Measure of a compact subset of $mathbb{R}^d$












0














Let $K subset mathbb{R}^d$ be closed and bounded.



Define $E_n = {x in mathbb{R}^d vert exists y in K$, $ vert x - y vert < frac{1}{n} }$



wish to show lim$_{n rightarrow infty}$ $m(E_n)$ = $m(K)$.



ATTEMPT:



So I wanted to use double sided inequality to show the equality holds.



NOTE: $E_1 supset E_2 supset E_2 ..$



I have a lemma stating: if additionally $m(E_1) < + infty$ then



lim$_{n rightarrow infty}$ $m(E_n)$ = $m(bigcap_{n = 1}^{infty} E_n$) **



(is ** lemma applicable her?)



using this, If I can make a claim stating $K = bigcap_{n=1}^{infty} E_n $ ( I doubt this is true) then I am done. But I feel like I am missing a subtlety.
Sorry for being a noob, any help is greatly appreciated, I am new to measure theory and trying to learn!



Also, I have an observation, I would like to know is true:



Do all compact subsets of $mathbb{R}^d$ have finite (Lebesgue) measure.










share|cite|improve this question


















  • 2




    If you have $x in bigcap_{n=1}^{infty}E_{n}$, then $d(x, K) = 0$, right? So since $K$ is closed, $x in K$. So $bigcap_{n=1}^{infty}E_{n} subseteq K$. Also, since $K subseteq E_{n}$ for all $n in mathbb{N}$, you also have $K subseteq bigcap_{n=1}^{infty}E_{n}$, so $K = bigcap_{n=1}^{infty}E_{n}$. Try to work out the details for yourself.
    – Matija Sreckovic
    Jan 3 at 22:19












  • oh yes true, this I can workout, its a simple analysis/point set topology problem. thank you so much!!
    – eyeheartmath
    Jan 3 at 22:26
















0














Let $K subset mathbb{R}^d$ be closed and bounded.



Define $E_n = {x in mathbb{R}^d vert exists y in K$, $ vert x - y vert < frac{1}{n} }$



wish to show lim$_{n rightarrow infty}$ $m(E_n)$ = $m(K)$.



ATTEMPT:



So I wanted to use double sided inequality to show the equality holds.



NOTE: $E_1 supset E_2 supset E_2 ..$



I have a lemma stating: if additionally $m(E_1) < + infty$ then



lim$_{n rightarrow infty}$ $m(E_n)$ = $m(bigcap_{n = 1}^{infty} E_n$) **



(is ** lemma applicable her?)



using this, If I can make a claim stating $K = bigcap_{n=1}^{infty} E_n $ ( I doubt this is true) then I am done. But I feel like I am missing a subtlety.
Sorry for being a noob, any help is greatly appreciated, I am new to measure theory and trying to learn!



Also, I have an observation, I would like to know is true:



Do all compact subsets of $mathbb{R}^d$ have finite (Lebesgue) measure.










share|cite|improve this question


















  • 2




    If you have $x in bigcap_{n=1}^{infty}E_{n}$, then $d(x, K) = 0$, right? So since $K$ is closed, $x in K$. So $bigcap_{n=1}^{infty}E_{n} subseteq K$. Also, since $K subseteq E_{n}$ for all $n in mathbb{N}$, you also have $K subseteq bigcap_{n=1}^{infty}E_{n}$, so $K = bigcap_{n=1}^{infty}E_{n}$. Try to work out the details for yourself.
    – Matija Sreckovic
    Jan 3 at 22:19












  • oh yes true, this I can workout, its a simple analysis/point set topology problem. thank you so much!!
    – eyeheartmath
    Jan 3 at 22:26














0












0








0







Let $K subset mathbb{R}^d$ be closed and bounded.



Define $E_n = {x in mathbb{R}^d vert exists y in K$, $ vert x - y vert < frac{1}{n} }$



wish to show lim$_{n rightarrow infty}$ $m(E_n)$ = $m(K)$.



ATTEMPT:



So I wanted to use double sided inequality to show the equality holds.



NOTE: $E_1 supset E_2 supset E_2 ..$



I have a lemma stating: if additionally $m(E_1) < + infty$ then



lim$_{n rightarrow infty}$ $m(E_n)$ = $m(bigcap_{n = 1}^{infty} E_n$) **



(is ** lemma applicable her?)



using this, If I can make a claim stating $K = bigcap_{n=1}^{infty} E_n $ ( I doubt this is true) then I am done. But I feel like I am missing a subtlety.
Sorry for being a noob, any help is greatly appreciated, I am new to measure theory and trying to learn!



