Any countable set has measurable zero












1












$begingroup$


To demonstrate that ANY countable set has measure zero, is it sufficient to show that the natural numbers have a measure zero? If so, why; and, if not, why not?



Thank you :)










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$endgroup$








  • 1




    $begingroup$
    Are you referring specifically to Lebesgue measure? Because this isn't true for an arbitrary measure.
    $endgroup$
    – carmichael561
    Dec 10 '18 at 2:04






  • 1




    $begingroup$
    No, it is not sufficient. But, if you have a countable set, you can write it as something similar to the natural numbers. Specifically, you can sequence the set, so that each element has a unique corresponding natural number, i.e $S = {a_1, a_2 ,a_3, cdots}$
    $endgroup$
    – rubikscube09
    Dec 10 '18 at 2:05










  • $begingroup$
    Hey Carmichael. I'm not sure. My book has only mentioned the term "measure zero," without additional qualifying adjectives. To quote, "A set $A subseteq mathbb{R}$ has measure zero if, for all $epsilon > 0,$ there exists a countable collection of open intervals $O_n$ with the property that $A$ is contained in the union of all of the intervals $O_n$ and the sum of the lengths of all of the intervals is less than or equal to $epsilon.$"
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:06








  • 1




    $begingroup$
    Yes it is identical. To answer the other question, your book is referring to the Lebesgue measure on $mathbb{R}$. There are other measures of course.
    $endgroup$
    – rubikscube09
    Dec 10 '18 at 2:14








  • 1




    $begingroup$
    Thanks, Rubik! :)
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:14
















1












$begingroup$


To demonstrate that ANY countable set has measure zero, is it sufficient to show that the natural numbers have a measure zero? If so, why; and, if not, why not?



Thank you :)










share|cite|improve this question









$endgroup$








  • 1




    $begingroup$
    Are you referring specifically to Lebesgue measure? Because this isn't true for an arbitrary measure.
    $endgroup$
    – carmichael561
    Dec 10 '18 at 2:04






  • 1




    $begingroup$
    No, it is not sufficient. But, if you have a countable set, you can write it as something similar to the natural numbers. Specifically, you can sequence the set, so that each element has a unique corresponding natural number, i.e $S = {a_1, a_2 ,a_3, cdots}$
    $endgroup$
    – rubikscube09
    Dec 10 '18 at 2:05










  • $begingroup$
    Hey Carmichael. I'm not sure. My book has only mentioned the term "measure zero," without additional qualifying adjectives. To quote, "A set $A subseteq mathbb{R}$ has measure zero if, for all $epsilon > 0,$ there exists a countable collection of open intervals $O_n$ with the property that $A$ is contained in the union of all of the intervals $O_n$ and the sum of the lengths of all of the intervals is less than or equal to $epsilon.$"
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:06








  • 1




    $begingroup$
    Yes it is identical. To answer the other question, your book is referring to the Lebesgue measure on $mathbb{R}$. There are other measures of course.
    $endgroup$
    – rubikscube09
    Dec 10 '18 at 2:14








  • 1




    $begingroup$
    Thanks, Rubik! :)
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:14














1












1








1


1



$begingroup$


To demonstrate that ANY countable set has measure zero, is it sufficient to show that the natural numbers have a measure zero? If so, why; and, if not, why not?



Thank you :)










share|cite|improve this question









$endgroup$




To demonstrate that ANY countable set has measure zero, is it sufficient to show that the natural numbers have a measure zero? If so, why; and, if not, why not?



