Question on Egoroff-like theorem
$begingroup$
Hi all I was tackled by this question from Folland's real analysis second edition in the second chapter, it looks like a modified Egoroff theorem but I cannot really tackle it, it is question 41 of chapter 2 which reads as follows:
Let $mu$ be a $sigma$-finite measure and $ f_n rightarrow f $ a.e. Then there exist measurable sets $E_1,E_2,ldots subset X $ such that
$$muleft(left(bigcup_{i=1}^infty E_iright)^Cright)=0$$ and $f_n rightarrow f $ uniformly on each $ E_i. $
I think this might have something to do with Egoroff's theorem but that theorem mentions nothing about complement having measure zero, only as small as you would like, which is what confuses me. Can anyone point out a proof of this with an explanation?
real-analysis measure-theory uniform-convergence
$endgroup$
|
show 7 more comments
$begingroup$
Hi all I was tackled by this question from Folland's real analysis second edition in the second chapter, it looks like a modified Egoroff theorem but I cannot really tackle it, it is question 41 of chapter 2 which reads as follows:
Let $mu$ be a $sigma$-finite measure and $ f_n rightarrow f $ a.e. Then there exist measurable sets $E_1,E_2,ldots subset X $ such that
$$muleft(left(bigcup_{i=1}^infty E_iright)^Cright)=0$$ and $f_n rightarrow f $ uniformly on each $ E_i. $
I think this might have something to do with Egoroff's theorem but that theorem mentions nothing about complement having measure zero, only as small as you would like, which is what confuses me. Can anyone point out a proof of this with an explanation?
real-analysis measure-theory uniform-convergence
$endgroup$
3
$begingroup$
The ideas in my answer here (math.stackexchange.com/questions/1068043/…) should be helpful.
$endgroup$
– PhoemueX
Jul 31 '15 at 10:10
1
$begingroup$
@PhoemueX thanks tried your other answer but I still cannot understand it
$endgroup$
– Don John Prep
Jul 31 '15 at 11:09
1
$begingroup$
If you start with the stronger assumption that $mu$ is a finite measure, can you see how the assertion is obtained from Egorov's theorem?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:21
2
$begingroup$
Let $F = bigcup E_n$. Then you have $Xsetminus F subset X setminus E_k$, and hence $mu(Xsetminus F) < 1/k$. Since that holds for all $k$, $mu(Xsetminus F) = 0$. Okay, the case of finite measure done. Now, for $sigma$-finite measure, write $X = bigcup A_m$, where each $A_m$ has finite measure. Perhaps it makes things easier to assume the $A_m$ disjoint, one can do that. So by the preceding, for each $m$, we have a sequence $E^{(m)}_n$ of subsets of $A_m$. How can you use that to get the desired sequence of subsets of $X$?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:48
3
$begingroup$
@AbeerAbassi: When you have got it worked out, you can post your own answer to this question, with your solution.
$endgroup$
– Nate Eldredge
Jul 31 '15 at 14:11
|
show 7 more comments
$begingroup$
Hi all I was tackled by this question from Folland's real analysis second edition in the second chapter, it looks like a modified Egoroff theorem but I cannot really tackle it, it is question 41 of chapter 2 which reads as follows:
Let $mu$ be a $sigma$-finite measure and $ f_n rightarrow f $ a.e. Then there exist measurable sets $E_1,E_2,ldots subset X $ such that
$$muleft(left(bigcup_{i=1}^infty E_iright)^Cright)=0$$ and $f_n rightarrow f $ uniformly on each $ E_i. $
I think this might have something to do with Egoroff's theorem but that theorem mentions nothing about complement having measure zero, only as small as you would like, which is what confuses me. Can anyone point out a proof of this with an explanation?
real-analysis measure-theory uniform-convergence
$endgroup$
Hi all I was tackled by this question from Folland's real analysis second edition in the second chapter, it looks like a modified Egoroff theorem but I cannot really tackle it, it is question 41 of chapter 2 which reads as follows:
Let $mu$ be a $sigma$-finite measure and $ f_n rightarrow f $ a.e. Then there exist measurable sets $E_1,E_2,ldots subset X $ such that
$$muleft(left(bigcup_{i=1}^infty E_iright)^Cright)=0$$ and $f_n rightarrow f $ uniformly on each $ E_i. $
I think this might have something to do with Egoroff's theorem but that theorem mentions nothing about complement having measure zero, only as small as you would like, which is what confuses me. Can anyone point out a proof of this with an explanation?
real-analysis measure-theory uniform-convergence
real-analysis measure-theory uniform-convergence
edited Jul 31 '15 at 13:25
Math1000
19.2k31745
19.2k31745
asked Jul 31 '15 at 8:41
Don John PrepDon John Prep
200111
200111
3
$begingroup$
The ideas in my answer here (math.stackexchange.com/questions/1068043/…) should be helpful.