Also, I have an observation, I would like to know is true:



Do all compact subsets of $mathbb{R}^d$ have finite (Lebesgue) measure.










share|cite|improve this question













Let $K subset mathbb{R}^d$ be closed and bounded.



Define $E_n = {x in mathbb{R}^d vert exists y in K$, $ vert x - y vert < frac{1}{n} }$



wish to show lim$_{n rightarrow infty}$ $m(E_n)$ = $m(K)$.



ATTEMPT:



So I wanted to use double sided inequality to show the equality holds.



NOTE: $E_1 supset E_2 supset E_2 ..$



I have a lemma stating: if additionally $m(E_1) < + infty$ then



lim$_{n rightarrow infty}$ $m(E_n)$ = $m(bigcap_{n = 1}^{infty} E_n$) **



(is ** lemma applicable her?)



using this, If I can make a claim stating $K = bigcap_{n=1}^{infty} E_n $ ( I doubt this is true) then I am done. But I feel like I am missing a subtlety.
Sorry for being a noob, any help is greatly appreciated, I am new to measure theory and trying to learn!



Also, I have an observation, I would like to know is true:



Do all compact subsets of $mathbb{R}^d$ have finite (Lebesgue) measure.







real-analysis measure-theory lebesgue-measure






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share|cite|improve this question











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asked Jan 3 at 22:09









eyeheartmath

657




657








  • 2




    If you have $x in bigcap_{n=1}^{infty}E_{n}$, then $d(x, K) = 0$, right? So since $K$ is closed, $x in K$. So $bigcap_{n=1}^{infty}E_{n} subseteq K$. Also, since $K subseteq E_{n}$ for all $n in mathbb{N}$, you also have $K subseteq bigcap_{n=1}^{infty}E_{n}$, so $K = bigcap_{n=1}^{infty}E_{n}$. Try to work out the details for yourself.
    – Matija Sreckovic
    Jan 3 at 22:19












  • oh yes true, this I can workout, its a simple analysis/point set topology problem. thank you so much!!
    – eyeheartmath
    Jan 3 at 22:26














  • 2




    If you have $x in bigcap_{n=1}^{infty}E_{n}$, then $d(x, K) = 0$, right? So since $K$ is closed, $x in K$. So $bigcap_{n=1}^{infty}E_{n} subseteq K$. Also, since $K subseteq E_{n}$ for all $n in mathbb{N}$, you also have $K subseteq bigcap_{n=1}^{infty}E_{n}$, so $K = bigcap_{n=1}^{infty}E_{n}$. Try to work out the details for yourself.
    – Matija Sreckovic
    Jan 3 at 22:19












  • oh yes true, this I can workout, its a simple analysis/point set topology problem. thank you so much!!
    – eyeheartmath
    Jan 3 at 22:26








2




2




If you have $x in bigcap_{n=1}^{infty}E_{n}$, then $d(x, K) = 0$, right? So since $K$ is closed, $x in K$. So $bigcap_{n=1}^{infty}E_{n} subseteq K$. Also, since $K subseteq E_{n}$ for all $n in mathbb{N}$, you also have $K subseteq bigcap_{n=1}^{infty}E_{n}$, so $K = bigcap_{n=1}^{infty}E_{n}$. Try to work out the details for yourself.
– Matija Sreckovic
Jan 3 at 22:19






If you have $x in bigcap_{n=1}^{infty}E_{n}$, then $d(x, K) = 0$, right? So since $K$ is closed, $x in K$. So $bigcap_{n=1}^{infty}E_{n} subseteq K$. Also, since $K subseteq E_{n}$ for all $n in mathbb{N}$, you also have $K subseteq bigcap_{n=1}^{infty}E_{n}$, so $K = bigcap_{n=1}^{infty}E_{n}$. Try to work out the details for yourself.
– Matija Sreckovic
Jan 3 at 22:19














oh yes true, this I can workout, its a simple analysis/point set topology problem. thank you so much!!
– eyeheartmath
Jan 3 at 22:26




oh yes true, this I can workout, its a simple analysis/point set topology problem. thank you so much!!
– eyeheartmath
Jan 3 at 22:26










2 Answers
2






active

oldest

votes


















2














For each $xinmathbb{R}^{d}$ and $r>0$, let $B(x,r)={ymid d(x,y)<r}$
be the open ball centered at $x$ with radius $r$. Note that
$$
E_{n}=bigcup_{xin K}B(x,frac{1}{n})
$$

which is a bounded (because $K$ is bounded) open set. In particular
$m(E_{n})<infty$.