Thank you :)







real-analysis general-topology elementary-set-theory






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Dec 10 '18 at 1:49









Rafael VergnaudRafael Vergnaud

348217




348217








  • 1




    $begingroup$
    Are you referring specifically to Lebesgue measure? Because this isn't true for an arbitrary measure.
    $endgroup$
    – carmichael561
    Dec 10 '18 at 2:04






  • 1




    $begingroup$
    No, it is not sufficient. But, if you have a countable set, you can write it as something similar to the natural numbers. Specifically, you can sequence the set, so that each element has a unique corresponding natural number, i.e $S = {a_1, a_2 ,a_3, cdots}$
    $endgroup$
    – rubikscube09
    Dec 10 '18 at 2:05










  • $begingroup$
    Hey Carmichael. I'm not sure. My book has only mentioned the term "measure zero," without additional qualifying adjectives. To quote, "A set $A subseteq mathbb{R}$ has measure zero if, for all $epsilon > 0,$ there exists a countable collection of open intervals $O_n$ with the property that $A$ is contained in the union of all of the intervals $O_n$ and the sum of the lengths of all of the intervals is less than or equal to $epsilon.$"
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:06








  • 1




    $begingroup$
    Yes it is identical. To answer the other question, your book is referring to the Lebesgue measure on $mathbb{R}$. There are other measures of course.
    $endgroup$
    – rubikscube09
    Dec 10 '18 at 2:14








  • 1




    $begingroup$
    Thanks, Rubik! :)
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:14














  • 1




    $begingroup$
    Are you referring specifically to Lebesgue measure? Because this isn't true for an arbitrary measure.
    $endgroup$
    – carmichael561
    Dec 10 '18 at 2:04






  • 1




    $begingroup$
    No, it is not sufficient. But, if you have a countable set, you can write it as something similar to the natural numbers. Specifically, you can sequence the set, so that each element has a unique corresponding natural number, i.e $S = {a_1, a_2 ,a_3, cdots}$
    $endgroup$
    – rubikscube09
    Dec 10 '18 at 2:05










  • $begingroup$
    Hey Carmichael. I'm not sure. My book has only mentioned the term "measure zero," without additional qualifying adjectives. To quote, "A set $A subseteq mathbb{R}$ has measure zero if, for all $epsilon > 0,$ there exists a countable collection of open intervals $O_n$ with the property that $A$ is contained in the union of all of the intervals $O_n$ and the sum of the lengths of all of the intervals is less than or equal to $epsilon.$"
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:06








  • 1




    $begingroup$
    Yes it is identical. To answer the other question, your book is referring to the Lebesgue measure on $mathbb{R}$. There are other measures of course.
    $endgroup$
    – rubikscube09
    Dec 10 '18 at 2:14








  • 1




    $begingroup$
    Thanks, Rubik! :)
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:14








1




1




$begingroup$
Are you referring specifically to Lebesgue measure? Because this isn't true for an arbitrary measure.
$endgroup$
– carmichael561
Dec 10 '18 at 2:04




$begingroup$
Are you referring specifically to Lebesgue measure? Because this isn't true for an arbitrary measure.
$endgroup$
– carmichael561
Dec 10 '18 at 2:04




1




1




$begingroup$
No, it is not sufficient. But, if you have a countable set, you can write it as something similar to the natural numbers. Specifically, you can sequence the set, so that each element has a unique corresponding natural number, i.e $S = {a_1, a_2 ,a_3, cdots}$
$endgroup$
– rubikscube09
Dec 10 '18 at 2:05




$begingroup$
No, it is not sufficient. But, if you have a countable set, you can write it as something similar to the natural numbers. Specifically, you can sequence the set, so that each element has a unique corresponding natural number, i.e $S = {a_1, a_2 ,a_3, cdots}$
$endgroup$
– rubikscube09
Dec 10 '18 at 2:05












$begingroup$
Hey Carmichael. I'm not sure. My book has only mentioned the term "measure zero," without additional qualifying adjectives. To quote, "A set $A subseteq mathbb{R}$ has measure zero if, for all $epsilon > 0,$ there exists a countable collection of open intervals $O_n$ with the property that $A$ is contained in the union of all of the intervals $O_n$ and the sum of the lengths of all of the intervals is less than or equal to $epsilon.$"
$endgroup$
– Rafael Vergnaud
Dec 10 '18 at 2:06






