
  
In general, <math> m(F(E)) \leq \int_{a}^{b}f^{+}(t) dt + \int_{a}^{b} f^{-}(t)dt =\int_{a}^{b}|f(t)|dt =\int_{E}|f(t)|dt </math>.
 
  
  
 
'''(Step 2)'''  <math>~E</math> : open set
 
'''(Step 2)'''  <math>~E</math> : open set
  
We can write <math>E=\bigcup_{n=1}^{\infty}I_{n} </math> when <math> I_{n}'s </math> are open intervals, so that
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Let <math>E=\bigcup_{n=1}^{\infty}I_{n} </math> when <math> ~I_{n} </math>'s are disjoint open intervals, so that
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<math> m(F(E))=m(F(\bigcup_{n=1}^{\infty}I_{n}))=m(\bigcup_{n=1}^{\infty}F(I_{n})) \leq \sum_{n=1}^{\infty} m(F(I_{n}))  \stackrel{(Step 1)}{\leq} \sum_{n=1}^{\infty} \int_{I_{n}}|f(t)| dt \stackrel{\rm LDCT}{=} \int_{E}|f(t)|dt </math>
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'''(Step 3)''' <math>~E</math> is a <math>~G_{\delta}</math>-set
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We can write <math> E= \bigcap_{n=1}^{\infty} G_{i}</math> when <math>~G_{n}</math>'s are nested open sets(<math>~G_{1} \supseteq G_{2} \supseteq ... </math>). Then,
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<math> m(F(E))=m(F(\bigcap_{n=1}^{\infty} G_{i})) \leq m(\bigcap_{n=1}^{\infty}F(G_{n}))=\lim_{n \to \infty}m(\bigcap_{i=1}^{n}F(G_{i}))=\lim_{n \to \infty}m(F(G_{n})) </math>
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<math>\stackrel{(Step 2)}{\leq} \lim_{n \to \infty} \int_{G_{n}}|f(t)|dt=\lim_{n \to \infty} \int_{0}^{1} |f(t)|\chi_{G_{n}}(t) dt \stackrel{\rm MCT} {=} \int_{0}^{1}|f(t)|\chi_{\bigcap_{n=1}^{\infty}G_{n}}(t)dt = \int_{0}^{1}|f(t)| \chi_{E}(t) dt = \int_{E}|f(t)| dt </math>
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'''(Step 4)''' <math>~E</math> is a measurable set.
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From a characterization of measurability, <math>~E=H-Z</math>, where <math>~H</math> is a <math>~G_{\delta}</math> set and <math>~m(Z)=0 </math>. Then,
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<math>m(F(E))=m(F(H-Z)) \leq m(F(H)) \stackrel{(Step 3)}{\leq} \int_{H}|f(t)|dt = \int_{E}|f(t)|dt </math>
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<math> m(F(E))=m(F(\bigcup_{n=1}^{\infty}I_{n}))=m(\bigcup_{n=1}^{\infty}F(I_{n})) \leq \sum_{n=1}^{\infty} m(F(I_{n}))  \leq \sum_{n=1}^{\infty} \int_{I_{n}}|f(t)| dt= \int_{E}|f(t)|dt </math>
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- JIN -

Latest revision as of 21:10, 21 July 2008


9.9. Let $ f \in L^{1}([0,1]) $ and let $ F(x)=\int_{0}^{x}f(t)dt $. If $ ~E $ is a measurable subset of $ ~[0,1] $, show that


(a) $ F(E)=\{y: \exist ~x \in E $ with $ ~y=F(x)\} $ is measurable.


Proof.

Let $ \int_{0}^{1}|f(t)|dt=M<\infty $.

$ \forall ~ x,y \in [0,1] (x \leq y) $,

$ |F(y)-F(x)|=\int_{x}^{y}f(t)dt=\int_{0}^{1}f(t) \chi_{[x,y]}(t) dt~ \stackrel{\rm Holder} {\leq} ~\left(\int_{0}^{1}|f(t)|dt\right)~||\chi_{[x,y]}||_{\infty} = M|x-y| $.

Hence $ ~F $ is a Lipschitz map, which preserves measurability. This proves (a).



(b) $ m(F(E)) \leq \int_{E}|f(t)| dt $.


Proof.

(Step 1) $ ~E=(a,b) $

Let $ ~ f=f^{+}-f^{-} $ and let $ F^{+}(x)=\int_{0}^{x}f^{+}(t)dt, ~F^{-}(x)=\int_{0}^{x}f^{-}(t)dt $, so that $ ~F^{+} $ and $ ~F^{-} $ are increasing.

Then, $ ~F(x)=\int_{0}^{x}f(t)dt=F^{+}(x)-F^{-}(x) $, and $ \int_{0}^{x}|f(t)|dt=F^{+}(x)+F^{-}(x) $.

$ m(F(E)) = \max_{x \in E}F(x)- \min_{x \in E}F(x) \leq \left( \max_{x \in E}F^{+}(x) - \min_{x \in E}F^{-}(t) \right)-\left( \min_{x \in E} F^{+}(x)-\max_{x \in E}F^{-}(x) \right) $

$ =(F^{+}(b)-F^{-}(a))-(F^{+}(a)-F^{-}(b))=(F^{+}(b)+F^{-}(b))-(F^{+}(a)+F^{-}(a))=\int_{a}^{b}|f(t)|dt =\int_{E}|f(t)|dt $


(Step 2) $ ~E $ : open set

Let $ E=\bigcup_{n=1}^{\infty}I_{n} $ when $ ~I_{n} $'s are disjoint open intervals, so that

$ m(F(E))=m(F(\bigcup_{n=1}^{\infty}I_{n}))=m(\bigcup_{n=1}^{\infty}F(I_{n})) \leq \sum_{n=1}^{\infty} m(F(I_{n})) \stackrel{(Step 1)}{\leq} \sum_{n=1}^{\infty} \int_{I_{n}}|f(t)| dt \stackrel{\rm LDCT}{=} \int_{E}|f(t)|dt $


(Step 3) $ ~E $ is a $ ~G_{\delta} $-set

We can write $ E= \bigcap_{n=1}^{\infty} G_{i} $ when $ ~G_{n} $'s are nested open sets($ ~G_{1} \supseteq G_{2} \supseteq ... $). Then,

$ m(F(E))=m(F(\bigcap_{n=1}^{\infty} G_{i})) \leq m(\bigcap_{n=1}^{\infty}F(G_{n}))=\lim_{n \to \infty}m(\bigcap_{i=1}^{n}F(G_{i}))=\lim_{n \to \infty}m(F(G_{n})) $

$ \stackrel{(Step 2)}{\leq} \lim_{n \to \infty} \int_{G_{n}}|f(t)|dt=\lim_{n \to \infty} \int_{0}^{1} |f(t)|\chi_{G_{n}}(t) dt \stackrel{\rm MCT} {=} \int_{0}^{1}|f(t)|\chi_{\bigcap_{n=1}^{\infty}G_{n}}(t)dt = \int_{0}^{1}|f(t)| \chi_{E}(t) dt = \int_{E}|f(t)| dt $


(Step 4) $ ~E $ is a measurable set.

From a characterization of measurability, $ ~E=H-Z $, where $ ~H $ is a $ ~G_{\delta} $ set and $ ~m(Z)=0 $. Then,

$ m(F(E))=m(F(H-Z)) \leq m(F(H)) \stackrel{(Step 3)}{\leq} \int_{H}|f(t)|dt = \int_{E}|f(t)|dt $


- JIN -

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