$endgroup$
– PhoemueX
Jul 31 '15 at 10:10
1
$begingroup$
@PhoemueX thanks tried your other answer but I still cannot understand it
$endgroup$
– Don John Prep
Jul 31 '15 at 11:09
1
$begingroup$
If you start with the stronger assumption that $mu$ is a finite measure, can you see how the assertion is obtained from Egorov's theorem?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:21
2
$begingroup$
Let $F = bigcup E_n$. Then you have $Xsetminus F subset X setminus E_k$, and hence $mu(Xsetminus F) < 1/k$. Since that holds for all $k$, $mu(Xsetminus F) = 0$. Okay, the case of finite measure done. Now, for $sigma$-finite measure, write $X = bigcup A_m$, where each $A_m$ has finite measure. Perhaps it makes things easier to assume the $A_m$ disjoint, one can do that. So by the preceding, for each $m$, we have a sequence $E^{(m)}_n$ of subsets of $A_m$. How can you use that to get the desired sequence of subsets of $X$?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:48
3
$begingroup$
@AbeerAbassi: When you have got it worked out, you can post your own answer to this question, with your solution.
$endgroup$
– Nate Eldredge
Jul 31 '15 at 14:11
|
show 7 more comments
3
$begingroup$
The ideas in my answer here (math.stackexchange.com/questions/1068043/…) should be helpful.
$endgroup$
– PhoemueX
Jul 31 '15 at 10:10
1
$begingroup$
@PhoemueX thanks tried your other answer but I still cannot understand it
$endgroup$
– Don John Prep
Jul 31 '15 at 11:09
1
$begingroup$
If you start with the stronger assumption that $mu$ is a finite measure, can you see how the assertion is obtained from Egorov's theorem?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:21
2
$begingroup$
Let $F = bigcup E_n$. Then you have $Xsetminus F subset X setminus E_k$, and hence $mu(Xsetminus F) < 1/k$. Since that holds for all $k$, $mu(Xsetminus F) = 0$. Okay, the case of finite measure done. Now, for $sigma$-finite measure, write $X = bigcup A_m$, where each $A_m$ has finite measure. Perhaps it makes things easier to assume the $A_m$ disjoint, one can do that. So by the preceding, for each $m$, we have a sequence $E^{(m)}_n$ of subsets of $A_m$. How can you use that to get the desired sequence of subsets of $X$?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:48
3
$begingroup$
@AbeerAbassi: When you have got it worked out, you can post your own answer to this question, with your solution.
$endgroup$
– Nate Eldredge
Jul 31 '15 at 14:11
3
3
$begingroup$
The ideas in my answer here (math.stackexchange.com/questions/1068043/…) should be helpful.
$endgroup$
– PhoemueX
Jul 31 '15 at 10:10
$begingroup$
The ideas in my answer here (math.stackexchange.com/questions/1068043/…) should be helpful.