$begingroup$
Hey Carmichael. I'm not sure. My book has only mentioned the term "measure zero," without additional qualifying adjectives. To quote, "A set $A subseteq mathbb{R}$ has measure zero if, for all $epsilon > 0,$ there exists a countable collection of open intervals $O_n$ with the property that $A$ is contained in the union of all of the intervals $O_n$ and the sum of the lengths of all of the intervals is less than or equal to $epsilon.$"
$endgroup$
– Rafael Vergnaud
Dec 10 '18 at 2:06






1




1




$begingroup$
Yes it is identical. To answer the other question, your book is referring to the Lebesgue measure on $mathbb{R}$. There are other measures of course.
$endgroup$
– rubikscube09
Dec 10 '18 at 2:14






$begingroup$
Yes it is identical. To answer the other question, your book is referring to the Lebesgue measure on $mathbb{R}$. There are other measures of course.
$endgroup$
– rubikscube09
Dec 10 '18 at 2:14






1




1




$begingroup$
Thanks, Rubik! :)
$endgroup$
– Rafael Vergnaud
Dec 10 '18 at 2:14




$begingroup$
Thanks, Rubik! :)
$endgroup$
– Rafael Vergnaud
Dec 10 '18 at 2:14










1 Answer
1






active

oldest

votes


















2












$begingroup$

(Asuuming we are talking about Lebesgue measur)



Let $a_i$ be a sequence of a countable set $A$, then $A_i=(a_i-2^{-i-1}varepsilon,a_i+2^{-i-1}varepsilon)$.



Now $Asubseteq bigcup A_i$, and $muleft(bigcup A_iright)lesum2^{-i}varepsilon=varepsilon$





Showing that $Bbb N$ is not enough because measure does not preserve by bijection, a simple example is the cantor set and the interval [0,1], where they have the same cardinality(hence there exists bijction between them) but one is measure 0 and the other 1.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Hey, Holo. Thanks for the response. My proof was the following: take the union of the intervals $(a_i - frac{epsilon}{2^{i+1}}, a_i + frac{epsilon}{2^{i+1}}).$ The sum would be $sum_{i = 1}^{infty} frac{epsilon}{2} (frac{1}{2})^{i} = epsilon.$ Is this correct? My summation looks different than your's!
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:11












  • $begingroup$
    @RafaelVergnaud you did the exact same things as I did, but you have a little(algebra) mistake, the summation of yours should be with $[...])^{i-1}$, then the $epsilon/2$ will make it $[...])^i$
    $endgroup$
    – Holo
    Dec 10 '18 at 2:14








  • 1




    $begingroup$
    Right. Got it! Thanks a lot Holo!
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:15






  • 1




    $begingroup$
    @RafaelVergnaud I added an example where 2 sets has the same cardinality but not the same measure to explain why to show that $mu(Bbb N)=0$ is not enough
    $endgroup$
    – Holo
    Dec 10 '18 at 2:29













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1 Answer
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1 Answer
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active

oldest

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active

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active

oldest

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2












$begingroup$

(Asuuming we are talking about Lebesgue measur)



Let $a_i$ be a sequence of a countable set $A$, then $A_i=(a_i-2^{-i-1}varepsilon,a_i+2^{-i-1}varepsilon)$.



Now $Asubseteq bigcup A_i$, and $muleft(bigcup A_iright)lesum2^{-i}varepsilon=varepsilon$





Showing that $Bbb N$ is not enough because measure does not preserve by bijection, a simple example is the cantor set and the interval [0,1], where they have the same cardinality(hence there exists bijction between them) but one is measure 0 and the other 1.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Hey, Holo. Thanks for the response. My proof was the following: take the union of the intervals $(a_i - frac{epsilon}{2^{i+1}}, a_i + frac{epsilon}{2^{i+1}}).$ The sum would be $sum_{i = 1}^{infty} frac{epsilon}{2} (frac{1}{2})^{i} = epsilon.$ Is this correct? My summation looks different than your's!
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:11












  • $begingroup$
    @RafaelVergnaud you did the exact same things as I did, but you have a little(algebra) mistake, the summation of yours should be with $[...])^{i-1}$, then the $epsilon/2$ will make it $[...])^i$
    $endgroup$
    – Holo
    Dec 10 '18 at 2:14