To show that $K=cap_{n}E_{n}$. Clearly $E_{1}supseteq E_{2}supseteqcdotssupseteq K$.
We only need to show that $cap_{n}E_{n}subseteq K$. Let $xincap_{n}E_{n}$.
For each $n$, $xin E_{n}Rightarrowexists y_{n}in K$ such that
$xin B(y_{n},frac{1}{n})$. Since $K$ is compact, for the sequence
$(y_{n})$, there exists a convergent subsequence $(y_{n_{k}})$ and
$yin K$ such that $y_{n_{k}}rightarrow y$ as $krightarrowinfty$.
Now
begin{eqnarray*}
d(x,y) & leq & d(x,y_{n_{k}})+d(y_{n_{k}},y)\
& < & frac{1}{n_{k}}+d(y_{n_{k}},y)\
& rightarrow & 0,
end{eqnarray*}

as $krightarrowinfty$. Therefore, $d(x,y)=0$ and hence $x=yin K$.
This shows that $cap_{n}E_{n}subseteq K$.






share|cite|improve this answer























  • got it thanks so much!!!
    – eyeheartmath
    Jan 3 at 22:29



















2















I doubt this is true




It is, and your method is fine, once you've also checked that $m(E_1) < +infty$.




Do all compact subsets of $mathbb{R}^d$ have finite (Lebesgue) measure.




Yes. This follows from the Heine-Borel Theorem: all compact subsets of $mathbb{R}^d$ are bounded, so are contained in some product of closed, bounded intervals, hence have measure bounded above by the measure of that product of closed, bounded intervals, which is finite. There's extra work to do to check that all compact subsets of $mathbb{R}^d$ are measurable, but indeed they are.






share|cite|improve this answer





















  • Awesome!! thank you so much!! I am always very very hesitant on making claims unless I'm 100000% certain. I thought the equality was true but wanted to be extra certain, and yes Heine-Borel, luckily enough my point-set topology is relatively strong specially in comparison to my measure theory. again, thanks!!!!
    – eyeheartmath
    Jan 3 at 22:23













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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









2














For each $xinmathbb{R}^{d}$ and $r>0$, let $B(x,r)={ymid d(x,y)<r}$
be the open ball centered at $x$ with radius $r$. Note that
$$
E_{n}=bigcup_{xin K}B(x,frac{1}{n})
$$

which is a bounded (because $K$ is bounded) open set. In particular
$m(E_{n})<infty$.



To show that $K=cap_{n}E_{n}$. Clearly $E_{1}supseteq E_{2}supseteqcdotssupseteq K$.
We only need to show that $cap_{n}E_{n}subseteq K$. Let $xincap_{n}E_{n}$.
For each $n$, $xin E_{n}Rightarrowexists y_{n}in K$ such that
$xin B(y_{n},frac{1}{n})$. Since $K$ is compact, for the sequence
$(y_{n})$, there exists a convergent subsequence $(y_{n_{k}})$ and
$yin K$ such that $y_{n_{k}}rightarrow y$ as $krightarrowinfty$.
Now
begin{eqnarray*}
d(x,y) & leq & d(x,y_{n_{k}})+d(y_{n_{k}},y)\
& < & frac{1}{n_{k}}+d(y_{n_{k}},y)\
& rightarrow & 0,
end{eqnarray*}

as $krightarrowinfty$. Therefore, $d(x,y)=0$ and hence $x=yin K$.
This shows that $cap_{n}E_{n}subseteq K$.






share|cite|improve this answer























  • got it thanks so much!!!
    – eyeheartmath
    Jan 3 at 22:29
















2














For each $xinmathbb{R}^{d}$ and $r>0$, let $B(x,r)={ymid d(x,y)<r}$
be the open ball centered at $x$ with radius $r$. Note that
$$
E_{n}=bigcup_{xin K}B(x,frac{1}{n})
$$

which is a bounded (because $K$ is bounded) open set. In particular
$m(E_{n})<infty$.