$endgroup$
– PhoemueX
Jul 31 '15 at 10:10
1
1
$begingroup$
@PhoemueX thanks tried your other answer but I still cannot understand it
$endgroup$
– Don John Prep
Jul 31 '15 at 11:09
$begingroup$
@PhoemueX thanks tried your other answer but I still cannot understand it
$endgroup$
– Don John Prep
Jul 31 '15 at 11:09
1
1
$begingroup$
If you start with the stronger assumption that $mu$ is a finite measure, can you see how the assertion is obtained from Egorov's theorem?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:21
$begingroup$
If you start with the stronger assumption that $mu$ is a finite measure, can you see how the assertion is obtained from Egorov's theorem?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:21
2
2
$begingroup$
Let $F = bigcup E_n$. Then you have $Xsetminus F subset X setminus E_k$, and hence $mu(Xsetminus F) < 1/k$. Since that holds for all $k$, $mu(Xsetminus F) = 0$. Okay, the case of finite measure done. Now, for $sigma$-finite measure, write $X = bigcup A_m$, where each $A_m$ has finite measure. Perhaps it makes things easier to assume the $A_m$ disjoint, one can do that. So by the preceding, for each $m$, we have a sequence $E^{(m)}_n$ of subsets of $A_m$. How can you use that to get the desired sequence of subsets of $X$?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:48
$begingroup$
Let $F = bigcup E_n$. Then you have $Xsetminus F subset X setminus E_k$, and hence $mu(Xsetminus F) < 1/k$. Since that holds for all $k$, $mu(Xsetminus F) = 0$. Okay, the case of finite measure done. Now, for $sigma$-finite measure, write $X = bigcup A_m$, where each $A_m$ has finite measure. Perhaps it makes things easier to assume the $A_m$ disjoint, one can do that. So by the preceding, for each $m$, we have a sequence $E^{(m)}_n$ of subsets of $A_m$. How can you use that to get the desired sequence of subsets of $X$?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:48
3
3
$begingroup$
@AbeerAbassi: When you have got it worked out, you can post your own answer to this question, with your solution.
$endgroup$
– Nate Eldredge
Jul 31 '15 at 14:11
$begingroup$
@AbeerAbassi: When you have got it worked out, you can post your own answer to this question, with your solution.
$endgroup$
– Nate Eldredge
Jul 31 '15 at 14:11
|
show 7 more comments
1 Answer
1
active
oldest
votes
$begingroup$
Since $mu$ is a $sigma-$finite measure, there exists ${A_n}_{nin mathbb{N}}$ such that $bigcup_{n}A_n=X$ and $mu(A_n)<infty$ $forall n$.
Let $epsilon>0$ be given. Note that $f_nrightarrow f$ a.e on each $A_n$. Therefore, by Egoroff's theorem, there exists $E_nsubset A_n$ such that $mu(A_nsetminus E_n)<frac{epsilon}{2^n}$ and $f_n rightarrow f$ uniformly on $E_n$ for each $nin mathbb{N}$.
Now, observe that
begin{align*}
mu(left(bigcup_{n}E_n right)^c)&= muleft( bigcup_n A_n setminus bigcup_n E_n right)\ &= muleft(bigcup_{n}(A_nsetminus E_n)right)\&leq sum_n mu(A_nsetminus E_n)\&<sum_nfrac{epsilon}{2^n}=epsilon hspace{1 in}forall nin mathbb{N}.
end{align*}
Since $epsilon >0$ is arbitrary, $mu(left(bigcup_{n}E_n right)^c)=0$.
Since $f_nrightarrow f$ uniformly on each $E_n$ by construction, we are done.
$endgroup$
1
$begingroup$
Hi, when you write $mu(E_n^c) < frac{1}{n}$ you actually mean $mu(A_n - E_n)$. Thus your argument doesn't work as stated. But you can actually prove that Egoroff works for $sigma- $ finite case by using your construction and using a standard $frac{epsilon}{2^n}$ argument.
$endgroup$
– Soham
Dec 11 '18 at 5:36
$begingroup$
@LucyferZedd I did not realize that! I fixed it :) Thank you!
$endgroup$
– Lev Ban
Dec 14 '18 at 16:06
add a comment |
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$begingroup$
Since $mu$ is a $sigma-$finite measure, there exists ${A_n}_{nin mathbb{N}}$ such that $bigcup_{n}A_n=X$ and $mu(A_n)<infty$ $forall n$.
Let $epsilon>0$ be given. Note that $f_nrightarrow f$ a.e on each $A_n$. Therefore, by Egoroff's theorem, there exists $E_nsubset A_n$ such that $mu(A_nsetminus E_n)<frac{epsilon}{2^n}$ and $f_n rightarrow f$ uniformly on $E_n$ for each $nin mathbb{N}$.
Now, observe that
begin{align*}
mu(left(bigcup_{n}E_n right)^c)&= muleft( bigcup_n A_n setminus bigcup_n E_n right)\ &= muleft(bigcup_{n}(A_nsetminus E_n)right)\&leq sum_n mu(A_nsetminus E_n)\&<sum_nfrac{epsilon}{2^n}=epsilon hspace{1 in}forall nin mathbb{N}.
end{align*}
Since $epsilon >0$ is arbitrary, $mu(left(bigcup_{n}E_n right)^c)=0$.