  • 1




    $begingroup$
    Right. Got it! Thanks a lot Holo!
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:15






  • 1




    $begingroup$
    @RafaelVergnaud I added an example where 2 sets has the same cardinality but not the same measure to explain why to show that $mu(Bbb N)=0$ is not enough
    $endgroup$
    – Holo
    Dec 10 '18 at 2:29


















2












$begingroup$

(Asuuming we are talking about Lebesgue measur)



Let $a_i$ be a sequence of a countable set $A$, then $A_i=(a_i-2^{-i-1}varepsilon,a_i+2^{-i-1}varepsilon)$.



Now $Asubseteq bigcup A_i$, and $muleft(bigcup A_iright)lesum2^{-i}varepsilon=varepsilon$





Showing that $Bbb N$ is not enough because measure does not preserve by bijection, a simple example is the cantor set and the interval [0,1], where they have the same cardinality(hence there exists bijction between them) but one is measure 0 and the other 1.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Hey, Holo. Thanks for the response. My proof was the following: take the union of the intervals $(a_i - frac{epsilon}{2^{i+1}}, a_i + frac{epsilon}{2^{i+1}}).$ The sum would be $sum_{i = 1}^{infty} frac{epsilon}{2} (frac{1}{2})^{i} = epsilon.$ Is this correct? My summation looks different than your's!
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:11












  • $begingroup$
    @RafaelVergnaud you did the exact same things as I did, but you have a little(algebra) mistake, the summation of yours should be with $[...])^{i-1}$, then the $epsilon/2$ will make it $[...])^i$
    $endgroup$
    – Holo
    Dec 10 '18 at 2:14








  • 1




    $begingroup$
    Right. Got it! Thanks a lot Holo!
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:15






  • 1




    $begingroup$
    @RafaelVergnaud I added an example where 2 sets has the same cardinality but not the same measure to explain why to show that $mu(Bbb N)=0$ is not enough
    $endgroup$
    – Holo
    Dec 10 '18 at 2:29
















2












2








2





$begingroup$

(Asuuming we are talking about Lebesgue measur)



Let $a_i$ be a sequence of a countable set $A$, then $A_i=(a_i-2^{-i-1}varepsilon,a_i+2^{-i-1}varepsilon)$.



Now $Asubseteq bigcup A_i$, and $muleft(bigcup A_iright)lesum2^{-i}varepsilon=varepsilon$





Showing that $Bbb N$ is not enough because measure does not preserve by bijection, a simple example is the cantor set and the interval [0,1], where they have the same cardinality(hence there exists bijction between them) but one is measure 0 and the other 1.






share|cite|improve this answer











$endgroup$



(Asuuming we are talking about Lebesgue measur)



Let $a_i$ be a sequence of a countable set $A$, then $A_i=(a_i-2^{-i-1}varepsilon,a_i+2^{-i-1}varepsilon)$.



Now $Asubseteq bigcup A_i$, and $muleft(bigcup A_iright)lesum2^{-i}varepsilon=varepsilon$





Showing that $Bbb N$ is not enough because measure does not preserve by bijection, a simple example is the cantor set and the interval [0,1], where they have the same cardinality(hence there exists bijction between them) but one is measure 0 and the other 1.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 10 '18 at 2:29

























answered Dec 10 '18 at 2:10









HoloHolo

5,75421131




5,75421131












  • $begingroup$
    Hey, Holo. Thanks for the response. My proof was the following: take the union of the intervals $(a_i - frac{epsilon}{2^{i+1}}, a_i + frac{epsilon}{2^{i+1}}).$ The sum would be $sum_{i = 1}^{infty} frac{epsilon}{2} (frac{1}{2})^{i} = epsilon.$ Is this correct? My summation looks different than your's!
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:11