To show that $K=cap_{n}E_{n}$. Clearly $E_{1}supseteq E_{2}supseteqcdotssupseteq K$.
We only need to show that $cap_{n}E_{n}subseteq K$. Let $xincap_{n}E_{n}$.
For each $n$, $xin E_{n}Rightarrowexists y_{n}in K$ such that
$xin B(y_{n},frac{1}{n})$. Since $K$ is compact, for the sequence
$(y_{n})$, there exists a convergent subsequence $(y_{n_{k}})$ and
$yin K$ such that $y_{n_{k}}rightarrow y$ as $krightarrowinfty$.
Now
begin{eqnarray*}
d(x,y) & leq & d(x,y_{n_{k}})+d(y_{n_{k}},y)\
& < & frac{1}{n_{k}}+d(y_{n_{k}},y)\
& rightarrow & 0,
end{eqnarray*}

as $krightarrowinfty$. Therefore, $d(x,y)=0$ and hence $x=yin K$.
This shows that $cap_{n}E_{n}subseteq K$.






share|cite|improve this answer























  • got it thanks so much!!!
    – eyeheartmath
    Jan 3 at 22:29














2












2








2






For each $xinmathbb{R}^{d}$ and $r>0$, let $B(x,r)={ymid d(x,y)<r}$
be the open ball centered at $x$ with radius $r$. Note that
$$
E_{n}=bigcup_{xin K}B(x,frac{1}{n})
$$

which is a bounded (because $K$ is bounded) open set. In particular
$m(E_{n})<infty$.



To show that $K=cap_{n}E_{n}$. Clearly $E_{1}supseteq E_{2}supseteqcdotssupseteq K$.
We only need to show that $cap_{n}E_{n}subseteq K$. Let $xincap_{n}E_{n}$.
For each $n$, $xin E_{n}Rightarrowexists y_{n}in K$ such that
$xin B(y_{n},frac{1}{n})$. Since $K$ is compact, for the sequence
$(y_{n})$, there exists a convergent subsequence $(y_{n_{k}})$ and
$yin K$ such that $y_{n_{k}}rightarrow y$ as $krightarrowinfty$.
Now
begin{eqnarray*}
d(x,y) & leq & d(x,y_{n_{k}})+d(y_{n_{k}},y)\
& < & frac{1}{n_{k}}+d(y_{n_{k}},y)\
& rightarrow & 0,
end{eqnarray*}

as $krightarrowinfty$. Therefore, $d(x,y)=0$ and hence $x=yin K$.
This shows that $cap_{n}E_{n}subseteq K$.






share|cite|improve this answer














For each $xinmathbb{R}^{d}$ and $r>0$, let $B(x,r)={ymid d(x,y)<r}$
be the open ball centered at $x$ with radius $r$. Note that
$$
E_{n}=bigcup_{xin K}B(x,frac{1}{n})
$$

which is a bounded (because $K$ is bounded) open set. In particular
$m(E_{n})<infty$.



To show that $K=cap_{n}E_{n}$. Clearly $E_{1}supseteq E_{2}supseteqcdotssupseteq K$.
We only need to show that $cap_{n}E_{n}subseteq K$. Let $xincap_{n}E_{n}$.
For each $n$, $xin E_{n}Rightarrowexists y_{n}in K$ such that
$xin B(y_{n},frac{1}{n})$. Since $K$ is compact, for the sequence
$(y_{n})$, there exists a convergent subsequence $(y_{n_{k}})$ and
$yin K$ such that $y_{n_{k}}rightarrow y$ as $krightarrowinfty$.
Now
begin{eqnarray*}
d(x,y) & leq & d(x,y_{n_{k}})+d(y_{n_{k}},y)\
& < & frac{1}{n_{k}}+d(y_{n_{k}},y)\
& rightarrow & 0,
end{eqnarray*}

as $krightarrowinfty$. Therefore, $d(x,y)=0$ and hence $x=yin K$.
This shows that $cap_{n}E_{n}subseteq K$.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Jan 3 at 22:30

























answered Jan 3 at 22:28









Danny Pak-Keung Chan

2,19038




2,19038












  • got it thanks so much!!!
    – eyeheartmath
    Jan 3 at 22:29


















  • got it thanks so much!!!
    – eyeheartmath
    Jan 3 at 22:29
















got it thanks so much!!!
– eyeheartmath
Jan 3 at 22:29




got it thanks so much!!!
– eyeheartmath
Jan 3 at 22:29











2















I doubt this is true




It is, and your method is fine, once you've also checked that $m(E_1) < +infty$.




