Power set of Cantor set has cardinality greater than that of reals












0












$begingroup$


I am reading the example of "Boreal measure is not complete" from Wiki:



https://en.wikipedia.org/wiki/Complete_measure



In the first example, it says




The power set of the Cantor set has cardinality strictly greater than that of the reals. Thus there is a subset of the Cantor set that is not contained in the Borel sets.




My questions are the following:




  1. Why "Cantor set has cardinality strictly greater than that of the reals"?

  2. Why "there is a subset of the Cantor set that is not contained in the Borel sets"? Can anyone give me an example of this?


Thanks!






real-analysis elementary-set-theory descriptive-set-theory







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$endgroup$








  • 6




    $begingroup$
    The Cantor set has cardinality equal to the cardinality of the reals, so its power set has greater cardinality. The cardinality of the collection of Borel sets is equal to the cardinality of the continuum.
    $endgroup$
    – T. Bongers
    Dec 13 '18 at 2:35










  • $begingroup$
    @T.Bongers Just sincerely ask a dumb question: "The Cantor set has cardinality equal to the cardinality of the reals" since both are equal to infinity?
    $endgroup$
    – sleeve chen
    Dec 13 '18 at 2:37








  • 3




    $begingroup$
    Since both are equal to continuum. Infinities don't all have the same size.
    $endgroup$
    – Mason
    Dec 13 '18 at 2:39






  • 3




    $begingroup$
    No - after all, that would trivialize the rest of your problem, because it would mean all infinite sets have equal cardinality. The Cantor set can be identified with the ternary numbers in $[0, 1]$ missing a $1$ in their base 3 expansion, which is naturally equinumerous with all of $[0, 1]$ by base 2 expansion.
    $endgroup$
    – T. Bongers
    Dec 13 '18 at 2:44
















0












$begingroup$


I am reading the example of "Boreal measure is not complete" from Wiki:



https://en.wikipedia.org/wiki/Complete_measure



In the first example, it says




The power set of the Cantor set has cardinality strictly greater than that of the reals. Thus there is a subset of the Cantor set that is not contained in the Borel sets.




My questions are the following:




  1. Why "Cantor set has cardinality strictly greater than that of the reals"?

  2. Why "there is a subset of the Cantor set that is not contained in the Borel sets"? Can anyone give me an example of this?


Thanks!






real-analysis elementary-set-theory descriptive-set-theory







share|cite|improve this question











$endgroup$








  • 6




    $begingroup$
    The Cantor set has cardinality equal to the cardinality of the reals, so its power set has greater cardinality. The cardinality of the collection of Borel sets is equal to the cardinality of the continuum.
    $endgroup$
    – T. Bongers
    Dec 13 '18 at 2:35










  • $begingroup$
    @T.Bongers Just sincerely ask a dumb question: "The Cantor set has cardinality equal to the cardinality of the reals" since both are equal to infinity?
    $endgroup$
    – sleeve chen
    Dec 13 '18 at 2:37








  • 3




    $begingroup$
    Since both are equal to continuum. Infinities don't all have the same size.
    $endgroup$
    – Mason
    Dec 13 '18 at 2:39






  • 3




    $begingroup$
    No - after all, that would trivialize the rest of your problem, because it would mean all infinite sets have equal cardinality. The Cantor set can be identified with the ternary numbers in $[0, 1]$ missing a $1$ in their base 3 expansion, which is naturally equinumerous with all of $[0, 1]$ by base 2 expansion.
    $endgroup$
    – T. Bongers
    Dec 13 '18 at 2:44














0












0








0





$begingroup$


I am reading the example of "Boreal measure is not complete" from Wiki:



https://en.wikipedia.org/wiki/Complete_measure



In the first example, it says




The power set of the Cantor set has cardinality strictly greater than that of the reals. Thus there is a subset of the Cantor set that is not contained in the Borel sets.




My questions are the following:




  1. Why "Cantor set has cardinality strictly greater than that of the reals"?

  2. Why "there is a subset of the Cantor set that is not contained in the Borel sets"? Can anyone give me an example of this?