Since $f_nrightarrow f$ uniformly on each $E_n$ by construction, we are done.
$endgroup$
1
$begingroup$
Hi, when you write $mu(E_n^c) < frac{1}{n}$ you actually mean $mu(A_n - E_n)$. Thus your argument doesn't work as stated. But you can actually prove that Egoroff works for $sigma- $ finite case by using your construction and using a standard $frac{epsilon}{2^n}$ argument.
$endgroup$
– Soham
Dec 11 '18 at 5:36
$begingroup$
@LucyferZedd I did not realize that! I fixed it :) Thank you!
$endgroup$
– Lev Ban
Dec 14 '18 at 16:06
add a comment |
$begingroup$
Since $mu$ is a $sigma-$finite measure, there exists ${A_n}_{nin mathbb{N}}$ such that $bigcup_{n}A_n=X$ and $mu(A_n)<infty$ $forall n$.
Let $epsilon>0$ be given. Note that $f_nrightarrow f$ a.e on each $A_n$. Therefore, by Egoroff's theorem, there exists $E_nsubset A_n$ such that $mu(A_nsetminus E_n)<frac{epsilon}{2^n}$ and $f_n rightarrow f$ uniformly on $E_n$ for each $nin mathbb{N}$.
Now, observe that
begin{align*}
mu(left(bigcup_{n}E_n right)^c)&= muleft( bigcup_n A_n setminus bigcup_n E_n right)\ &= muleft(bigcup_{n}(A_nsetminus E_n)right)\&leq sum_n mu(A_nsetminus E_n)\&<sum_nfrac{epsilon}{2^n}=epsilon hspace{1 in}forall nin mathbb{N}.
end{align*}
Since $epsilon >0$ is arbitrary, $mu(left(bigcup_{n}E_n right)^c)=0$.
Since $f_nrightarrow f$ uniformly on each $E_n$ by construction, we are done.
$endgroup$
1
$begingroup$
Hi, when you write $mu(E_n^c) < frac{1}{n}$ you actually mean $mu(A_n - E_n)$. Thus your argument doesn't work as stated. But you can actually prove that Egoroff works for $sigma- $ finite case by using your construction and using a standard $frac{epsilon}{2^n}$ argument.
$endgroup$
– Soham
Dec 11 '18 at 5:36
$begingroup$
@LucyferZedd I did not realize that! I fixed it :) Thank you!
$endgroup$
– Lev Ban
Dec 14 '18 at 16:06
add a comment |
$begingroup$
Since $mu$ is a $sigma-$finite measure, there exists ${A_n}_{nin mathbb{N}}$ such that $bigcup_{n}A_n=X$ and $mu(A_n)<infty$ $forall n$.
Let $epsilon>0$ be given. Note that $f_nrightarrow f$ a.e on each $A_n$. Therefore, by Egoroff's theorem, there exists $E_nsubset A_n$ such that $mu(A_nsetminus E_n)<frac{epsilon}{2^n}$ and $f_n rightarrow f$ uniformly on $E_n$ for each $nin mathbb{N}$.
Now, observe that
begin{align*}
mu(left(bigcup_{n}E_n right)^c)&= muleft( bigcup_n A_n setminus bigcup_n E_n right)\ &= muleft(bigcup_{n}(A_nsetminus E_n)right)\&leq sum_n mu(A_nsetminus E_n)\&<sum_nfrac{epsilon}{2^n}=epsilon hspace{1 in}forall nin mathbb{N}.
end{align*}
Since $epsilon >0$ is arbitrary, $mu(left(bigcup_{n}E_n right)^c)=0$.
Since $f_nrightarrow f$ uniformly on each $E_n$ by construction, we are done.
$endgroup$
Since $mu$ is a $sigma-$finite measure, there exists ${A_n}_{nin mathbb{N}}$ such that $bigcup_{n}A_n=X$ and $mu(A_n)<infty$ $forall n$.
Let $epsilon>0$ be given. Note that $f_nrightarrow f$ a.e on each $A_n$. Therefore, by Egoroff's theorem, there exists $E_nsubset A_n$ such that $mu(A_nsetminus E_n)<frac{epsilon}{2^n}$ and $f_n rightarrow f$ uniformly on $E_n$ for each $nin mathbb{N}$.