Do all compact subsets of $mathbb{R}^d$ have finite (Lebesgue) measure.




Yes. This follows from the Heine-Borel Theorem: all compact subsets of $mathbb{R}^d$ are bounded, so are contained in some product of closed, bounded intervals, hence have measure bounded above by the measure of that product of closed, bounded intervals, which is finite. There's extra work to do to check that all compact subsets of $mathbb{R}^d$ are measurable, but indeed they are.






share|cite|improve this answer





















  • Awesome!! thank you so much!! I am always very very hesitant on making claims unless I'm 100000% certain. I thought the equality was true but wanted to be extra certain, and yes Heine-Borel, luckily enough my point-set topology is relatively strong specially in comparison to my measure theory. again, thanks!!!!
    – eyeheartmath
    Jan 3 at 22:23


















2















I doubt this is true




It is, and your method is fine, once you've also checked that $m(E_1) < +infty$.




Do all compact subsets of $mathbb{R}^d$ have finite (Lebesgue) measure.




Yes. This follows from the Heine-Borel Theorem: all compact subsets of $mathbb{R}^d$ are bounded, so are contained in some product of closed, bounded intervals, hence have measure bounded above by the measure of that product of closed, bounded intervals, which is finite. There's extra work to do to check that all compact subsets of $mathbb{R}^d$ are measurable, but indeed they are.






share|cite|improve this answer





















  • Awesome!! thank you so much!! I am always very very hesitant on making claims unless I'm 100000% certain. I thought the equality was true but wanted to be extra certain, and yes Heine-Borel, luckily enough my point-set topology is relatively strong specially in comparison to my measure theory. again, thanks!!!!
    – eyeheartmath
    Jan 3 at 22:23
















2












2








2







I doubt this is true




It is, and your method is fine, once you've also checked that $m(E_1) < +infty$.




Do all compact subsets of $mathbb{R}^d$ have finite (Lebesgue) measure.




Yes. This follows from the Heine-Borel Theorem: all compact subsets of $mathbb{R}^d$ are bounded, so are contained in some product of closed, bounded intervals, hence have measure bounded above by the measure of that product of closed, bounded intervals, which is finite. There's extra work to do to check that all compact subsets of $mathbb{R}^d$ are measurable, but indeed they are.






share|cite|improve this answer













I doubt this is true




It is, and your method is fine, once you've also checked that $m(E_1) < +infty$.




Do all compact subsets of $mathbb{R}^d$ have finite (Lebesgue) measure.




Yes. This follows from the Heine-Borel Theorem: all compact subsets of $mathbb{R}^d$ are bounded, so are contained in some product of closed, bounded intervals, hence have measure bounded above by the measure of that product of closed, bounded intervals, which is finite. There's extra work to do to check that all compact subsets of $mathbb{R}^d$ are measurable, but indeed they are.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jan 3 at 22:20









user3482749

2,738414




2,738414












  • Awesome!! thank you so much!! I am always very very hesitant on making claims unless I'm 100000% certain. I thought the equality was true but wanted to be extra certain, and yes Heine-Borel, luckily enough my point-set topology is relatively strong specially in comparison to my measure theory. again, thanks!!!!
    – eyeheartmath
    Jan 3 at 22:23




















  • Awesome!! thank you so much!! I am always very very hesitant on making claims unless I'm 100000% certain. I thought the equality was true but wanted to be extra certain, and yes Heine-Borel, luckily enough my point-set topology is relatively strong specially in comparison to my measure theory. again, thanks!!!!
    – eyeheartmath
    Jan 3 at 22:23


















Awesome!! thank you so much!! I am always very very hesitant on making claims unless I'm 100000% certain. I thought the equality was true but wanted to be extra certain, and yes Heine-Borel, luckily enough my point-set topology is relatively strong specially in comparison to my measure theory. again, thanks!!!!
– eyeheartmath
Jan 3 at 22:23






Awesome!! thank you so much!! I am always very very hesitant on making claims unless I'm 100000% certain. I thought the equality was true but wanted to be extra certain, and yes Heine-Borel, luckily enough my point-set topology is relatively strong specially in comparison to my measure theory. again, thanks!!!!
– eyeheartmath
Jan 3 at 22:23




















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