Thanks!






real-analysis elementary-set-theory descriptive-set-theory







share|cite|improve this question











$endgroup$




I am reading the example of "Boreal measure is not complete" from Wiki:



https://en.wikipedia.org/wiki/Complete_measure



In the first example, it says




The power set of the Cantor set has cardinality strictly greater than that of the reals. Thus there is a subset of the Cantor set that is not contained in the Borel sets.




My questions are the following:




  1. Why "Cantor set has cardinality strictly greater than that of the reals"?

  2. Why "there is a subset of the Cantor set that is not contained in the Borel sets"? Can anyone give me an example of this?


Thanks!






real-analysis elementary-set-theory descriptive-set-theory




real-analysis elementary-set-theory descriptive-set-theory






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 13 '18 at 3:18









Andrés E. Caicedo

65.5k8159250




65.5k8159250










asked Dec 13 '18 at 2:32









sleeve chensleeve chen

3,14041853




3,14041853








  • 6




    $begingroup$
    The Cantor set has cardinality equal to the cardinality of the reals, so its power set has greater cardinality. The cardinality of the collection of Borel sets is equal to the cardinality of the continuum.
    $endgroup$
    – T. Bongers
    Dec 13 '18 at 2:35










  • $begingroup$
    @T.Bongers Just sincerely ask a dumb question: "The Cantor set has cardinality equal to the cardinality of the reals" since both are equal to infinity?
    $endgroup$
    – sleeve chen
    Dec 13 '18 at 2:37








  • 3




    $begingroup$
    Since both are equal to continuum. Infinities don't all have the same size.
    $endgroup$
    – Mason
    Dec 13 '18 at 2:39






  • 3




    $begingroup$
    No - after all, that would trivialize the rest of your problem, because it would mean all infinite sets have equal cardinality. The Cantor set can be identified with the ternary numbers in $[0, 1]$ missing a $1$ in their base 3 expansion, which is naturally equinumerous with all of $[0, 1]$ by base 2 expansion.
    $endgroup$
    – T. Bongers
    Dec 13 '18 at 2:44














  • 6




    $begingroup$
    The Cantor set has cardinality equal to the cardinality of the reals, so its power set has greater cardinality. The cardinality of the collection of Borel sets is equal to the cardinality of the continuum.
    $endgroup$
    – T. Bongers
    Dec 13 '18 at 2:35










  • $begingroup$
    @T.Bongers Just sincerely ask a dumb question: "The Cantor set has cardinality equal to the cardinality of the reals" since both are equal to infinity?
    $endgroup$
    – sleeve chen
    Dec 13 '18 at 2:37








  • 3




    $begingroup$
    Since both are equal to continuum. Infinities don't all have the same size.
    $endgroup$
    – Mason
    Dec 13 '18 at 2:39






  • 3




    $begingroup$
    No - after all, that would trivialize the rest of your problem, because it would mean all infinite sets have equal cardinality. The Cantor set can be identified with the ternary numbers in $[0, 1]$ missing a $1$ in their base 3 expansion, which is naturally equinumerous with all of $[0, 1]$ by base 2 expansion.
    $endgroup$
    – T. Bongers
    Dec 13 '18 at 2:44








6




6




$begingroup$
The Cantor set has cardinality equal to the cardinality of the reals, so its power set has greater cardinality. The cardinality of the collection of Borel sets is equal to the cardinality of the continuum.
$endgroup$
– T. Bongers
Dec 13 '18 at 2:35




$begingroup$
The Cantor set has cardinality equal to the cardinality of the reals, so its power set has greater cardinality. The cardinality of the collection of Borel sets is equal to the cardinality of the continuum.
$endgroup$
– T. Bongers
Dec 13 '18 at 2:35












$begingroup$
@T.Bongers Just sincerely ask a dumb question: "The Cantor set has cardinality equal to the cardinality of the reals" since both are equal to infinity?
$endgroup$
– sleeve chen
Dec 13 '18 at 2:37