  • $begingroup$
    @RafaelVergnaud you did the exact same things as I did, but you have a little(algebra) mistake, the summation of yours should be with $[...])^{i-1}$, then the $epsilon/2$ will make it $[...])^i$
    $endgroup$
    – Holo
    Dec 10 '18 at 2:14








  • 1




    $begingroup$
    Right. Got it! Thanks a lot Holo!
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:15






  • 1




    $begingroup$
    @RafaelVergnaud I added an example where 2 sets has the same cardinality but not the same measure to explain why to show that $mu(Bbb N)=0$ is not enough
    $endgroup$
    – Holo
    Dec 10 '18 at 2:29




















  • $begingroup$
    Hey, Holo. Thanks for the response. My proof was the following: take the union of the intervals $(a_i - frac{epsilon}{2^{i+1}}, a_i + frac{epsilon}{2^{i+1}}).$ The sum would be $sum_{i = 1}^{infty} frac{epsilon}{2} (frac{1}{2})^{i} = epsilon.$ Is this correct? My summation looks different than your's!
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:11












  • $begingroup$
    @RafaelVergnaud you did the exact same things as I did, but you have a little(algebra) mistake, the summation of yours should be with $[...])^{i-1}$, then the $epsilon/2$ will make it $[...])^i$
    $endgroup$
    – Holo
    Dec 10 '18 at 2:14








  • 1




    $begingroup$
    Right. Got it! Thanks a lot Holo!
    $endgroup$
    – Rafael Vergnaud
    Dec 10 '18 at 2:15






  • 1




    $begingroup$
    @RafaelVergnaud I added an example where 2 sets has the same cardinality but not the same measure to explain why to show that $mu(Bbb N)=0$ is not enough
    $endgroup$
    – Holo
    Dec 10 '18 at 2:29


















$begingroup$
Hey, Holo. Thanks for the response. My proof was the following: take the union of the intervals $(a_i - frac{epsilon}{2^{i+1}}, a_i + frac{epsilon}{2^{i+1}}).$ The sum would be $sum_{i = 1}^{infty} frac{epsilon}{2} (frac{1}{2})^{i} = epsilon.$ Is this correct? My summation looks different than your's!
$endgroup$
– Rafael Vergnaud
Dec 10 '18 at 2:11






$begingroup$
Hey, Holo. Thanks for the response. My proof was the following: take the union of the intervals $(a_i - frac{epsilon}{2^{i+1}}, a_i + frac{epsilon}{2^{i+1}}).$ The sum would be $sum_{i = 1}^{infty} frac{epsilon}{2} (frac{1}{2})^{i} = epsilon.$ Is this correct? My summation looks different than your's!
$endgroup$
– Rafael Vergnaud
Dec 10 '18 at 2:11














$begingroup$
@RafaelVergnaud you did the exact same things as I did, but you have a little(algebra) mistake, the summation of yours should be with $[...])^{i-1}$, then the $epsilon/2$ will make it $[...])^i$
$endgroup$
– Holo
Dec 10 '18 at 2:14






$begingroup$
@RafaelVergnaud you did the exact same things as I did, but you have a little(algebra) mistake, the summation of yours should be with $[...])^{i-1}$, then the $epsilon/2$ will make it $[...])^i$
$endgroup$
– Holo
Dec 10 '18 at 2:14






1




1




$begingroup$
Right. Got it! Thanks a lot Holo!
$endgroup$
– Rafael Vergnaud
Dec 10 '18 at 2:15




$begingroup$
Right. Got it! Thanks a lot Holo!
$endgroup$
– Rafael Vergnaud
Dec 10 '18 at 2:15




1




1




$begingroup$
@RafaelVergnaud I added an example where 2 sets has the same cardinality but not the same measure to explain why to show that $mu(Bbb N)=0$ is not enough
$endgroup$
– Holo
Dec 10 '18 at 2:29






$begingroup$
@RafaelVergnaud I added an example where 2 sets has the same cardinality but not the same measure to explain why to show that $mu(Bbb N)=0$ is not enough
$endgroup$
– Holo
Dec 10 '18 at 2:29




















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