Now, observe that
begin{align*}
mu(left(bigcup_{n}E_n right)^c)&= muleft( bigcup_n A_n setminus bigcup_n E_n right)\ &= muleft(bigcup_{n}(A_nsetminus E_n)right)\&leq sum_n mu(A_nsetminus E_n)\&<sum_nfrac{epsilon}{2^n}=epsilon hspace{1 in}forall nin mathbb{N}.
end{align*}
Since $epsilon >0$ is arbitrary, $mu(left(bigcup_{n}E_n right)^c)=0$.
Since $f_nrightarrow f$ uniformly on each $E_n$ by construction, we are done.
edited Dec 14 '18 at 16:05
answered Aug 6 '18 at 14:31
Lev BanLev Ban
1,0671317
1,0671317
1
$begingroup$
Hi, when you write $mu(E_n^c) < frac{1}{n}$ you actually mean $mu(A_n - E_n)$. Thus your argument doesn't work as stated. But you can actually prove that Egoroff works for $sigma- $ finite case by using your construction and using a standard $frac{epsilon}{2^n}$ argument.
$endgroup$
– Soham
Dec 11 '18 at 5:36
$begingroup$
@LucyferZedd I did not realize that! I fixed it :) Thank you!
$endgroup$
– Lev Ban
Dec 14 '18 at 16:06
add a comment |
1
$begingroup$
Hi, when you write $mu(E_n^c) < frac{1}{n}$ you actually mean $mu(A_n - E_n)$. Thus your argument doesn't work as stated. But you can actually prove that Egoroff works for $sigma- $ finite case by using your construction and using a standard $frac{epsilon}{2^n}$ argument.
$endgroup$
– Soham
Dec 11 '18 at 5:36
$begingroup$
@LucyferZedd I did not realize that! I fixed it :) Thank you!
$endgroup$
– Lev Ban
Dec 14 '18 at 16:06
1
1
$begingroup$
Hi, when you write $mu(E_n^c) < frac{1}{n}$ you actually mean $mu(A_n - E_n)$. Thus your argument doesn't work as stated. But you can actually prove that Egoroff works for $sigma- $ finite case by using your construction and using a standard $frac{epsilon}{2^n}$ argument.
$endgroup$
– Soham
Dec 11 '18 at 5:36
$begingroup$
Hi, when you write $mu(E_n^c) < frac{1}{n}$ you actually mean $mu(A_n - E_n)$. Thus your argument doesn't work as stated. But you can actually prove that Egoroff works for $sigma- $ finite case by using your construction and using a standard $frac{epsilon}{2^n}$ argument.
$endgroup$
– Soham
Dec 11 '18 at 5:36
$begingroup$
@LucyferZedd I did not realize that! I fixed it :) Thank you!
$endgroup$
– Lev Ban
Dec 14 '18 at 16:06
$begingroup$
@LucyferZedd I did not realize that! I fixed it :) Thank you!
$endgroup$
– Lev Ban
Dec 14 '18 at 16:06
add a comment |
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3
$begingroup$
The ideas in my answer here (math.stackexchange.com/questions/1068043/…) should be helpful.
$endgroup$
– PhoemueX
Jul 31 '15 at 10:10
1
$begingroup$
@PhoemueX thanks tried your other answer but I still cannot understand it
$endgroup$
– Don John Prep
Jul 31 '15 at 11:09
1
$begingroup$
If you start with the stronger assumption that $mu$ is a finite measure, can you see how the assertion is obtained from Egorov's theorem?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:21
2
$begingroup$
Let $F = bigcup E_n$. Then you have $Xsetminus F subset X setminus E_k$, and hence $mu(Xsetminus F) < 1/k$. Since that holds for all $k$, $mu(Xsetminus F) = 0$. Okay, the case of finite measure done. Now, for $sigma$-finite measure, write $X = bigcup A_m$, where each $A_m$ has finite measure. Perhaps it makes things easier to assume the $A_m$ disjoint, one can do that. So by the preceding, for each $m$, we have a sequence $E^{(m)}_n$ of subsets of $A_m$. How can you use that to get the desired sequence of subsets of $X$?
$endgroup$
– Daniel Fischer♦
Jul 31 '15 at 13:48
3
$begingroup$
@AbeerAbassi: When you have got it worked out, you can post your own answer to this question, with your solution.
$endgroup$
– Nate Eldredge
Jul 31 '15 at 14:11