$begingroup$
@T.Bongers Just sincerely ask a dumb question: "The Cantor set has cardinality equal to the cardinality of the reals" since both are equal to infinity?
$endgroup$
– sleeve chen
Dec 13 '18 at 2:37






3




3




$begingroup$
Since both are equal to continuum. Infinities don't all have the same size.
$endgroup$
– Mason
Dec 13 '18 at 2:39




$begingroup$
Since both are equal to continuum. Infinities don't all have the same size.
$endgroup$
– Mason
Dec 13 '18 at 2:39




3




3




$begingroup$
No - after all, that would trivialize the rest of your problem, because it would mean all infinite sets have equal cardinality. The Cantor set can be identified with the ternary numbers in $[0, 1]$ missing a $1$ in their base 3 expansion, which is naturally equinumerous with all of $[0, 1]$ by base 2 expansion.
$endgroup$
– T. Bongers
Dec 13 '18 at 2:44




$begingroup$
No - after all, that would trivialize the rest of your problem, because it would mean all infinite sets have equal cardinality. The Cantor set can be identified with the ternary numbers in $[0, 1]$ missing a $1$ in their base 3 expansion, which is naturally equinumerous with all of $[0, 1]$ by base 2 expansion.
$endgroup$
– T. Bongers
Dec 13 '18 at 2:44










1 Answer
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As mentioned in the comments, the Cantor set has cardinality $2^{aleph_0}$ (i.e. the same cardinality as the reals). Thus its power set has cardinality $2^{2^{aleph_0}},$ which by Cantor's theorem is greater than the cardinality of the reals. On the other hand, it can be shown that there are only $2^{aleph_0}$-many Borel sets (this isn't trivial). Thus, there are more subsets of the Cantor set than there are Borel sets, so there is a subset of the Cantor set that is not a Borel set.






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    $begingroup$

    As mentioned in the comments, the Cantor set has cardinality $2^{aleph_0}$ (i.e. the same cardinality as the reals). Thus its power set has cardinality $2^{2^{aleph_0}},$ which by Cantor's theorem is greater than the cardinality of the reals. On the other hand, it can be shown that there are only $2^{aleph_0}$-many Borel sets (this isn't trivial). Thus, there are more subsets of the Cantor set than there are Borel sets, so there is a subset of the Cantor set that is not a Borel set.






    share|cite|improve this answer











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      4












      $begingroup$

      As mentioned in the comments, the Cantor set has cardinality $2^{aleph_0}$ (i.e. the same cardinality as the reals). Thus its power set has cardinality $2^{2^{aleph_0}},$ which by Cantor's theorem is greater than the cardinality of the reals. On the other hand, it can be shown that there are only $2^{aleph_0}$-many Borel sets (this isn't trivial). Thus, there are more subsets of the Cantor set than there are Borel sets, so there is a subset of the Cantor set that is not a Borel set.






      share|cite|improve this answer











      $endgroup$
















        4












        4








        4





        $begingroup$

        As mentioned in the comments, the Cantor set has cardinality $2^{aleph_0}$ (i.e. the same cardinality as the reals). Thus its power set has cardinality $2^{2^{aleph_0}},$ which by Cantor's theorem is greater than the cardinality of the reals. On the other hand, it can be shown that there are only $2^{aleph_0}$-many Borel sets (this isn't trivial). Thus, there are more subsets of the Cantor set than there are Borel sets, so there is a subset of the Cantor set that is not a Borel set.






        share|cite|improve this answer











        $endgroup$



        As mentioned in the comments, the Cantor set has cardinality $2^{aleph_0}$ (i.e. the same cardinality as the reals). Thus its power set has cardinality $2^{2^{aleph_0}},$ which by Cantor's theorem is greater than the cardinality of the reals. On the other hand, it can be shown that there are only $2^{aleph_0}$-many Borel sets (this isn't trivial). Thus, there are more subsets of the Cantor set than there are Borel sets, so there is a subset of the Cantor set that is not a Borel set.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Dec 13 '18 at 3:13

























        answered Dec 13 '18 at 2:44









        spaceisdarkgreenspaceisdarkgreen

        33.3k21753




        33.3k21753






























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